Combined effects of phosphorus nutrition and elevated carbon dioxide concentration on chlorophyll fluorescence, photosynthesis, and nutrient efficiency of cotton

To examine the combined effects of phosphorus (P) nutrition and CO2 on photosynthesis, chlorophyll fluorescence (CF), and nutrient utilization and uptake, two controlled-environment experiments were conducted using 0.01, 0.05 and 0.20 mM external phosphate each at ambient and elevated CO2 (aCO2: 400 and eCO2: 800 mmol mol –1, respectively). The CF parameters were affected more by P nutrition than by CO2 treatment. Photoinhibition of photosystem II (PSII) was due to increased minimal CF (Fo¢) and decreased maximal CF (Fm¢), and efficiency of energy harvesting (Fv¢/Fm¢). In addition, reduced electron transport rate (ETR), the quantum yield of PSII (FPSII) and CO2 assimilation (FCO2 ), and overall photochemical quenching in the P-deficient leaves led to reduction in the efficiency of energy transfer to the PSII reaction center. Stimulation in the FPSII/FCO2 and photorespiration (ETR/Pnet) was found under P deficiency, whereas the opposite was the case under CO2 enrichment. On average, photosynthetic rate (Pnet) and stomatal conductance declined by 50–53% at 0.05 mM P and by 70–72% at 0.01 mM P as compared to the 0.20 mM P treatment. However, P deficiency, especially at eCO2, tended to increase the intrinsic water-use efficiency. In the P-deficient plants, the decline in the P and N utilization efficiency (up to 91%) of biomass production was mainly associated with greater reduction in the biomass relative to the tissue P concentration as the P supply was reduced. However, it was significantly stimulated by eCO2 especially at higher P supply. The CO2 · P interaction was observed for some parameters such as Fo¢, Fm¢, P utilization efficiencies of photosynthesis and biomass production that might be attributed to the irresponsiveness of these parameters to eCO2 under low P treatment. Thus, P deficiency limited the beneficial effect of eCO2. A close relationship between total biomass and photosynthesis with the P and N utilization or uptake efficiencies was found. The P utilization efficiency of Pnet appeared to be stable across a range of leaf P concentrations, whereas the N-utilization efficiency markedly increased with leaf P and differed between CO2 levels. An apparent effect of both the treatments (P and CO2) on N-uptake and utilization efficiency also indicated the alteration in N acquisition and assimilation in cotton plants.