Stomatal closure response to soil drying at different vapor pressure deficit conditions in maize

A plant transpiration rate under progressive soil drying remains constant until a threshold fraction of transpirable soil water (FTSW) is reached, and subsequently decreases linearly. The sensitivity of this function and the involvement of abscisic acid (ABA) and aquaporins in such responses have not been compared at various levels of atmospheric evaporative demand conditions. This study was conducted in controlled environment chambers with a drought-tolerant maize hybrid imposing progressive drought stress under three levels of vapor pressure deficit (VPD- 1.2, 2.3, and 3.5 kPa). A shift in threshold-FTSW from 1.2 kPa (FTSW-0.42) VPD to 3.5 kPa(FTSW-0.51) VPD was observed, showing an effect of VPD on stomatal closure response under soil drought conditions. Foliar ABA showed a substantial rise approximately at the same time as of stomatal closure initiated (FTSW-threshold), indicating ABA involvement. As the drought progressed, an increase in plasma membrane intrinsic protein and a decrease in tonoplast intrinsic protein expression levels were observed. Overall, this study suggests the influence of evaporative demand on the initiation of stomatal closure of drought-tolerant maize subjected to soil drying. The sensitivity of stomatal closure was associated with foliar ABA under drought stress but not under high evaporative demand conditions, indicating alternative water conservative mechanisms.