Leaf gas exchange and chlorophyll fluorescence parameters in Phaseolus vulgaris as affected by nitrogen and phosphorus deficiency

The effects of N and P deficiency, isolated or in combination, on leaf gas exchange and fast chlorophyll (Chl) fluorescence emission were studied in common bean cv. Negrito. 10-d-old plants grown in aerated nutrient solution were supplied with high N (HN, 7.5 mol m(-3)) or low N (LN, 0.5 mol m(-3)), and also with high P (HP, 0.5 mol m(-3)) or low P (LP, 0.005 mol m(-3)). Regardless of the external P supply, in LN plants the initial fluorescence (F-0) increased 12 % in parallel to a quenching of about 14 % in maximum fluorescence (F-m). As a consequence, the variable to maximum fluorescence ratio (F-v/F-m) decreased by about 7 %, and the variable to initial fluorescence ratio (F-v/F-0) was lowered by 25 % in relation to control plants. In LP plants, F-v/F-m remained unchanged whilst F-v/F-0 decreased slightly as a result of 5 % decline in F-m. Under N deficiency, the net photosynthetic rate (PN) halved at 6 d after imposition of treatment and so remained afterwards. As compared to LN plants, PN declined in LP plants latter and to a less extent. From 12 d of P deprivation onwards, PN fell down progressively to display rates similar to those of LN plants only at the end of the experiment. The greater PN in LP plants was not reflected in larger biomass accumulation in relation to LN beans. In general, P and N limitation affected photosynthesis parameters and growth without showing any synergistic or additive effect between deficiency of both nutrients.