Higher PEPC activity and vein density contribute to improve cotton leaf water use efficiency under water stress

Abstract Plants with the C 4 photosynthetic pathway can withstand water stress better than plants with C 3 metabolism. However, it is unclear whether C 4 photosynthesis can be preliminarily activated in droughted cotton leaves, and if this contributes to increase in water use efficiency (WUE). An upland cotton ( Gossypium hirsutum L., Xinluzao45) was used to determine gas exchange, stomatal and vein anatomy, phosphoenolpyruvate carboxykinase (PEPC) and Rubisco enzyme activity, and carbon isotope composition ( δ 13 C) under well‐watered, mild or moderate water stress. Water stress triggered reduced photosynthesis, stomatal conductance, and Rubisco activity, but higher vein density (VD), PEPC activity, and WUE. The correlations between δ 13 C and each of VD and PEPC activity implied that these coordinately contributed to higher leaf WUE via a preliminary induction of C 4 photosynthetic pathway. Preliminary C 4 photosynthesis indicated by more PEPC enzyme and veins offers an effective way to improve leaf WUE and potentially aids in acclimation to adverse growing conditions.