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

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