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 c ommon 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)). R egardless 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 flu orescence (F-m). As a consequence, the variable to maximum fluorescence rat io (F-v/F-m) decreased by about 7 %, and the variable to initial fluorescen ce 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 ra te (PN) halved at 6 d after imposition of treatment and so remained afterwa rds. As compared to LN plants, PN declined in LP plants latter and to a les s 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 experim ent. The greater PN in LP plants was not reflected in larger biomass accumu lation in relation to LN beans. In general, P and N limitation affected pho tosynthesis parameters and growth without showing any synergistic or additi ve effect between deficiency of both nutrients.