Vapour pressure deficit and endogenous ABA level modulate stomatal responses of tomato plants to soil water deficit

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High vapour pressure deficit (VPD) in the air and water deficits in the soil are considered two key stress factors limiting crop growth and production. To explore their interactive effects on stomatal behaviour and plant water use and the role of abscisic acid (ABA) in mediating these responses, two tomato (Solanum lycopersicum) genotypes, Ailsa Craig (AC) and its ABA-deficient mutant (flacca), were grown at two VPD levels (VPD1, 0.71 kPa and VPD2, 1.25 kPa) and were exposed to progressive soil drying. The results showed that the relatively higher VPD, i.e. VPD2, decreased stomatal conductance only in AC, and increased transpiration rate and water consumption in both genotypes, which was associated with altered stomatal morphological traits, including lowered stomatal density, reduced stomatal size and pore aperture. During progressive soil drying, both AC and flacca grown at VPD2 closed stomata earlier than VPD1-plants with a low leaf ABA concentration, indicating that rising VPD advanced stomatal drought response in an ABA-independent way. At the end of soil drying, the stomata of VPD1-AC plants remained widely opened resulting in fast desiccation of the leaves excised from the plants upon exposure to VPD2, which implied an interaction of VPD and ABA in mediating short-term stomatal response. VPD2 increased the intrinsic water use efficiency (WUE) in AC plants but decreased the whole-leaf and plant WUE in both genotypes, as affirmed by the corresponding changes in leaf δ13C and δ 18O. Compared to δ 18O, δ13C was a more robust indicator of stomatal movement when plants were exposed to combined atmospheric and soil droughts. Collectively, these findings provide some novel insights into the stomatal regulation of tomato plants confronting combined atmospheric and edaphic droughts.