Cotton Gas Exchange, Fluorescence and Nutrient Use Efficiency As Affected By Phosphorus Nutrition and Carbon Dioxide Enrichment

To study the cotton (Gossypium hirsutum L.) photosynthetic response and nutrient use efficiency to the combined effect of phosphorus and carbon dioxide (CO₂) concentration, two experiments were conducted in six growth chambers with three levels of phosphate (Pi) supply (0.20, sufficient; 0.05 and 0.01 mM, deficient) under current (aCO₂, 400 µmol mol^-1) and projected (eCO₂, 800 µmol mol^-1) CO₂ concentration. Plant photosynthetic rate (P_net), stomatal conductance (g_s) , chlorophyll fluorescence (Fvʹ/Fmʹ), electron transport rate (ETR), intrinsic water use efficiency (WUE), and phosphorus (PUE) and nitrogen (NUE) use efficiency of photosynthesis (PUE_Pnet and NUEP_net) and biomass production (PUE_Bio and NUE_Bio) were assessed periodically. Pi deficiency significantly affected and decreased P_net and g_s (65%), Fvʹ/Fmʹ (43%), and ETR (54%) averaged across both lower Pi supply. The WUE was increased by 23% in the lowest Pi supplied plants; however, this increase was only observed in plants grown under eCO₂. Elevated CO₂ showed increase in P_net (12.5%) and WUE (89%), whereas exhibited a decrease in g_s (30%), Fvʹ/Fmʹ (5%) and ETR (12%) when averaged across Pi supply. The response of P_net to g_s was higher at eCO₂ versus aCO₂ resulting in higher WUE in plants grown under eCO₂. Biomass production and P_net exhibited a linear response to both PUE and NUE. The Pi deficient plants showed on average of 78-85% and 36-67% lower nutrient use efficiency (P and N) of biomass and P_net, respectively as compared to the sufficient Pi supply. The eCO₂ significantly stimulated an average of 46-62% and 35-48% nutrient use efficiency for biomass production and P_net, respectively. The study suggested a definite loss in the nutrient use efficiency under in Pi deficient cotton and eCO₂ stimulates growths even under Pi deficiency by increasing P_net and efficiency of water and nutrient use.