Blue light is a key factor in regulating stomatal movement, transpiration rate and ABA metabolism under varying vapour pressure

Vapour pressure deficit (VPD) and blue light (BL) are import signals for stomatal regulation and highly relevant during cultivation of economically important crops such as cucumber in controlled environments. However, the information about combined effects of VPD and BL and the regulation of abscisic acid (ABA) in this respect, is limited and seems to vary between species. This study investigated how moderate (1.12 kPa) or low (0.28 kPa) VPD combined with 5% or 30% BL affect the foliar ABA metabolism, transpiration rate, nutrient uptake, and growth in well-watered cucumber plants (Cucumis sativus ‘Quatro’). Additional BL, regardless of VPD, resulted in a decrease in the nighttime ABA content (52%), as well as increased adaxial stomatal density (8–19%), transpiration rate during the day (40–50%) and foliar nitrogen content (30%). ABA regulation was unaffected by VPD, but the BL levels during the day influenced the ABA inactivation at night; both the transpiration rate and the ABA-glucose ester (ABA-GE) content then increased in 30% BL compared to 5% BL. An increase in starch degradation due to additional BL led to higher content of the transport carbohydrate raffinose in source leaves and increased the fruit appearance rate (fruit day⁻¹) in low VPD but reduced it in moderate VPD. This study provides novel insight into the regulation of ABA under different VPDs and BL proportions in cucumber, and reveals that under well-watered conditions, BL is a stronger determinant of stomatal aperture regulation than ABA. Also, a combination of high VPD and high BL can act as a weak stressor and affect cucumber growth and yield.