Phosphatase and phytase activities in nodules of common bean genotypes at different levels of phosphorus supply

Plants grown at limited P supply can increase the activity of phosphatases in roots to hydrolyse organic-P compounds in the soil thus improving plant P acquisition, but little information is available about the role of these enzymes for internal plant metabolism at limited-P conditions. This work intended to measure the activities of acid phosphatases and phytases in nodules of common bean (Phaseolus vulgaris) genotypes at different levels of P supply. The experiment was carried out in a 5 x 5 factorial design with four replicates, comprising five bean genotypes and five P levels (20, 40, 80, 160 and 320 mu mol P plant(-1) week(-1)) in nutrient solution. Root seedlings were inoculated with Rhizobium tropici and plants were grown in 1-l bottles. Nodule samples were detached from 39-day-old plants and enzyme activities were determined in crude extracts. Plants were harvested at the stage of pod setting. Polynomial models fitted to data indicated maximal values at the level of 194 mu mol P for shoot mass, at 206 mu mol P for nodule mass and at 221 mu mol P for shoot N. Whereas shoot mass was 1.7 times lower at 20 than at 160 mu mol P, nodule mass was 7.5 times lower. Concentration of P in nodules increased from 40 to 320 mu mol P but remained stable between 20 and 40 mu mol P, suggesting a minimal threshold concentration of 3 mg P g(-1) for nodule growth. Activities of phosphatases and phytases in nodules decreased strongly as P supply was raised from 20 to 80 mu mol P, remaining almost stable at higher P levels. Phosphatase activity ranged from 1,154 to 406 nmol min(-1) g(-1) (nodule fresh mass) from 20 to 80 mu mol P respectively, while the phytase activity ranged from 55 to 14 nmol min(-1) g(-1) from 20 to 80 mu mol P. Bean genotypes differed in shoot and nodule mass at the levels of 80 and 160 mu mol P, whilst they differed in nodule enzyme activities only at the lowest P level, the relationship between nodule enzyme activities and growth of different bean genotypes was not evident. It is concluded that bean plants at P-deficient conditions increase the activities of phosphatases and phytases in nodules. This may constitute an adaptive mechanism for N-2-fixing legumes to tolerate P deficiency, by increasing the utilisation of the scarce P within the nodules.