Subcellular Localization of Zinc and Calcium in Bean ( Phaseolus vulgaris L.) Tissues

Abstract

Two bean (Phaseolus vulgaris L.) cultivars differing in growth responses to zinc were examined for differences in uptake and subcellular localization of (65)Zn during a 15-day growth period. The zinc-sensitive cultivar Sanilac showed initially a much higher rate of absorption, which declined after 24 hours. The zinc-tolerant cultivar Saginaw showed a slow but steady rate of absorption for 10 days. In roots as well as in stem callus tissues of both cultivars, three-fourths of the absorbed (65)Zn was localized in the "cytoplasmic" supernatant fractions (containing ribosomes and vacuolar sap). Very little (less than 7%) (65)Zn was localized in the cell wall fraction. There was a much greater proportion of the absorbed (65)Zn localized in root mitochondria and nuclei of the zinc-sensitive Sanilac than in the zinc-tolerant Saginaw. Stem callus tissues, however, did not show such cultivar differences in zinc accumulation at the sub-cellular level.Calcium-45 distribution in the cultivar Redkote showed preferential affinity for cell walls both in roots and stem cellus tissue. The percentage of absorbed (45)Ca associated with cell wall fractions of roots gradually increased with a corresponding decline in the percentage of radioactivity associated with the cytoplasmic fraction. At 15 days, 57% of the total (45)Ca in the roots was localized in the cell walls. A similar, albeit less pronounced, preferential (45)Ca sorption on cell walls occurred for the callus tissue. Calcium-45 also accumulated at later stages in the nuclear fraction, but there was no mitochondrial accumulation.