Two cotton cultivars, Deltapines 50 and 90 (G. hirsutum, L.), were subjected to two levels each of three major global climate factors, water stress, CO₂, O₃. Physiological measurements were recorded at 4 plant ages, 65, 78, 85, and 92 days after emergence (d.a.e). Water stress had significantly reduced photosynthesis (PS) at the last three plant ages, which coincided with flowering and fruit setting, and increased photorespiration (Pr) at all plant ages, except at 92 d.a.e. Plants under water stress exhibited capacity to adjust to depletion of available soil moisture through stomata with significant reductions in both stomatal conductance and transpiration rates at all plant ages.

Ambient O₃, levels had a significant effect on PS, only at 85 d.a.e,, where plants exposed to nonfiltered (NF) air had higher PS compared with plant exposed to charcoal filtered (CF) air. Plants under NF, also had significantly lower Pr at 65 d.a.e. Stomata conductance and transpiration rate responses to O₃, were different with plant age, where at 65 d.a.e ambient O₃ (NF) had reduced both measurements, while at later ages had stimulatory effect.

The higher CO₂ level (500 umol mol^-1), stimulate PS rates at all plant ages compared with ambient [CO₂], while reduced significantly Pr rates at 65 and 78 d.a.e. Stomata conductance and transpiration rates responses to CO₂, were different at different plant ages. Plants under CO₂ enrichment had significantly higher stomata conductance and transpiration rate at 65 d.a.e, while significantly lower values were found at 78 d.a.e. Varietal differences were observed in PS and stomata conductance at all plant ages, where DP 50 had higher PS, and lower stomatal conductance. No differences were recorded between the cultivars in Pr and transpiration rate.