Response of carbon assimilation and chlorophyll fluorescence to soybean leaf phosphorus across CO2: Alternative electron sink, nutrient efficiency and critical concentration

To evaluate the response of CO2 assimilation rate (P-N) and various chlorophyll fluorescence (CF) parameters to phosphorus (P) nutrition, soybean plants were grown in controlled environment with sufficient (0.50 mM) and deficient (0.10 and 0.01 mM) phosphate (P) supply under ambient and elevated CO2 (aCO(2), 400 and eCO(2), 800 mu mol mol(-1), respectively). Measurements were made at ambient (21%) and low (2%) O-2 concentrations. Results showed strong correlation of leaf P concentration with P-N and CF parameters. The P deficiency showed parallel decreases in P-N, and CF parameters including quantum efficiency (F-v'/F-m'), quantum yield of photosystem II (Phi(PSII)), electron transport rate (J(F)), and photochemical quenching (q(p)). The F-v'/F-m' decreased as a result of greater decline in maximal (F-m') than minimal (F-o') fluorescence. The eCO(2) stimulated P-N especially under higher leaf P concentrations. Low O-2 also stimulated P-N but only at aCO(2). The photosynthetic carbon reduction (PCR, signified by P-N) and photorespiratory carbon oxidation cycles (PCO, signified photorespiration as indicated by ratio of J(F) to gross P-N and % increase in P-N at 2% O-2) was the major electron sinks. However, the presence of alternative electron sink was also evident as determined by the difference between the electron transport calculated from chlorophyll fluorescence and gas exchange measurements. Alternative electron sink declined at lower leaf P concentration suggesting its minor role in photochemical energy consumption, thus dissipation of the excess excitation pressure of PSII reaction center under P deficiency. The J(F)/P-G and % increase in P-N at 2 versus 21% O-2 remained consistent across leaf P concentration suggesting PCO cycle as an important mechanism to dissipate excess excitation energy in P deficient leaves. The severe decline of F-v'/F-m' Phi(PSII), J(F)and q(p) under P deficiency also suggested the occurrences of excess radiant energy dissipation by non-photochemical quenching mechanisms. Critical leaf P concentration (CLPC) needed to achieve 90% of the maximum value was greater for P-N than CF parameters. Moreover, CLPC was always higher at eCO(2) suggesting increased sensitivity of soybean to P deficiency under eCO(2). An increased phosphorus utilization efficiency of P-N and CF parameters was also achieved but with the expense of net CO2 assimilation in P-deficient leaves. (C) 2015 Elsevier B.V. All rights reserved.