ABA regulated stomatal control and photosynthetic water use efficiency of potato (Solanum tuberosum L.) during progressive soil drying

Glasshouse experiments at two developmental stages (tuber initiation and tuber bulking) of potatoes (Solanum tuberosum L.) were conducted to investigate plant water relation and leaf gas exchange characteristics during progressive soil drying. Leaf relative water content (RWC), leaf water potential (Ψl), root water potential (Ψr), stomatal conductance (gs), photosynthesis (A), and xylem sap abscisic acid (ABA) concentration ([ABA]xylem) were determined in well-watered (WW) and drought-stressed (DS) plants. At both stages, RWC and Ψl were hardly affected, significant decreases of the two parameters occurred only at severe soil water deficits; however, gs decreased much early at 2 and 1 days after imposition of stress (DAIS) at tuber initiation and tuber bulking, respectively, and coincided with decrease of Ψr and increase of [ABA]xylem; while A decreased 2 days later than gs at each stage. Analyses of the pooled data of the two stages showed that gs was linearly correlated with [ABA]xylem at mild soil water deficits (i.e. Ψr > −0.3 MPa); photosynthetic water use efficiency, viz. A/gs, increased linearly with decreasing gs until the latter reached 0.2 mol m−2 s−1, below this point, it decreased sharply. The results suggest that at mild soil water deficits, gs of potato is seemingly controlled by xylem-borne ABA. As a consequence of A being less sensitive than gs to soil drying, photosynthetic water use efficiency, i.e. A/gs, is increased at mild soil water deficits.