2018 ASHS Annual Conference

Blue light is known to affect leaf stomatal development and aperture and has been shown to play a role regulating plant-water relations. The objective of this study was to quantify the effects of blue light on growth and morphology, water-use efficiency (WUE), stomatal conductance ((g_s), SPAD index, and shoot nitrogen uptake by ‘Cherokee’ and ‘Waldmann’s Green’ lettuce (Lactuca sativa) grown under different red-to-blue-light ratios. Seven treatments were evaluated in the study: 100% red; 7% blue + 93% red; 26% blue + 74% red; 42% blue + 58% red; 66% blue + 34% red; 100% blue; and broad-band white light (containing 19% blue, control). All treatments provided an average daily light integral (DLI) of 17.5 mol·m^‒2·d^‒1 (270 ± 5 µmol·m^‒2·s^‒1 over an 18-h photoperiod). The experiment was replicated three times over time; each experimental replication was terminated 21 d after treatment initiation. Regardless of cultivar, no treatment differences were measured for leaf area and specific leaf area (SLA). In contrast, for every 10% increase in blue light, leaf number and shoot dry mass (DM) decreased by 0.02 leaves and 0.2 g, respectively. Water-use efficiency also decreased linearly in response to blue light. Conversely, g_s and SPAD index increased by 0.05 mmol·m^‒2·s^‒1 and 0.02 units, respectively, for every 10% increase in blue light. Although tissue nitrogen content (mg·g^-‒1) increased with higher blue light, nitrogen uptake (mg) was unaffected by light quality. Our findings indicate that the significant decrease in WUE under higher blue light could be attributed to a reduction in plant growth (leaf number and DM) and an increase in g_s, which may be the result of higher stomatal aperture and leaf conductance, and could lead to higher water loss through transpiration.