Aims: In protected cultivation, a microclimate with low vapour pressure deficit (VPD) is common due to limited ventilation, high plant density and low air movement. The aim of this study was to investigate how blue light (BL) interacts with VPD on growth, stomatal regulation and transpiration rate in light and darkness. To improve the understanding of signals involved, changes in abscisic acid (ABA) metabolism and content of carbohydrates and flavonoids were studied.
Methods: Cucumber plants (Cucumis sativus 'Quatro') were exposed to moderate (1.12 kPa) and low VPD (0.28 kPa) in combination with a photosynthetically active radiation of 200 μmol m -2 s -1 with either 5% or 30% BL.
Results: Additional BL reduced the ABA content (+ its metabolites) during light and darkness, increased the adaxial stomatal density, transpiration rate and foliar nitrogen (N) content irrespective of VPD. ABA regulation was not affected by VPD, but the amount of BL provided during the day affected the ABA catabolic pathway at night. In the high BL (30%) treatment, higher transpiration and higher content of ABA-glucose ester (ABA-GE) - a storage form of ABA - were found during night. By contrast, in low BL (5%), more ABA was converted to phaseic acid (PA). Increased starch degradation in response to increased BL resulted in a higher content of transport carbohydrates such as sucrose, raffinose and stachyose in source leaves and was accompanied by a higher fruit appearance rate in low VPD, in contrast to moderate VPD where additional BL reduced the fruit appearance rate.
Conclusion: Our results provide new insight into the regulation of ABA under different VPDs and BL proportions and show that additional BL act as a weak stressor in moderate VPD but can be a cultivation strategy in low VPD to increase growth through increase in transpiration and N content.