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- Materials and methods
Seeds perceive the light environment through photoreceptors, especially the phytochrome family (Bliss & Smith 1985; Hartmann & Nezadal 1990; Casal, Sánchez & Botto 1998). Phytochromes have two photo-interconvertible forms: Pfr (usually the active far-red absorbing form), with maximum absorption at 735 nm, and Pr (the inactive red-absorbing form), with maximum absorption at 665 nm (Borthwick et al. 1954; Taylorson & Borthwick 1969; Mancinelli 1994). Exposure of seeds to light with a high red (R) to far-red (FR) ratio (R : FR) leads to the formation of Pfr : P ratios that might be high enough to trigger germination, depending on the dormancy level in the seed population. Phytochrome-mediated responses can be classified physiologically into three classes (Kronenberg & Kendrick 1986). Two of these classes, the very low fluence responses (VLFR) and the low fluence responses (LFR), are characterized by the correlation between the physiological effect and the level of Pfr formed by the light exposure. They differ in that an extremely low level of Pfr saturates VLFR, while higher Pfr levels are needed to bring about LFR (Mancinelli 1994; Casal, Sánchez & Botto 1998). High irradiance responses (HIR) are the third class, showing no simple relationship between Pfr levels, and may involve additional components of the phytochrome system (Heim & Schäfer 1982, 1984; Mancinelli 1994). The HIR of germination has maximum activity at 710–720 nm (Hartmann 1966; Hendricks, Toole & Borthwick 1968; Mohr 1972) and the inhibitory effect of continuous FR can be secured for R-promoted seeds, even after lapse of the escape time (Mohr & Appuhn 1963; Hartmann 1966; Frankland & Taylorson 1983).
The presence of a crop canopy reduces the photon flux of all wavelengths relative to full daylight, much more in the photosynthetically active part of the spectrum (400–700 nm) than in the near infra-red (700–1000 nm), because of strong absorption by chlorophylls (Taylorson & Borthwick 1969; Smith 1982; Pons 1992). Hence canopy shade light is rich in FR and poor in R. For example, unfiltered daylight has an R : FR ratio of approximately 1·2 and leaf canopies may reduce this value to 0·2, depending on the leaf area index (LAI; Federer & Tanner 1966; Holmes & Smith 1977; Pons 1983). Moreover, the presence of a crop canopy reduces the thermal amplitude of the soil, thus preventing germination of species that require fluctuating temperatures to terminate dormancy (Thompson & Grime 1983; Benech-Arnold et al. 1990). Although it is well known that canopy presence interferes with seed germination through modifications of the above-mentioned environmental cues, the issue has only been investigated through static studies (i.e. the effect of an established canopy). The changing effect of a growing canopy has not been assessed previously, thus precluding the identification of shading-intensity thresholds for triggering the different types of canopy-detection mechanisms in different seed populations.
Knowledge of the regulatory effects of the crop canopy on weed seed germination is necessary to understand fully the behaviour of weed seed banks during a crop cycle. In this context the following questions arise. (i) Do seeds from the different predominant weed species require light and/or fluctuating temperatures to have their dormancy terminated? (ii) With what LAI is a crop able to diminish light quality and/or thermal amplitude to prevent the germination of seeds requiring these factors for dormancy termination? Location of the weed seeds in the soil, among other things, will determine changes in the factors that terminate dormancy. For example, modifications in the light environment would only be perceived by seeds that are located at the soil surface or very close to it (Tester & Morris 1987; Kasperbauer & Hunt 1988), whereas variations in the thermal amplitude must be registered independently of the position that seeds occupy in the soil profile (Ghersa, Benech-Arnold & Martinez-Ghersa 1992). In this work we tried to clarify whether the germination–emergence pattern of weed species, accompanying a wheat canopy, can be modified by alterations in the light and thermal environments within this crop. For this purpose experiments under field conditions were carried out, to modify artificially the thermal and light environments under a wheat canopy, until light and thermal conditions prevailing at bare soil (i.e. soil without vegetation) were matched. It was also intended to elucidate the mechanisms through which germination of the different species is regulated by the presence of the crop.
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- Materials and methods
It is well known that crops accompanying the weed community can modulate environmental factors such as light and fluctuating temperatures that terminate dormancy (Holmes & Smith 1977; Deregibus et al. 1994). These modifications in the seed environment are dynamic during the crop cycle and their influence varies with crop structure and the different phenological stages (Cumming 1963). In weed species requiring environmental signals to terminate dormancy, the strength of the regulation of the emergence by the presence of a crop canopy will depend on the overlap between the ‘emergence window’ of the weed (i.e. the period at which the seed population displays minimum dormancy; Kruk & Benech-Arnold 2000; Vleeshouwers & Kropff 2000) and the density of the crop canopy modifying these signals. The capacity to detect these changes in the thermal and light environments will also depend on the position of the seeds within the soil (Bliss & Smith 1985; Ghersa, Benech-Arnold & Martinez-Ghersa 1992), which will determine the probability of a seed germinating, emerging successfully and its relative time of emergence (Grundy, Mead & Burston 2003).
Two seed locations in the soil were studied in this work: (i) a buried seed bank in experiment 1 and (ii) seeds at the soil surface in experiment 2. In experiment 1 no modifications were observed in the emergence pattern of the different species beneath a crop canopy if related to that observed at bare soil in a buried seed bank (Fig. 2). This could be because during the crop establishment stages (i.e. when most of the weed emergence takes place) the changes produced in the thermal environment were not sufficiently pronounced to modify the emergence pattern of the different species. Galinsoga parviflora presented a particular behaviour: the life cycle of this weed is very short (Rai & Tripathi 1983) and the plants emerging together with the crop were able to disperse seeds when the crop was at anthesis (Fig. 2f). Ivany & Sweet (1973) had reported that freshly dispersed seeds from this species show poor dormancy and germinate in the field during spring and summer, immediately after dispersal. In experiment 1 the seeds dispersed at crop anthesis were located at the soil surface, so they should have been able to perceive the light environment below the crop canopy. The number of seedlings produced by this second generation of seeds at bare soil was higher than that recorded beneath the crop canopy (Fig. 3). Filtering FR in the inter-rows by means of CuSO4 solution produced a similar amount of emergence as that recorded at bare soil, thus eliminating most of the inhibition caused by the canopy (Fig. 3). In contrast, polycarbonate racks containing distilled water did not produce the same effect, thus suggesting that this influence reflects changes in the R : FR ratio of the light that penetrates the crop canopy. In other words, the situation prevailing during the initial crop stages does not allow buried seeds to perceive environmental light changes and the crop canopy does not produce important modifications in the thermal environment. However, canopy structure at anthesis markedly modified the light environment and this could be perceived by seeds located at the soil surface, inhibiting their germination. This conclusion is supported by the fact that Galinsoga parviflora seeds have been reported to respond to light in LFR mode (Batlla, Kruk & Benech-Arnold 2000).
In experiment 2 the seeds were located at the soil surface and had been exposed to the thermal and light modifications caused by the canopy. The germination of most of the studied species was inhibited by the presence of the establishing crop and, interestingly, this influence occurred very early in the crop cycle, with a crop LAI smaller than 1 and an R : FR ratio around 0·9 (Figs 1a, 1b, 4 and 6), which means a moderate modification of the light environment. Germination of Portulaca oleracea and Amaranthus quitensis took place at later stages of the crop cycle, also below the crop canopy (c. 30 days after crop emergence; Fig. 4). At that moment the LAI of the crop was near 2 (Fig. 1a) and the R : FR ratio beneath the canopy was down to 0·4–0·6 (Figs 1b and 6). As concluded from experiment 1, modification of the thermal regime by an incipient canopy is not enough to alter the weed emergence patterns observed at bare soil. On the other hand, R : FR ratios of the light reaching the soil beneath the canopy (c. 0·9–1·0) should be unable to inhibit germination via phytochrome shift towards its inactive form (Pr). For example, Frankland & Poo (1980) have shown that germination of Plantago major seeds is inhibited when the R : FR ratio caused by the canopy of Sinapis alba drops below 0·6, resulting from a LAI near 1·4. Deregibus et al. (1994) found that a R : FR ratio threshold of 0·5–0·8 inhibited the germination of Lolium multiflorum below a canopy. These R : FR ratios require a relatively dense canopy. However, the inhibiting influence of the growing canopy was eliminated for those treatments when the FR was filtered out by CuSO4 solution (Fig. 4), indicating that an increase of the R : FR ratios nullifies the inhibition. This is good evidence that the inhibition of germination by the crop was caused by small modifications of the light quality arriving at the soil surface. Batlla, Kruk & Benech-Arnold (2000) reported that, besides moderate FR enrichment, prolonged exposure is needed to inhibit germination; indeed, exposure to canopy-filtered light pulses of 1 h, presumably saturating the low fluence response of phytochrome (LFR), did not inhibit but rather promoted germination compared with dark controls.
These results, together with the relatively high R : FR ratios measured below the early stages of established canopy, suggest that an HIR might be also be involved here, because the light environment can be perceived by different plant photoreceptors. All cases so far investigated are consistent with the notion that phytochrome A mediates VLFR and HIR while phytochrome B, and in some cases other stable phytochrome(s), mediate LFR (Shinomura et al. 1996; Casal, Sánchez & Botto 1998; Hartmann & Mollwo 2002).
Photocontrol of weeds by soil cultivation in darkness is a preventive weed control method. The idea is to reduce the germination of light-sensitive weed seeds by excluding light during soil disturbance (Hartmann & Nezadal 1990). However, the results of this method are very variable (Hartmann et al. 2003; Juroszek & Gerhards 2004). Another possibility for photocontrol of weed germination is shading of seeds located on the soil surface by vegetation or mulches to avoid their germination (Ghersa et al. 1994).
Understanding the seed responses to modifications in the photothermal environment below the crop canopy (e.g. wheat crop) should allow us to improve weed management strategies in manipulating crop canopy attributes. This could be achieved by modification of sowing date, crop density, spatial arrangement and genotype. For example, increase in the crop plant density would diminish the number of weeds emerging during the first phase of crop establishment. This strategy would be feasible in a situation with weed seeds predominantly located at the soil surface, typically found in a no-till cropping system (Yenish, Doll & Buhler 1992). This soil surface layer, which represents the interface between the seed bank and the environment, is a crucial and variable zone in weed population dynamics (Grundy, Mead & Burston 2003). Furthermore, it is vulnerable to rapid changes in moisture and temperature (Fenner 1985). Ormerod et al. (2003) reported that agricultural production is more complex because of the need to balance global food security, optimum production, technological innovation, preservation of environmental functions and protection of biodiversity. The ecological knowledge achieved in the present study enables considerable compliance with most of those objectives. Furthermore, this information could be incorporated into weed-seed germination models as proposed by Grundy, Mead & Burston (1999).