Response to Phosphate Deficiency in Bean (Phaseolus vulgarisL.) Roots. Respiratory Metabolism, Sugar Localization and Changes in Ultrastructure of Bean Root Cells

Abstract

Bean plants (Phaseolus vulgarisL. ‘Zlota Saxa’) were cultured on complete (+P) or phosphate-deficient (−P) nutrient medium. A large increase in glucose concentration was found in the meristematic zone of −P roots compared to control roots. The increased glucose concentration in the meristematic zone did not influence total respiration rate. Glucose or uncoupler (carbonyl cyanide m-chlorophenylhydrazone) failed to increase the respiration rate in −P root segments, but stimulated respiration in +P roots. The ultrastructure of cortical cells from the meristematic root zone showed marked differences between +P and −P plants. Large vacuoles, invaginations of the plasmalemma and condensed forms of mitochondria were dominating features in cortical cells of −P roots. Analysis of extracts after treating roots with dimethylsulfoxide (DMSO) indicated different localization of sugars in the cell compartments. In roots of −P plants, most of the reducing sugars were detected in the cytoplasm fraction while most sucrose was in the vacuole. Observations of the effect of 10% DMSO on cell ultrastructure indicated partial destruction of the plasmalemma but not the tonoplast. The localization of reducing sugars in secondary vacuoles or plasmalemma invaginations in the cells from the meristematic region of −P roots is discussed.