Physiological responses of Lupinus mutabilis to phosphorus nutrition and season of growth

The response to phosphorus (P) concentration in the nutrient solution (0-0. 5 mol P m(-3)) was studied in Lupinus mutabilis Sweet cv. Potosi in two dif ferent seasons (winter and spring). Phosphorus deficiency was more severe o n growth than on photosynthesis and the season of growth dramatically influ enced the optimal concentration of P for plant growth; root biomass was pro portionally less affected than shoot biomass. During winter, growth and pho tosynthesis of plants supplied with 0.02-0.5 mol P m(-3) were not significa ntly different, whereas in spring, rates of growth and photosynthesis were faster at the 0.5 mol P m(-3) level. Stomatal conductance decreased with de ficient P independently of leaf water relations. Severe P deficiency limite d carbon (C) assimilation rates due to reduction in stomatal conductance an d mesophyll photosynthetic capacity. Decreased sucrose/starch in P-deficien t leaves was a consequence of the observed source/sink imbalance which was more marked in winter. Hydraulic conductance was not a limiting factor for leaf expansion under low P. In conclusion, growth and metabolic changes obs erved in lupins grown at low P supply can be ascribed to an adjustment at t he whole plant level, preventing a large drop in leaf P, reducing shoot gro wth and facilitating P uptake through higher root biomass.