Grain filling of cereals under soil drying

Authors


Abstract

Contents

  • Summary 223

  • I. Introduction 224
  • II. Problems in grain filling: unfavorably delayed whole-plant senescence 224
  • III. Controlled soil drying improves carbon remobilization and grain filling as a result of enhanced whole-plant senescence 225
  • IV. Hormonal regulation of whole-plant senescence and grain filling 229
  • V. Activities of key enzymes involved in carbon remobilization and grain filling 230
  • VI. Conclusions 232
  • Acknowledgements 232

  • References 232

Summary

Monocarpic plants require the initiation of whole-plant senescence to remobilize and transfer assimilates pre-stored in vegetative tissues to grains. Delayed whole-plant senescence caused by either heavy use of nitrogen fertilizer or adoption of lodging-resistant cultivars/hybrids that remain green when the grains are due to ripen results in a low harvest index with much nonstructural carbohydrate (NSC) left in the straw. Usually, water stress during the grain-filling period induces early senescence, reduces photosynthesis, and shortens the grain-filling period; however, it increases the remobilization of NSC from the vegetative tissues to the grain. If mild soil drying is properly controlled during the later grain-filling period in rice (Oryza sativa) and wheat (Triticum aestivum), it can enhance whole-plant senescence, lead to faster and better remobilization of carbon from vegetative tissues to grains, and accelerate the grain-filling rate. In cases where plant senescence is unfavorably delayed, such as by heavy use of nitrogen and the introduction of hybrids with strong heterosis, the gain from the enhanced remobilization and accelerated grain-filling rate can outweigh the loss of reduced photosynthesis and the shortened grain-filling period, leading to an increased grain yield, better harvest index and higher water-use efficiency.

I. Introduction

Grain filling is the final stage of growth in cereals where fertilized ovaries develop into caryopses. Its duration and rate determine the final grain weight, a key component of the total yield. In today's crop production systems with their high yield outputs, improvement in grain filling has become more challenging than ever (Venkateswarlu & Visperas, 1987; Saini & Westgate, 2000; Zahedi & Jenner, 2003).

Grain filling in cereals depends on carbon from two resources: current assimilates transferred directly to the grain and assimilates redistributed from reserve pools in vegetative tissues either pre- or post-anthesis (Pheloung & Siddique, 1991; Kobata et al., 1992; Schnyder, 1993). Reserve pools provide the substrate needed to maintain transport and supply of assimilate to grains during the dark period of the diurnal cycle and during the later grain-filling period, when the photosynthetic apparatus is senescing and the rate of dry matter accumulation of grains exceeds the rate of dry matter accumulation of the total crop (Schnyder, 1993). Under adequate moisture conditions, pre-anthesis assimilate reserves in the stems and sheaths of wheat (Triticum aestivum) and rice (Oryza sativa) contribute 10–40% of the final grain weight (Rawson & Evans, 1971; Gallagher et al., 1976; Bidinger et al., 1977; Schnyder, 1993; Gebbing & Schnyder, 1999). Remobilization of reserves to the grain is critical for grain yield if the plants are subjected to water stress (unfavorable to the point of being stressful) or if the yield potential is largely based on high biomass accumulation (Yoshida, 1972; Nicolas et al., 1985a,b; Palta et al., 1994; Ehdaie & Waines, 1996; Asseng & van Herwaarden, 2003; Plaut et al., 2004).

It is generally believed that the process of grain filling is regulated either genetically or environmentally (Yoshida, 1972; Venkateswarlu & Visperas, 1987; Saini & Westgate, 2000). In rice, most indica cultivars usually exhibit a faster grain filling rate than japonica cultivars (Zhu et al., 1997; Cao et al., 1992). The hybrid of japonica/indica rice usually shows a slower grain filling rate than other hybrid rice varieties, for example the indica/indica hybrids (Yuan, 1994, 1998; Peng et al., 1999, 2003). Environmental disturbances also have a great influence on grain filling. The sedentary nature of plants exposes them constantly to variations in environmental conditions (Saini & Westgate, 2000). Water or drought stress is one of the most important factors limiting crop yields world-wide (Jones & Corlett, 1992; Beltrano et al., 1999; Boyer & Westgate, 2004). Water stress occurring during early grain development curtails the kernel sink potential by reducing the number of endosperm cells and amyloplasts formed (Nicolas et al., 1985a; Ober et al., 1991; Saini & Westgate, 2000), thus reducing grain weight as a result of a reduction in the capacity of the endosperm to accumulate starch, in terms of both rate and duration (Yoshida, 1972; Brocklehurst, 1977; Nicolas et al., 1985b).

Recently it has been proposed that grain filling is closely linked to the whole-plant senescence process (Zhang et al., 1998; Yang et al., 2000b; Mi et al., 2002). Unfavorably delayed senescence, which in practice can be induced by either heavy use of nitrogen (N) fertilizer or adoption of lodging-resistant cultivars that stay ‘green’ for too long (i.e. plants remain green when grains are due to ripen), results in a low grain-filling rate, leading to many poorly filled grains. Usually, water stress at grain filling induces early senescence and shortens the grain-filling period but increases remobilization of assimilates from the straw to the grains (Cock & Yoshida, 1972; Yoshida, 1972; Kobata & Takami, 1981; Nicolas et al., 1985b; Palta et al., 1994; Ehdaie & Waines, 1996; Asseng & van Herwaarden, 2003; Plaut et al., 2004). Soil drying is unfavorable to plant growth but may not be unfavourable to the point of producing stress. The question arises as to whether it is possible to take advantage of soil drying-induced whole-plant senescence and better carbon remobilization to improve grain yield in situations where slow grain filling, as a result of delayed senescence, is a problem. Research relate to this question is the focus of this review. The discussions here will cover:

  • • problems in grain filling and the relationship between whole-plant senescence and carbon remobilization;
  • • the possibility that controlled soil drying during grain filling induces whole-plant senescence and thereby enhances carbon remobilization and grain filling;
  • • hormonal regulation of whole-plant senescence and grain filling;
  • • enzymatic activity during carbohydrate remobilization and grain filling.

II. Problems in grain filling: unfavorably delayed whole-plant senescence

Remobilization and transfer of assimilates stored in vegetative tissues to the grain in monocarpic plants such as rice and wheat require the initiation of whole-plant senescence. Delayed whole-plant senescence, which leads to poorly filled grains and unused carbohydrate in straw, is a new problem increasingly recognized in rice and wheat production in recent years. Slow grain filling is almost always associated with delayed whole-plant senescence (Mi et al., 2002; Gong et al., 2005). Although farmers can choose cultivars with early maturation, there are still situations in which delayed senescence is a serious problem requiring attention.

  • 1Heavy use of N fertilizers is well known to lead to delayed senescence and, in the worst cases, canopy lodging. Although farmers are generally aware of this, the problem still occurs every year in some countries, especially in highly productive areas (e.g. Yue, 1997; Dobermann et al., 2002; Buresh et al., 2004). This is possibly related to the intensive nature of agriculture today, with less arable land being used to feed increasing numbers of people.
  • 2The First Green Revolution made a breakthrough in wheat and rice breeding for semidwarf cultivars that are lodging-resistant and can accommodate more chemical fertilizers and produce higher grain yields (Berry et al., 2004). Selection of lodging-resistant cultivars, however, has led to another problem in some cases, namely that stems are short and strong but stored nonstructural carbohydrate (NSC) is poorly used because the plants may remain green when the grains are due to ripen, particularly in the cases of some short-grain rice cultivars (Yuan, 1994, 1998; Zhu et al., 1997). If weather conditions are favorable, and if senescence is also ‘functionally’ delayed, that is, photosynthesis and phloem translocation are functional, such delayed senescence may help achieve higher dry mass production and possibly higher grain yield (Thomas & Smart, 1993). However, in many cases, kernels and their connecting rachis or rachilla seem to mature or senesce earlier than the stem and leaves. Much unused NSC is left in the stem and sheath as a result (e.g. Liang et al., 1994; Ricciardi & Stelluti, 1995; Yang et al., 2002a).
  • 3Introduction of hybrid rice has been a fantastic success in China since the 1970s and in other parts of Asia since the 1990s (Yuan, 1998; Virmani, 2003). Hybrid rice has a yield advantage of more than 20% over conventional rice and helped China to increase the annual grain yield 8.3 million tons from 1976 to 1995 (Yuan, 1998). Utilization of even stronger heterosis from a hybrid between the japonica and indica subspecies (or ecotypes) of rice has, however, also encountered the problem of delayed senescence. Many grains are poorly filled or unfilled (Yang et al., 2002a; Peng et al., 2003; Yuan, 2003). Such hybrid genotypes seem too vigorous in terms of staying ‘young’, and produce high biomass with a much lower harvest index. Hybrid wheat creates a similar problem (Gong et al., 2005): biomass production was 40% higher but grain yield only 15% higher compared with ordinary wheat in a yield trial.

We may define the cases described above as ‘senescence unfavorably delayed’, which means that no gain is obtainable from the extended grain-filling period. We must distinguish this situation from that found in favorable conditions, where early senescence should be avoided because it reduces photosynthesis during the grain-filling period and therefore reduces grain weight (e.g. Zhang et al., 1998; Plaut et al., 2004).

It should be noted that too much N fertilization is a problem in China that has led to the occurrence of unfavorably delayed senescence in both wheat and rice. This is mainly a result of the intensive nature of China's crop production and government-subsidized prices of chemical fertilizers. In Europe and other more developed areas, however, this may not be a problem. Use of chemical fertilizers has very much stabilized over the last few decades (e.g. Peng et al., 1999).

Delayed senescence in situations (2) and (3) described above is basically a genetically controlled character and there are no practical measures that can be taken to deal with this problem. However, it is possible that lodging-resistant varieties may be given more N fertilizer which may cause the canopy to live longer. In northern Europe, where wheat lodging is a serious problem, lodging-prone varieties can be expected to senesce later than lodging-resistant varieties 7(Berry et al., 2004). Situation (1) is managed in practice with so-called ‘appropriate control of N fertilizers and canopy density’, which is often carried out too late when crops are in the grain-filling stage. In all of these three situations, slow grain filling is a yield-limiting factor, and any way in which the rate of grain filling can be enhanced should be beneficial to the final yield formation.

III. Controlled soil drying to improve carbon remobilization and grain filling as a result of enhanced whole-plant senescence

Our experience with field-grown wheat has indicated that soil drying during the grain-filling period can greatly enhance early senescence (Table 1). The grain-filling period was shortened by 10 d (from 41 to 31 d) in unwatered (during the grain-filling period) plots, but a faster grain-filling rate and enhanced remobilization of pre-stored carbohydrate were achieved (Table 1). It seems possible that controlled soil drying at the later grain-filling stage may promote whole-plant senescence, leading to increased re-translocation of the pre-stored carbon reserve in the stem and sheath.

Table 1. Comparison between wheat (Triticum aestivum) plots that were well watered or unwatered during the grain-filling stage
Watering treatmentTime from anthesis to maturation (d)Fate of fed 14C (14CO2 applied 10 d early) Total sugars left in stem (%) (day 26 from anthesis)
% in kernels% in stem
  1. The fate of fed 14C was measured on day 18 from anthesis.

  2. The data are taken from Zhang et al. (1998).

Well watered4141.340.529
Unwatered3181.3 9.6 8

The mechanisms by which the utilization of pre-stored assimilates was enhanced are not known. Many processes are likely to be involved, including the hydrolysis of stored carbohydrate, phloem loading, long-distance translocation and phloem unloading into the kernels. Whatever the mechanisms may be, it is of interest to determine whether such a treatment (i.e. soil drying) can be beneficial to grain filling in cases where grain filling is slow, apparently as a result of delayed senescence. The rationale behind such an approach is as follows.

  • 1Mild soil drying may not seriously disrupt phloem function. It has been shown that phloem translocation may be less susceptible to drought than leaf photosynthesis (Boyer & McPherson, 1975; Kozlowski, 1978).
  • 2The period during which a developing grain receives assimilates, that is the filling period, may be limited by the life-span of its phloem link. Faster filling will certainly have some advantages if the season is limited. It has been shown that ‘stay-green’ (or delayed senescence) is not necessarily associated with the full function of photosynthesis (Thomas & Smart, 1993). It is also possible that the phloem link to the grains may lose its function before chlorophyll disappears from the leaves of ‘stay-green’ genotypes.
  • 3Even in weather conditions that may permit delayed senescence to produce increased photosynthetic assimilation (e.g. in the functionally ‘stay-green’ varieties), the gain from accelerated grain filling from the pre-anthesis carbon reserve may outweigh any loss of photosynthesis as a result of imposed soil drying. Some delayed senescence seems to be genetically controlled, for example in lodging-resistant cultivars that stay green too long or in hybrid cultivars with heterosis that is too strong. These cultivars always leave a substantial amount (200–280 g m−2) of NSC unused in their straw. It has been shown in maize (Zea mays) that it is the fast reallocation of stem carbohydrate that is responsible for the high grain weight, rather than the ‘stay-green’ characteristics (Dwyer et al., 1995).

Here, the term ‘controlled soil drying’ refers to situations in which crops are not soil-dried to such an extent that overnight rehydration cannot be completed and photosynthesis is severely inhibited. It should be stressed that the soil drying should be at the later stage of grain filling because the early development of the embryo (at the rapid cell division stage), that is the ‘grain-setting’ stage, is very susceptible to water stress (Nicolas et al., 1985a; Boyle et al., 1991; Saini & Westgate, 2000; Boyer & Westgate, 2004). Severe water stress immediately after fertilization can cause serious abortion of kernel development in maize (Boyle et al., 1991) and spikelet sterility in rice (Ekanayake et al., 1989, 1993).

It should be noted that wheat grain yield is more vulnerable to a shortened grain-filling period than rice grain yield. In wheat, grain size can be greatly decreased by reduced irrigation during grain filling (Zhang et al., 1998). However, in cases where stay-green is a problem as a result of heavy use of N, moderate soil drying may not necessarily reduce the grain yield of wheat (see discussion in section III).

In field experiments with highly lodging-resistant cultivars (total precipitation, mean solar radiation and daily temperature during the grain-filling period were 105 and 142 mm, 16.8 and 18.4 MJ m−2 d−1, and 19.2 and 24.8°C, respectively, for wheat and rice), our results (Yang et al., 2000b, 2001b,c,d, 2004a) showed that, if soil drying (soil water potential of −0.05 MPa for rice and −0.08 MPa for wheat) during the grain-filling periods of rice and wheat is controlled properly so that plants can rehydrate overnight (i.e. the predawn leaf water potential is not statistically different from that of well-watered plants), photosynthesis should not be severely inhibited (Figs 1 and 2). One benefit of such soil drying is that it can enhance plant senescence and lead to faster and better remobilization of pre-stored carbon from vegetative tissues to the grains (Table 2). The early senescence induced by soil drying does not necessarily reduce the grain yield (on a dry weight basis) even when the plants are grown under normal N conditions. Furthermore, in cases where plant senescence is unfavorably delayed, such as by heavy use of N, the gain from the enhanced remobilization and accelerated grain-filling rate may outweigh the loss of photosynthesis and the shortened grain-filling period and increase the grain yield and harvest index (Table 3).

Figure 1.

Changes in leaf water potentials of rice, Oryza sativa (cv. Wuyujing 3) (a) and wheat, Triticum aestivum (cv. Yangmai 158) (b) during the first 30 d (a) or 24 d (b) after withholding water. Treatments were normal amounts of nitrogen (NN) + well watered (WW); high amounts of nitrogen (HN) + WW; NN + soil drying (SD); and HN + SD. Measurements were made on the flag leaves predawn (PD, 06:00 h) and at midday (MD, 11:30 h). Vertical bars represent ± the standard error of the mean (n = 6) where they exceed the size of the symbol. Data are adapted from Yang et al. (2001d, 2004a).

Figure 2.

Photosynthetic rates in the flag leaves of rice, Oryza sativa (cv. Wuyujing 3) (a) and wheat, Triticum aestivum (cv. Yufu 188) (b) during the grain-filling period. Treatments were normal amounts of nitrogen (NN) + well watered (WW); high amounts of nitrogen (HN) + WW; NN + soil drying (SD); and HN + SD. Arrows indicate the start of the period in which water was withheld. Vertical bars represent ± the standard error of the mean (n = 6) where they exceed the size of the symbol. Data are adapted from Yang et al. (2000b, 2001d).

Table 2. Remobilization of pre-stored assimilates in the straw of rice, Oryza sativa (cv. Wuyujing 3) and wheat, Triticum aestivum (cv. Yanfu 188) subjected to various nitrogen and soil moisture treatments
CropWater treatmentNitrogen appliedRemobilized carbon reserve (%)Contribution to grain (%)NSC in residue (mg g−1 d. wt)Total dry matter (g m−2)Harvest index
  1. The cultivars used were highly lodging-resistant, that is they stay green when grains are mature.

  2. NN and HN indicate normal and high amounts of nitrogen application at heading time, respectively. WW and SD indicate the well-watered and soil-drying treatments during the grain-filling period, respectively. Values are means of 20 plants. Different letters indicate statistical significance at P < 0.05 within a column and within the same crop. NSC, nonstructural carbohydrate in straw; d. wt, dry weight.

  3. Data are taken from Yang et al. (2000b, 2001d).

RiceWWNN48 c14 c142 b1707 a0.47 c
WWHN25 d 7.6 d219 a1741 a0.41 d
SDNN75 a29 a 65 d1538 b0.52 a
SDHN61 b22 b104 c1717 a0.50 b
WheatWWNN57 c23 c134 b1764 a0.42 c
WWHN33 d22 c231 a1503 b0.38 d
SDNN79 a44 a 67 d1477 b0.44 a
SDHN65 b36 b112 c1714 a0.43 b
Table 3. Grain-filling rate and grain yield of rice, Oryza sativa (cv. Wuyujing 3) and wheat, Triticum aestivum (cv. Yangmai 158) subjected to various nitrogen and soil moisture treatments
CropWater treatmentNitrogen appliedActive grain-filling period (d)Grain-filling rate (mg d−1 per grain)Total number of spikelets/grains × 103 m−2Ripened grain (%)Grain weight (mg per grain)Grain yield (g m−2)
  1. The cultivars used were highly lodging-resistant, that is they stay green when grains are mature.

  2. NN and HN indicate normal and high amounts of nitrogen application at heading time, respectively. WW and SD indicate well-watered and soil-drying treatments during the grain-filling period, respectively. The active grain-filling period and grain filling were calculated according to the Richards (1959) equation. Values for total number of spikelets or grains, grain weight and percentage of ripened grains were means of plants harvested from 2 m2 of each treatment. The grain yield was the mean from 15–18 m2 of each treatment and was calculated on a dry weight basis. Different letters indicate statistical significance at P < 0.05 within a column and within the same crop.

  3. Data are taken from Yang et al. (2001d, 2004a).

RiceWWNN20 b1.21 c33.73 a90.8 b26 b802 b
WWHN25 a0.91 d33.78 a84.2 c25 c714 c
SDNN17 c1.39 a33.71 a90.2 b26 b780 b
SDHN19 b1.28 b33.62 a94.2 a27 a858 a
WheatWWNN27 b1.40 c17.18 a42 b701 b
WWHN34 a0.95 d17.81 a36 c628 c
SDNN19 d1.94 a16.68 a41 b679 b
SDHN24 c1.69 b17.90 a45 a785 a

When senescence is unfavorably delayed, rice and wheat will show prolonged, slow grain filling, for example under high-N conditions (Fig. 3). Controlled soil drying increases grain-filling rates and shortens grain-filling periods. The increased rate and the shortened period are especially remarkable in high-N conditions (Figs 3c,d). Our results (Yang et al., 2001d, 2003c, 2004a) with lodging-resistant cultivars also showed that the final grain weight was not significantly different between well-watered and soil-drying treatments when a normal amount of N was applied. However, it was significantly increased under soil drying plus high-N treatment, implying that the gain from accelerated grain-filling rate outweighed the possible loss of photosynthesis as a result of a shortened grain-filling period when plants were subjected to soil drying during grain filling.

Figure 3.

The grain-filling process (a, b) and grain-filling rate (c, d) of rice, Oryza sativa (cv. Wuyujing 3) (a, c) and wheat, Triticum aestivum (cv. Yangmai 158) (b, d) subjected to various nitrogen and soil moisture treatments. Treatments were normal amounts of nitrogen (NN) + well watered (WW); high amounts of nitrogen (HN) + WW; NN + soil drying (SD); and HN + SD. Grain-filling rate was calculated according to the Richards (1959) equation. Arrows indicate the start of the period in which water was withheld. Vertical bars in (a) and (b) represent ± the standard error of the mean (n = 3) where they exceed the size of the symbol. Data are adapted from Yang et al. (2001c, 2004a).

The hybrid of japonica/indica rice possesses stronger heterosis in biomass production than other hybrid rice varieties, for example the currently used indica/indica hybrid rice (Yuan, 1994, 2003; Peng et al., 1999, 2003). Too many poorly filled grains, however, hinder the exploitation of such heterosis (Yuan, 1994, 1998), and have been attributed to many factors (e.g. Yuan, 1994, 1998; Peng et al., 1999, 2003; Yang et al., 2002a). The slow grain filling that results from delayed whole-plant senescence is considered the main cause of poor grain filling (Yuan, 1994, 1998; Zhu et al., 1997; Peng et al., 1999, 2003; Yang et al., 2002a). Controlled soil drying imposed during grain filling substantially enhanced the remobilization of pre-stored carbohydrate (98–124 g m−2) to the grains, accelerated the grain-filling rate, and increased the grain yield of japonica/indica hybrids (Yang et al., 2002c). Similar findings were obtained in two-line hybrid rice that shows slow grain filling mainly as a result of delayed senescence (Yang et al., 2003a). Moderate soil drying imposed during the grain-filling period substantially accelerated the grain-filling rate. The grain weight and grain yield were actually improved, rather than reduced, by the moderate soil drying (Table 4).

Table 4. Grain-filling rate and grain yield of rice (Oryza sativa) subjected to various soil moisture treatments
HybridWater deficit treatmentActive grain-filling period (d)Grain-filling rate (mg d−1 per grain)Total number of spikelets × 103 m−2Ripened grain (%)Grain weight (mg per grain)Grain yield (g m−2)
  1. Two two-line indica hybrids Pei-Ai 64S/Yangdao 6 and Pei-Ai 64S/E 32 were field grown. WW, MD and SD indicate well-watered conditions, moderate soil drying and severe soil drying, respectively, during grain filling. The active grain-filling period and grain-filling rate were calculated according to the Richards (1959) equation. The total number of spikelets, the percentage of ripe grains, and grain weight were means of plants harvested from 3 m2 of each treatment. Grain yield was the mean of plants harvested from 12 m2 of each treatment and was calculated on a dry weight basis. Different letters indicate statistical significance at P < 0.05 within a column and within the same hybrid.

  2. Data are taken from Yang et al. (2003a).

Pei-Ai 64S/Yangdao 6WW44 a0.52 c52.9 a73.4 b25.6 b 994 b
MD39 b0.61 b51.7 a77.8 a26.8 a1077 a
SD32 c0.71 a51.9 a72.6 b25.2 b 949 b
Pei-Ai 64S/E32WW46 a0.49 c49.9 a70.2 b25.3 b 886 b
MD40 b0.59 b49.1 a74.1 a26.4 a 959 a
SD33 c0.68 a49.8 a69.7 b24.9 b 863 b

In theory, N metabolism and respiration should affect grain filling (Ntanos & Koutroubas, 2002). However, information linking N/protein metabolism and respiration under soil drying to grain filling is lacking in the literature. It is possible that soil N availability is reduced under soil drying and N uptake is curtailed. Advancing senescence may therefore inhibit canopy protein remobilization, reduce grain protein synthesis, and hence increase yield by a reduction in respiratory requirement. This possibility is worthy of investigation.

IV. Hormonal regulation of whole-plant senescence and grain filling

Extensive studies have demonstrated that post-anthesis water deficits result in early senescence and more remobilization of pre-anthesis stored assimilates to grains in cereals (Gallagher et al., 1976; Johnson & Moss, 1976; Jones & Rawson, 1979; Austin et al., 1980; Lauer & Simmons, 1985; Nicolas et al., 1985b; Kobata et al., 1992; Palta et al., 1994; Zhang et al., 1998). However, the mechanisms by which plant senescence promotes remobilization of assimilates are rather obscure. Very little is known about the relationship between these two processes, a surprisingly neglected subject given its importance to yield formation.

Many factors, either endogenous or environmental, contribute to plant senescence (Nooden, 1988a; Smart, 1994; Lers et al., 1998; Pic et al., 2002), but abscisic acid (ABA) and cytokinins are generally believed to be two of the major regulators of plant senescence (Biswas & Choudhuri, 1980; Nooden, 1988b; Zeevaart & Creelman, 1988; Haberer & Kieber, 2002). High concentrations of cytokinins in plants can delay senescence (Buchanan-Wollaston, 1997), whereas a high ABA concentration has the opposite effect (Nooden, 1988b; Madhu et al., 1999; Tadas et al., 1999). ABA also has been reported as an important regulator in the transport of assimilates to the developing seeds or fruits (Dewdney & McWha, 1979; Tietz et al., 1981; Clifford et al., 1986; Brenner & Cheikh, 1995). However, contradictory data on the involvement of ABA in the regulation of both senescence and assimilate remobilization have been reported (Setter et al., 1980; Nooden, 1988a; Barratt et al., 1989; Ober & Setter, 1990; Brown et al., 1991; de Bruijn & Vreugdenhil, 1992). Our work (Yang et al., 2001b, 2002d, 2003d) on both rice and wheat demonstrated that the remobilization of pre-stored assimilates was closely associated to whole-plant senescence. Soil drying during the grain-filling period enhanced senescence and led to more and faster remobilization of pre-stored carbon assimilates from the stems to grains. In contrast, heavy use of N slowed senescence and retarded such remobilization. Such results indicate that senescence and remobilization are coupled processes in rice and wheat. Our results (Yang et al., 2002d, 2003d) showed that soil drying substantially increased ABA accumulation (concentration) in the leaves and stems or root exudates, and markedly reduced cytokinins (zeatin and zeatin riboside) in the leaves. ABA was significantly and negatively correlated with the photosynthetic rate and chlorophyll content of the flag leaves, while cytokinins were positively correlated with these variables. Spraying 20 × 10−6 m ABA significantly reduced the chlorophyll content, whereas spraying 40 × 10−6 m kinetin increased it. These results suggest that both ABA and cytokinins are involved in controlling senescence in rice and wheat under conditions of soil drying.

We observed that elevated ABA concentrations in the stems or root exudates were associated with the partitioning of pre-feed 14C in the grains in soil drying treatments. ABA in both the leaves and stems, but not cytokinins, was significantly and positively correlated with remobilization of pre-stored carbon, and such remobilization was enhanced by exogenous ABA, suggesting that enhanced remobilization by soil drying during grain filling can be attributed, at least partly, to an elevated ABA concentration in the plant.

It is well known that plant hormones are involved in grain filling and seed development (e.g. Davies, 1987; Brenner & Cheikh, 1995; Yang et al., 2000a, 2002b, 2003b). There are many reports of auxins, gibberellins (GAs) and ABA regulating grain development (Karssen, 1982; Davies, 1987; Kende & Zeevaart, 1997). In rice grains, hormone content (per grain) and hormone concentration (per unit weight) are significantly correlated, and both vary with grain-filling stage (Yang et al., 2000a). We (Yang et al., 2001c) observed that cytokinins and indole-3-acetic acid contents in rice grains transiently increased at the early filling stage and coincided with the rapid increase in grain-filling rate. The heights and timings of such peaks were regulated by N nutrition status, with high N being associated with lower peaks and their later appearance. Controlled soil drying only hastened declines in cytokinins and indole-3-acetic acid contents at the late grain-filling stage.

Contents of GAs (GA1 + GA4) in rice grains were also higher at the early grain-filling stage than at the late stage. High N enhanced, while soil drying substantially reduced, GA accumulation. ABA content in the rice grains was low at the early grain-filling stage, reaching a maximum when the grain-filling rate was highest. Soil drying greatly increased ABA accumulation in the grains. The peak values of ABA in the grains were significantly correlated with the maximum grain-filling rates. Our results (Yang et al., 2001c) suggest that an altered hormonal balance in the grains produced by soil drying during grain filling, especially a decrease in GAs and an increase in ABA, enhances the remobilization of pre-stored carbon to the grains and accelerates the grain-filling rate.

It was reported that an antagonistic relationship between ABA and ethylene mediates root and shoot growth in maize (Spollen et al., 2000) and tomato (Lycopersicon esculentum) (Hussain et al., 2000; Sharp et al., 2000; Sharp, 2002) when plants are subjected to water stress. We observed that, in contrast to that of ABA, the concentration (evolution rate) of ethylene in rice grains was very high at the early grain-filling stage and sharply decreased during the linear period of grain growth (Yang et al., 2004c). Moderate soil drying reduced, whereas severe soil drying increased, ethylene accumulation. The ethylene concentration was significantly and negatively correlated with the grain-filling rate. Application of cobalt ion (an inhibitor of ethylene synthesis) or ABA at the early grain-filling stage significantly increased the grain-filling rate. Spraying with ethephon (an ethylene-releasing agent) or fluridone (an inhibitor of ABA synthesis) had the opposite effect (Yang et al., 2004c). The results indicate that the effect of ABA and ethylene on grain-filling rate depends not only on their concentrations but also on their balance. Correlation coefficients between hormone concentrations/ratios and grain-filling rate based on the hormone measurements we have made in various studies are listed in Table 5.

Table 5. Correlation coefficients between hormone concentrations/ratios in the grains and grain-filling rates during the active grain-filling period (the linear increase period for grain dry weight) of rice (Oryza sativa)
Hormone concentration/ratioCoefficientsReferences
  1. Data are taken from the references listed in the table. *, ** indicate statistical significance at the P < 0.05 and P < 0.01 levels, respectively.

Cytokinins (CK)   0.69* Yang et al. (2000a)
Indole-3-acetic acid (IAA)   0.46, 0.42 Yang et al. (2000a, 2001c)
Gibberellins (GAs)   0.38, −0.14 Yang et al. (2000a, 2001c)
Abscisic acid (ABA)   0.95**, 0.98**, 0.74** Yang et al. (2001c, 2003b, 2004c)
Ethylene (Ethy)−0.78** Yang et al. (2004c)
CK:IAA   0.37 Yang et al. (2000a)
CK:GAs   0.52 Yang et al. (2000a)
IAA:GAs   0.45 Yang et al. (2000a)
Ethy:ABA   0.79** Yang et al. (2004c)
GAs:ABA   0.63* Yang et al. (2001c)

V. Activities of key enzymes involved in carbon remobilization and grain filling

The main storage form of NSC in the stem (culm + sheath) of rice is starch (Murayama et al., 1961; Murata & Matsushima, 1975; Cao et al., 1992). Starch needs to be degraded as glucose first and then sucrose is re-synthesized when the carbon is remobilized from the stem to the grains (Murayama et al., 1961; Venkateswarlu & Visperas, 1987; Beck & Ziegler, 1989). Starch degradation can occur via hydrolytic and phosphorolytic reactions, or probably the concerted action of several enzymes (Beck & Ziegler, 1989; Nielsen et al., 1997). These enzymes are likely to include α-amylase, β-amylase, α-glucosidase, and starch phosphorylase. Based on its high affinity, sucrose-phosphate synthase (SPS) is expected to play a major role in the re-synthesis of sucrose (Whittingham et al., 1979; Wardlaw & Willenbrink, 1994) and sustain the assimilatory carbon flux from source to sink (Isopp et al., 2000). There are reports that starch degradation in stolons of white clover is controlled by α-amylase activity (Gallagher et al., 1997). Water stress can induce α-amylase in barley leaves (Jacobsen et al., 1986) and enhance β-amylase activity in cucumber cotyledons (Todaka et al., 2000). We (Yang et al., 2001a) also investigated all the possible enzymes involved in starch degradation and sucrose re-synthesis in rice stems during grain filling. Our results showed that both α- and β-amylase activities were enhanced by soil drying, with the former enhanced more than the latter, and significantly correlated with the concentrations of soluble sugars in the stems. The other two possible starch-breaking enzymes, α-glocosidase and starch phosphorylase, showed no significant differences in activities between the well-watered and water stress treatments. Soil drying also increased the SPS activity that is responsible for sucrose production. These results suggest that the fast hydrolysis of starch and increased carbon remobilization are attributable to enhanced α-amylase and SPS activities in rice stems under soil drying.

In contrast to those in rice stems, the main storage forms of water-soluble carbohydrates (WSCs) in wheat stems are fructans and sucrose (Judel & Mengel, 1982; Blacklow et al., 1984; Hendrix et al., 1986; Kühbauch & Thome, 1989; Wardlaw & Willenbrink, 1994; Yukawa et al., 1995). This storage peaks well into the period of grain filling under adequate moisture conditions and declines during the later stages of kernel development to supply a high proportion of the assimilates needed for concurrent kernel development (e.g. Wardlaw & Willenbrink, 1994). At the stage of maximum WSC content, fructans and sucrose represented 85% and 10%, respectively, of the WSCs in wheat stem internodes (Blacklow et al., 1984). The reserve of WSCs is likely to be determined by the activity of enzymes associated with both fructan and sucrose metabolism in the wheat stem. It has been reported that the synthesis of high-molecular-weight fructan is catalyzed by sucrose-sucrose fructosyl transferase (SST) and fructan-fructan fructosyl transferase (Nelson & Spollen, 1987; Housley et al., 1989; Pollock & Cairns, 1991). SST is considered to be the most important enzyme for fructan synthesis as it increases concomitantly with fructan accumulation (Wagner et al., 1986; Dubois et al., 1990; Yukawa et al., 1995). Fructan exohydrolase (FEH) catalyzes the hydrolysis of fructans, leading to the release of fructose which, in turn, has to be converted to the precursors required for the re-synthesis of sucrose, catalyzed by enzymes involved in the synthesis of sucrose, mainly SPS (Huber & Huber, 1996), before phloem loading (Simpson & Bonnett, 1993; Willienbrink et al., 1998). Our results (Yang et al., 2004b) showed that both FEH and SPS activities were substantially enhanced by controlled soil drying during the grain-filling period and positively correlated with the total NSC and fructan remobilization from wheat stems. The activity of acid invertase, a hydrolase cleaving sucrose irreversibly into glucose and fructose (Sturm & Tang, 1999; Wang et al., 2000), was also enhanced by soil drying and associated with the change in fructan concentration, but not correlated with the total NSC remobilization and pre-feed 14C increase (indicating fast filling) in the grains. SST activity was inhibited by soil drying and negatively correlated with the remobilization of carbon reserves. All of these results demonstrate that source-supplying capacity is increased by regulation of key enzymes involved in the hydrolysis of fructans and the synthesis of sucrose when controlled soil drying accelerates carbon remobilization.

On the sink side, grain filling is a process of active metabolism of carbohydrate and starch accumulation in kernels. It is generally accepted that four enzymes may play a key role in this process: sucrose synthase (SuSase), ADP glucose pyrophosphorylase (AGPase), starch synthase (StSase), and starch branching enzyme (SBE) (Hawker & Jenner, 1993; Ahmadi & Baker, 2001; Hurkman et al., 2003). SuSase catalyses the cleavage of sucrose, the main transported form of assimilates in wheat plants (Fisher & Gifford, 1986), to form UDP-glucose and fructose, which is thought to be the first step in sucrose-to-starch conversion. Its activity is considered to be linked to sink strength in the developing rice grain and tomato fruit (Sun et al., 1992; Wang et al., 1993; Kato, 1995). AGPase produces ADP-glucose, the primer of the starch chain (Smith & Denyer, 1992), and is regarded as the rate-limiting enzyme in starch biosynthesis (Preiss, 1988). StSase, composed of both soluble and granule-bound isoforms, elongates the amylose and amylopectin chains (Déjardin et al., 1997). Soluble StSase (SSS) activity is reported to be positively correlated with the rate of starch synthesis in wheat grains (Keeling et al., 1993). SBE forms branches on the polymers. It cleaves α-1,4 bonds on both amylose and amylopectin molecules and reattaches the released glucan segments to the same or another glucan chain through the formation of α-1,6 linkages (Hurkman et al., 2003). Its activity is closely associated with the increase in starch content during the development of the rice endosperm (Nakamura et al., 1989; Nakamura & Yuki, 1992).

Numerous studies have been performed on the effects of heat stress on the activities of enzymes involved in sucrose-to-starch metabolism in cereals (Caley et al., 1990; Hawker & Jenner, 1993; Keeling et al., 1993; Jenner, 1994; Cheih & Jones, 1995; Duke & Doehlert, 1996; Wilhelm et al., 1999; Hurkman et al., 2003). A correlation of reduction in starch content with declines in SuSase and SSS activities in heat-treated grains has been reported in these studies. With the exception of the work by Ahmadi & Baker (2001), who reported that reduction in grain growth rate of water-stressed wheat plants resulted from reduced SSS activity, whereas growth cessation resulted mainly from the inactivation of AGPase, information is scarce on changes in activities of key enzymes in the sucrose-to-starch catalytic pathway in water-stressed wheat and rice grains during the grain-filling period.

In our studies of rice and wheat grain filling under controlled soil drying (Yang et al., 2003c, 2004a), activities of SuSase, SSS and SBE in the grains were substantially enhanced by soil drying and positively correlated with the starch accumulation rate (SAR) in grains. AGPase activity was also enhanced by soil drying and correlated with SAR with a smaller coefficient. Activities of granule-bound starch synthase and soluble and insoluble acid invertase in the grains were less affected by soil drying. These results indicate that increased grain filling rate is mainly attributable to enhanced sink activity through regulation of key enzymes involved in sucrose-to-starch conversion, especially SuSase, SSS and SBE, in rice and wheat grains when subjected to mild soil drying during grain filling.

We (Yang et al., 2001a, 2003c, 2004a,b) found that enhanced activities of SPS and FEH in stems and SuSase, SSS and SBE in grains were closely associated with elevated ABA concentrations in grains. Our spraying experiments (Yang et al., 2001a, 2003c, 2004a,b) showed that application of ABA to well-watered plants gave similar results to those obtained by soil drying. Spraying with fluridone, an ABA synthesis inhibitor, had the opposite effect. These results suggest that ABA plays a vital role in the regulation of the key enzymes involved in carbon remobilization and metabolism of carbohydrate in grains, although exogenously applied ABA could produce multiple effects such as stomatal closure, which leads to a reduction in photosynthesis, and ABA above normal concentrations in the grains under severe water stress may have an inverse effect on filling as a result of a shortened grain-filling period.

VI. Conclusions

If soil drying during the grain-filling period of rice and wheat is controlled properly so that plants can rehydrate overnight, photosynthesis should not be severely inhibited. A benefit of such soil drying is that it can enhance whole-plant senescence and lead to faster and better remobilization of pre-stored carbon from vegetative tissues to the grains. The gain from enhanced remobilization and accelerated grain-filling rate may outweigh the loss of photosynthesis and the shortened grain-filling period and increase grain yield and harvest index in cases where plant senescence is unfavorably delayed. This practice has great significance for agriculture for several reasons. First, seasons for crop production are limited by adverse weather conditions in many parts of the world. Continental hot, dry winds common in early June in the north and middle of China can dehydrate wheat in 1 or 2 d before it matures, while the cold front in late autumn abruptly marks the end of the rice season in east Asia. Controlled soil drying may accelerate whole-plant senescence so that the wheat and rice plants can mature before the adverse conditions occur. Secondly, the heavy use of N results in unfavorably delayed senescence and slow grain filling, leading to low grain weight. Early senescence induced by soil drying could increase the rate of grain filling and improve grain weight in this case. Thirdly, some lodging-resistant rice and wheat cultivars or hybrids have a low harvest index and poor grain filling because the plants remain ‘green’ when the grains are due to ripen and poorly remobilize assimilates to the grains. Controlled soil drying may induce these cultivars or hybrids to mobilize more pre-stored assimilates to grains. In addition, controlled soil drying may contribute to water saving in rice and wheat production, which is urgently needed to develop sustainable agriculture in many parts of the world where the rapid depletion of water resources is threatening crop production.

How can controlled soil drying be applied during the grain-filling period, if needed? We applied soil drying in the field experiment by giving the crop small amounts of water and monitoring soil water potentials. In commercial production, control of soil drying may be achieved by monitoring leaf rolling and wilting of older leaves as a sign of plant water stress and applying small amounts of irrigation at a time. Plants develop sequential responses, and the older leaves usually show wilting first under prolonged soil drying (Zhang & Davies, 1989). In agriculture, better regulation of the use of N fertilizers has also become an urgent issue. In areas such as China where food demand is high and chemical fertilizers are relatively cheap, over-application of N fertilizers may not only lead to serious environmental problems, but also cause reduced yield as a result of delayed senescence, as discussed above. Such a problem may also have implications for plant water use efficiency. Over-expanded leaf areas and delayed senescence will result in plants consuming water resources that could be used for other purposes. Water shortage has become the most limiting factor in crop production in many parts of the world (e.g. Zhang & Yang, 2004).

Acknowledgements

We gratefully acknowledge support from the Research Grant Council of Hong Kong (RGC 2165/05M; RGC 2149/04M), the Area of Excellence for Plant and Fungal Biotechnology in the Chinese University of Hong Kong, the National Natural Science Foundation of China (grant no. 30270778, 30370828), and the State Key Project (grant no. 2004BA520A12-5).

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