This investigation was part of a general characterization of Zn tolerance among cultivars of an agriculturally important plant species. Greenhouse studies were conducted to further evaluate the effect of soil Zn additions on soybean (Glycine max L. Merr.) cultivar differences in Zn tolerance, as well as plant Zn, Mn, Fe, and P contents. Four cultivars selected from a previous Zn tolerance screening trial (‘Wye’ and ‘Hudson Manchu’, tolerant; ‘York’, sensitive; ‘Columbus’, normal) were grown for 4 weeks on a fertilized Sassafras sandy loam (Fragiudult) amended with a wide range of Zn additions at pH 5.5 (nine additions between 1 and 131 ppm Zn) and 6.5 (10 additions between 1 and 524 ppm Zn).
Leaf dry weight reductions were curvilinear across the Zn additions at each pH, with yield increases occurring at the lower Zn treatments. For all cultivars, root dry weight was unaffected by the Zn additions at the lower pH and significantly reduced at the higher pH by Zn additions greater than 262 ppm. Significant cultivar differences in leaf dry weight yields were observed at the higher Zn treatments.
Leaf and root Zn contents increased linearly with increasing soil Zn at each soil pH (root r2 > 0.92, leaf r2 > 0.93 for all cultivars). The soil pH strongly influenced plant Zn contents; at an equivalent Zn addition, tissue values at pH 6.5 were approximately one-third as great as at pH 5.5. Differential absorption and translocation of Zn was shown by the cultivars; at both pH levels, Hudson Manchu absorbed more Zn but translocated less than the other cultivars.
Generally, leaf yield reductions of 20% corresponded to leaf Zn contents greater than 620 ppm at both pH levels; however Hudson Manchu had only 370 ppm Zn in its leaves at 20% yield reduction at pH 6.5. Cultivar differences in both soil and leaf Zn contents at a 20% reduction for leaf dry weight illustrate the inherent weakness of foliar analysis as a monitoring tool for available metals at toxic levels.
Root Mn was increased linearly by soil Zn additions at both pH levels, while leaf Mn increases were curvilinear at pH 5.5 and linear at pH 6.5. Leaf Mn concentrations at both pH levels reached reported toxic levels (> 500 ppm) illustrating the complex nature of Zn phytotoxicity. DTPA-extractable Mn was unaffected by soil Zn additions at both pH levels.
Foliar (combined trifoliolate leaves) Fe, at pH 5.5 and 6.5, and P at pH 6.5, decreased with increasing soil Zn, but did not approach reported deficiency levels. Similar amounts of Fe and P were found hi Zn-tolerant and Znsensitive cultivars.
These results illustrate the complex nature of Zn phytotoxicity and the importance of cultivar selection for cropping soils amended with metal-containing wastes.