LED pre-exposure shines a new light on drought tolerance complexity in lettuce (Lactuca sativa) and rocket (Eruca sativa)

Abstract

We investigated the morphological and physiological bases of drought tolerance in rocket (Eruca sativa) and lettuce (Lactuca sativa) seedlings as induced by pre-exposure to different ratios of red (R) and blue (B) LED light. Seedlings were grown for 17–18 days under white (control), 100R:0B, 75R:25B, 50R:50B, 25R:75B, or 0R:100B LED lighting at 156 μmol/m2/s. All seedlings were moved under white fluorescent lights 17–18 days after sowing. Half the seedlings were then droughted for 7 days, followed by re-irrigation for 7 days, while the other seedlings remained fully irrigated. Exposure to combined red and blue wavelengths promoted morphological development (leaf area, number of open leaves, leaf specific weight), as well as pigment (chlorophyll, carotenoids) and secondary metabolite (flavonoid) biosynthesis in both species. Morphological and physiological characteristics were more enhanced by monochromatic red or blue light exposure in lettuce than in rocket. Rocket grown under combined R:B lighting, and lettuce grown under 100 % red or blue LED light suffered most from drought (in the form of lower relative water content (RWC) and increased membrane leakage), despite increased ABA concentrations and antioxidant activity at mid-drought. In lettuce, but not in rocket, there was a positive residual effect of LED pre-treatment on fresh weight of droughted and re-irrigated plants. Increased concentrations of antioxidant metabolites did not correlate with tolerance to drought. All combined R:B light treatments increased stomatal density in both rocket and lettuce. All LED regimes which resulted in increased stomatal aperture area per unit leaf area (AALA) (μm2 aperture/cm2 leaf) resulted in decreased RWC at the end of drought. Stomatal AALA was negatively correlated with end-drought RWC (r = -0.89, p = 0.02) in rocket, but not in lettuce. The relationship between stomatal development and antioxidant activity on drought tolerance appears to be species-specific. The benefits provided to two leafy green species when grown under LED lights must be balanced against the resulting vulnerability to drought.