Characterization of Zinc Uptake, Binding, and Translocation in
Intact Seedlings of Bread and Durum Wheat Cultivars |
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Authors: | Jonathan J Hart Wendell A Norvell Ross M Welch Lori A Sullivan and Leon V Kochian |
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Institution: | Plant, Soil, and Nutrition Laboratory, United States Department of Agriculture-Agricultural Research Service, Cornell University, Ithaca, New York 14853 |
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Abstract: | Durum wheat (Triticum
turgidum L. var durum) cultivars exhibit lower
Zn efficiency than comparable bread wheat (Triticum
aestivum L.) cultivars. To understand the physiological
mechanism(s) that confers Zn efficiency, this study used
65Zn to investigate ionic Zn2+ root uptake,
binding, and translocation to shoots in seedlings of bread and durum
wheat cultivars. Time-dependent Zn2+ accumulation during 90
min was greater in roots of the bread wheat cultivar. Zn2+
cell wall binding was not different in the two cultivars. In each
cultivar, concentration-dependent Zn2+ influx was
characterized by a smooth, saturating curve, suggesting a
carrier-mediated uptake system. At very low solution Zn2+
activities, Zn2+ uptake rates were higher in the bread
wheat cultivar. As a result, the Michaelis constant for
Zn2+ uptake was lower in the bread wheat cultivar (2.3
μm) than in the durum wheat cultivar (3.9
μm). Low temperature decreased the rate of
Zn2+ influx, suggesting that metabolism plays a role in
Zn2+ uptake. Ca inhibited Zn2+ uptake equally
in both cultivars. Translocation of Zn to shoots was greater in the
bread wheat cultivar, reflecting the higher root uptake rates. The
study suggests that lower root Zn2+ uptake rates may
contribute to reduced Zn efficiency in durum wheat varieties under
Zn-limiting conditions.Soils that contain insufficient levels of the essential plant
micronutrient Zn are common throughout the world. As a result, Zn
deficiency is a widespread problem in crop plants, especially cereals
(Graham et al., 1992). The importance of plant foods as sources of Zn,
particularly in the marginal diets of developing countries, is well
established (Welch, 1993). The development of crop plants that are
efficient Zn accumulators is therefore a potentially important
endeavor. In addition to its effects on nutrition, Zn deficiency in
crops is relevant to other areas of human health. Another consequence
of Zn-deficient soils is the tendency for plants grown in such soils to
accumulate heavy metals. For example, in the Great Plains region of
North America, where soil Zn levels are low and naturally occurring Cd
is present, durum wheat (Triticum turgidum L. var
durum) grains accumulate Cd to relatively high
concentrations (Wolnik et al., 1983). The presence of Cd in food
represents a potential human health hazard and, in response,
international trade standards have been proposed to limit the levels of
Cd in exported grain (Codex Alimentarius Commission, 1993).
Thus, there is a need to understand the physiological processes that
control acquisition of Zn from soil solution by roots and mobilization
of Zn within plants.It has been demonstrated in recent years that crop plants vary in their
ability to take up Zn, particularly when its availability to roots is
limited. Zn efficiency, defined as the ability of a plant to grow and
yield well in Zn-deficient soils, varies among wheat cultivars (Graham
and Rengel, 1993). In field trials, durum wheat cultivars have been
shown to be consistently less Zn efficient than bread wheat
(Triticum aestivum L.) cultivars (Graham et al., 1992).
Similarly, durum wheat varieties were reported to be less Zn efficient
than bread wheat varieties when grown in chelate-buffered hydroponic
nutrient culture (Rengel and Graham, 1995a).The physiological mechanism(s) that confers Zn efficiency has not been
identified. Processes that could influence the ability of a plant to
tolerate limited amounts of available Zn include higher root uptake,
more efficient utilization of Zn, and enhanced Zn translocation within
the plant. Cakmak et al. (1994) showed that a Zn-inefficient durum
wheat cultivar exhibited Zn-deficiency symptoms earlier and more
intensely than a Zn-efficient bread wheat cultivar even though the Zn
tissue concentrations were similar in both lines, suggesting
differential utilization of Zn in the two cultivars. Rates of Zn
translocation to shoots were shown to vary among sorghum cultivars,
although correlations with Zn efficiency were not established (Ramani
and Kannan, 1985). Root uptake kinetics have been reported to vary
between rice cultivars having different Zn requirements, with
high-Zn-requiring cultivars exhibiting consistently higher root uptake
rates (Bowen, 1986). In contrast, a correlation between Zn efficiency
and rates of root Zn uptake in bread and durum wheat cultivars could
not be demonstrated (Rengel and Graham, 1995b).In grasses Zn influx into the root symplasm has been hypothesized to
occur as the free Zn2+ ion (Halvorson and
Lindsay, 1977), as well as in the form of Zn complexes with nonprotein
amino acids known as phytosiderophores (Tagaki et al., 1984) or
phytometallophores (Welch, 1993). Concentration-dependent uptake of
free Zn2+ ions has been shown to be saturable in
several species, including maize (Mullins and Sommers, 1986), barley
(Veltrup, 1978), and wheat (Chaudhry and Loneragan, 1972), suggesting
that ionic uptake in grasses occurs via a carrier-mediated system.
However, several of these studies have been criticized on the basis
that excessively high (and physiologically unrealistic)
Zn2+ concentrations were used (Kochian, 1993).This study was undertaken to examine unidirectional
Zn2+ influx and translocation to shoots in
Zn-efficient bread wheat lines and Zn-inefficient durum wheat lines.
Experiments were performed in the absence of added phytometallophores
and results are presumed to represent influx of ionic
Zn2+. Zn activities in the nanomolar range were
used to more closely mimic free Zn2+ levels
occurring naturally in soil solution. The results presented here
indicate that a Zn-efficient bread wheat cultivar maintained higher
rates of Zn uptake than a Zn-inefficient durum wheat cultivar,
particularly at low (and physiologically relevant) solution
Zn2+ activities. |
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