RESPONSES OF RHIZOBIUM-INOCULATED AND NITROGEN-SUPPLIED COWPEA PLANTS TO INCREASING PHOSPHORUS CONCENTRATIONS IN SOLUTION CULTURE

ABSTRACT

The main objective of this investigation was to analyze the responses of Rhizobium-inoculated and nitrogen (N)-supplied cowpea (Vigna unguiculata L. Walp) plants to increasing phosphorus concentrations (Pc) in the nutrient solution. For this purpose, cowpea plants inoculated with Bradyrhizobium I-125 (R+) or provided with 15 mM N as KNO₃ (N+) were grown in nutrient solutions supplied with the following Pc: 0.05 mM (P1), 0.1 mM (P2), 0.25 mM (P3), 0.5 mM (P4), 1 mM (P5), 2 mM (P6), 3 mM (P7), and 4 mM (P8). The experiment was conducted in a growth chamber under controlled conditions and the pH of the solution was maintained at 6.2. Increasing Pc up to P6 enhanced the aerial and reduced the root mass of R+ and N+ plants. Nodulation was also increased by increasing Pc up to P4. In R+ and N+ plants, the concentration of total reducing sugars (TRS) decreased in leaves, shoots and roots, while there were increases in the α-amino-N concentrations with increasing Pc. In nodules, TRS concentration was not affected by increasing P. Increasing Pc up to P6 concomitantly decreased the starch concentration in leaves but not in roots of R+ and N+ plants. In R+ and N+ plants, chlorophyll concentration decreased with increasing P c, whereas, the N concentration in leaves was not affected. The ureides in R+ plants as well as the nitrate concentration in N+ plants remained constant between P1 and P6. Nevertheless, the aerial mass in R+ plants was always smaller than that of N+ plants in spite of the higher TRS, starch, α-amino-N, chlorophyll, and P concentrations in the leaf tissues as well as the two-fold increase in photosynthesis. Metabolic constraints for ureide degradation in leaves seem to be the mechanism underlying the poor response of plants to rhizobial inoculation at high Pc. The P7 and P8 concentrations proved to be toxic for R+ and N+ plants.

ACKNOWLEDGMENTS

We thank Dr. T. R. Sinclair for his valuable comments on the manuscript. We are also grateful to Dr. N. Suarez, Lic. C. Alceste, and Lic. S. Flores for the photosynthesis, phosphorus, and nitrogen measurements, respectively.