Root-based alterations in soil Zn availability Many of the earlier studies on ZE focused, quite naturally, on comparisons of root systems in efficient and inefficient genotypes of a specific plant species (often wheat) with regards to their ability to ‘mine’ Zn from the soil by increasing Zn availability in the rhizosphere (see for example, Dong et al., 1995; Cakmak et al., 1996a; Rengel et al., 1998). Several chemical (soil pH, redox potential or nutrient interactions), physical (organic matter content, soil texture, or clay content and type) and biological factors (mycorrhizae formation, phytosiderophore release) operating at the root–soil interface may affect availability and absorption of zinc and subsequently, zinc efficiency.
Root architecture varies among plant species and cultivars within plants species and has been implicated in influencing plant Zn availability and zinc efficiency (Dong et al., 1995). As a consequence, thinner roots with increased root surface area may increase the availability of Zn along with other nutrients due to a more thorough exploration of the soil (Rengel & Graham, 1995). Furthermore, roots of some plant species are colonized by mycorrhizal fungi that increase the uptake of diffusion-limited nutrients from soil including phosphorus, zinc and copper. In particular, vesicular-arbuscular mycorrhizae (VAM) have been cited to benefit plants by expanding the volume of soil explored by root systems and consequently increasing Zn uptake (Kothari et al., 1991). However, as genetic differences in ZE found in the field also can be seen in hydroponically grown plants that form no mycorrhizal associations, the role and importance of mycorrhizae in ZE is unclear.
With regards to root-mediated alterations in rhizosphere chemistry, this could involve changes in rhizosphere pH or the release of organic ligands that could chelate soil Zn and increase its availability. As soil pH increases, Zn availability for plants decreases and when soil pH rises above pH 5.5, Zn is adsorbed by soil constituents such as hydrous oxides of aluminium, iron and manganese or is precipitated by specific compounds (Marschner, 1995). Therefore, root-mediated processes that can lower the rhizosphere pH increase the plant Zn availability by solubilizing Zn from organic and inorganic soil solid phases. However, there have been no definitive studies implicating root-mediated changes in rhizosphere pH in ZE.
When the role of root exudates in ZE is looked at, there have been a number of studies that have focused on phytosiderophore release. Phytosiderophores (PS) are nonprotein amino acids that chelate a number of micronutrients, including Fe and Zn and are released from the roots of grasses under iron deficiency (Marschner, 1995). It is widely accepted that PS release and root absorption of Fe(III)-PS are key factors in Fe nutrition in grasses. Several researchers have suggested that phytosiderophores also can play a role in grass Zn nutrition and thus, quite possibly, Zn efficiency. With regard to the role of PS in plant Zn nutrition, Zhang et al. (1991) reported that Zn deficient graminaceous species released pytosiderophores and thus increased the mobilization of Zn and Fe in soil. Evidence in support of the possibility that roots of grass species could absorb the Zn-PS complex was presented by von Wiren et al. (1996), who investigated Zn2+ and Zn-PS uptake in roots of the maize yellow-stripe 1 (ys1) mutant, which is defective in Fe-PS uptake. These researchers showed that Zn uptake from Zn-PS was significantly reduced in the ys1 mutant, compared with wild type maize. Thus, it was speculated that two root Zn uptake pathways exist in grasses, one that involves free Zn2+, and a second based on Zn-PS uptake. The literature on the role of root PS release in ZE is both contradictory and somewhat controversial. Cakmak et al. (1996b) reported that Zn-efficient bread wheat genotypes had higher PS release than Zn-inefficient durum wheat genotypes under Zn deficiency. However, in another study, they found that phytosiderophore release did not correlate with differences in zinc efficiency of efficient and inefficient bread wheat genotypes under Zn deficiency conditions (Erenoglu et al., 1996). In a study with bread and durum wheat cultivars, Rengel & Romheld (2000) reported that Zn-efficient bread wheat cultivars released more pytosiderophores than Zn-inefficient cultivars. There have been several other studies that contradict the role of PS in both Zn nutrition and zinc efficiency. Both Gries et al. (1995) and Pedler et al. (2000) found no significant Zn-deficiency induced PS release in barley and wheat cultivars, that had been reported by others, in response to Zn deficiency. Furthermore, in the work by Pedler et al. (2000), they found no differences in PS release in barley and wheat cultivars that have been reported to differ significantly in ZE. They did find that Fe deficiency induced a large PS release in all barley and wheat genotypes studied. These researchers suggested that the previous observations of Zn-deficiency induced PS release may be dependent on the growth methods used, and might be explained by an induced physiological deficiency of Fe, and not Zn. In a recent review on this topic, Rengel (2001) indicated that further work is needed to arrive at any definitive conclusions about the possible role of PS release in ZE. From our point of view, when all of the available literature on the role of root-based changes in soil Zn availability in ZE is considered, it does not appear that these processes play a key role in ZE. Certainly the above mentioned observation that genotypic differences in ZE determined on Zn deficient soils are also seen when the same plants are grown under hydroponic culture where Zn availability is not an issue, argues against an important role for rhizosphere Zn availability in ZE. Although, root exudation does not seem to have a major importance in ZE, it cannot be ruled out, and a final determination on the role of rhizosphere bioavailability of Zn in ZE awaits the results of further study (Rengel, 2001).
Root Zn uptake and translocation to the shoot Zn first enters the root cell wall free space by diffusion and then moves across the plasma membrane via the action of ion transport proteins. Zn is mainly taken up by plant roots as the Zn2+ ion, although in grasses a second pathway involving the uptake of Zn-PS could also occur (Kochian, 1993; von Wiren et al., 1996). It appears that root Zn2+ uptake is mediated by two different transport systems: a high-velocity, low-affinity system (Km = 2–5 µm), and a low velocity, high affinity system (Km = 0.6–2 nm) that probably is the dominant transport system under low soil Zn conditions (Hacisalihoglu et al., 2001; Hacisalihoglu, 2002).
There are at least six different families of transporters that could facilitate plant Zn transport, including ZIP (Zrt- and Irt-related proteins), CDF (Cation Diffusion Facilitator proteins), P-type ATPase (metal transporting ATPases), NRAMP (natural resistance-associated macrophage proteins), and CAX (calcium and other divalent cation exchange antiporters) proteins. Recent studies suggest that the ZIP family of micronutrient transporters probably include some of the important Zn transporters that facilitate Zn entry into plant cells. Collectively, over 85 ZIP family members have been identified in different organisms, including 18 different ZIP genes in Arabidopsis; other members of this gene family have been identified in rice, the metal-accumulator plant Thlaspi caerulescens, animals, insects, fungi, and archae (Guerinot, 2000). In Arabidopsis, it is believed that the ZIP1-4 genes are involved in plant Zn transport but it is not known which ZIP transporters actually mediate root Zn uptake from the soil (Guerinot, 2000).
The long distance transport of Zn is known to occur primarily via the flux of Zn2+ ions in the xylem stream; Zn concentrations in xylem sap have been shown to range from 2 to nearly 100 µm (Kochian, 1993; Marschner, 1995). Leaf Zn concentrations below 10–15 µg Zn g−1 dry weight are considered Zn deficient; whereas 20–100 µg Zn g−1 leaf dry weight is sufficient for normal plant growth (Marschner, 1995). Under low Zn conditions, zinc is not usually effectively mobilized from the older leaves, and because of this, it is generally believed that the phloem mobility of Zn is fairly low. This has been suggested to explain why zinc deficiency symptoms appear primarily on young leaves (Marschner, 1995). However, a recent study in wheat has suggested that under certain conditions, Zn may be able to move fairly readily in the phloem (Haslett et al., 2001).
With regards to Zn translocation to the shoot, the relevant literature on this topic suggests that this transport process also does not correlate with Zn efficiency. In a field study of 37 bread wheat and 3 durum wheat cultivars differing in ZE, no significant differences in shoot Zn concentration were found for the different cultivars grown on either Zn-deficient or -sufficient soils (Kalayci et al., 1999). In a recent study from our laboratory, where 65Zn translocation was directly measured in both wheat and bean cultivars differing in ZE, again no correlation between Zn translocation to the shoot and differential ZE was found (Hacisalihoglu, 2002; Hacisalihoglu et al., 2003a).