VEGETATIVE GROWTH OF THE COMMON BEAN IN RESPONSE TO PHOSPHORUS-NUTRITION

Common bean (Phaseolus vulgaris L.) is often grown in soils of low P availability. It has been proposed that P deficiency may reduce crop growth by inhibiting leaf expansion and photosynthesis. Our objective was to determine how P applied to the root medium or leaves influences vegetative growth, leaf area development, and photosynthesis of bean. Plants were grown in solid-phase-buffered sand culture at 0.4, 1.0, and 27.0-mu-M P with or without foliar P (ammonium polyphosphate) and with or without mycorrhizal inoculation. Leaf area development was very sensitive to P supply. Phosphorus deficiency primarily reduced leaf area by diminishing the number of leaves through effects on the number of nodes, branching, and relative leaf appearance rate, and secondarily by reducing leaf expansion. The effects of mycorrhizae and increased root P availability were similar. Foliar P application stimulated leaf area development in P-deficient plants by increasing leaf appearance rate, branching, the number of mainstem nodes, and overall shoot growth, although expansion of individual leaves was reduced slightly. Mycorrhizal and foliar P effects were most pronounced at low root P availability. Although severe P deficiency decreased leaf photosynthesis in nonmycorrhizal plants, intermediate P stress in nonmycorrhizal plants did not reduce photosynthesis, and photosynthesis in mycorrhizal plants was inversely correlated with P stress. Root P availability influenced leaf photosynthesis largely by influencing leaf P concentration. We conclude that P availability affects bean growth primarily through effects on leaf appearance and biomass partitioning between photosynthetic and respiring organs, rather than through effects on leaf photosynthesis.