THE EFFECT OF PHOSPHATE DEFICIENCY ON MITOCHONDRIAL ACTIVITY AND ADENYLATE LEVELS IN BEAN ROOTS

Bean plants (Phaseolus vulgaris) were grown for 16-20 days with or without phosphate in Knop nutrient medium. It was found in previous experiments that for roots grown on a P1-deficient medium respiration is mainly carried out by the cyanide-insensitive pathway. Mitochondria isolated from -P(i) roots had poor respiratory control and their respiration exhibited 62% inhibition by cyanide and was inhibited (30%) by salicylhydroxamic acid (SHAM). In contrast, mitochondria obtained with control (+P(i)) roots had respiratory control and ADP/O ratios typical for succinate as the substrate; their respiration was inhibited to 95% by cyanide and insensitive to SHAM. The integrity of mitochondrial membranes was similar in both types of mitochondria. Cytochrome oxidase activity, however, was about 20% lower in -P(i) mitochondria, but the cytochrome composition was the same in both types of mitochondria. The cytochrome pathway was not operating at full capacity in mitochondria isolated from -P(i) roots but the alternative oxidation pathway participated in a great part in mitochondrial respiration, similar to in vivo whole roots. The participation of the non-phosphorylating, alternative pathway decreased the respiratory control ratio in mitochondria and had an effect on the total adenine nucleotide pool and energy charge values which were lower (16 and 13% respectively) in -P(i) roots. About 50% lower ADP and 20% lower ATP levels were observed whereas AMP levels were several times higher.