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

Plants with the C₄ photosynthetic pathway can withstand water stress better than plants with C₃ metabolism. However, it is unclear whether C₄ 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 (δ¹³C) 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 δ¹³C and each of VD and PEPC activity implied that these coordinately contributed to higher leaf WUE via a preliminary induction of C₄ photosynthetic pathway. Preliminary C₄ 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.