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 CO 2 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 CO 2 (aCO 2 : 400 and eCO 2 : 800 µmol mol −1 , respectively). The CF parameters were affected more by P nutrition than by CO 2 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 (Φ PSII ) and CO 2 assimilation ( Φ CO₂ ), 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 Φ PSII / Φ CO₂ and photorespiration (ETR/P net ) was found under P deficiency, whereas the opposite was the case under CO 2 enrichment. On average, photosynthetic rate (P net ) 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 eCO 2 , 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 eCO 2 especially at higher P supply. The CO 2 × 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 eCO 2 under low P treatment. Thus, P deficiency limited the beneficial effect of eCO 2 . A close relationship between total biomass and photosynthesis with the P and N utilization or uptake efficiencies was found. The P utilization efficiency of P net appeared to be stable across a range of leaf P concentrations, whereas the N‐utilization efficiency markedly increased with leaf P and differed between CO 2 levels. An apparent effect of both the treatments (P and CO 2 ) on N‐uptake and utilization efficiency also indicated the alteration in N acquisition and assimilation in cotton plants.