Changes in cotton water use efficiency and its mechanism during drought and subsequent rehydration

There is limited information on the changes in cotton water use efficiency and its affecting factors under drought and rehydration. To end this, a greenhouse experiment was conducted with Dexiamian 1 (a drought-tolerant cultivar) and Yuzaomian 9110 (a drought-sensitive cultivar) under three different soil relative water content (SRWC) including well-watered SRWC(75 ± 5)%, mild drought SRWC(60 ± 5)% and moderate drought SRWC(45 ± 5)% during the reproductive stage. The changes of characteristics associated with instantaneous water use efficiency (WUEi) were studied at 10-, 17-, and 24-day of drought treatments and subsequent rehydration. Under 10-day drought, the decreased osmotic potential and increased GhPIP2;3 expression helped maintain root water uptake, while the increased total root length and root surface area also participated in mediating water uptake under 17-day drought. Besides maintaining root water uptake, cotton also closed stomata to reduce transpirational water loss, thus WUEi was increased under 10- and 17-day drought. However, under 24-day drought, the weakened osmotic adjustment, down-regulated GhPIP2;3 expression and suppressed root growth inhibited water uptake, and photosynthesis was more suppressed compared with transpiration, resulting in the decreased WUEi. When rehydrated after 10- and 17-day drought, the compensatory effects of root growth and physiological characteristics enhanced water uptake, and photosynthesis was also promoted, contributing to the WUEi improvement. However, under the rehydration after 24-day drought, root water uptake was irreversibly inhibited, and photosynthesis was still suppressed because of the weak recovery of stomatal conductance, thus WUEi was not restored. In addition, more efficient water transport mediated by increased root vessel numbers improved cotton WUEi, which effect was pronounced during the rehydration period rather than the drought period. In brief, under dynamic drought and subsequent rehydration, the changed water uptake and transport regulated by root morphological and anatomical traits, osmotic adjustment and aquaporin expression, and the varied transpiration efficiency for carbon fixation which was mediated by stomatal conductance, coordinately affected cotton WUEi.