2019 ASHS Annual Conference

A rapid screening test for fast growth can facilitate cultivar development and selection for controlled environmental agriculture. An ideal screening method can rapidly and non-invasively detect desirable phenotypes, based on physiological and/or morphological characteristics. We developed a screening protocol by testing a combination of canopy imaging and chlorophyll fluorescence-light response curves. Ten plants of 11 lettuce cultivars (Lactuca sativa; ‘Aquino’, ‘Barlach’, ‘Carmessi’, ‘Crunchita’, ‘Locarno’, ‘Lotus’, 'Rex', ‘Rouxai’, ‘Seurat’, ‘Teodore’, and ‘Xandra’) were planted in 0.5 L pots containing a soilless substrate. Plants were subirrigated with hydroponic solution containing 100 mg·L^-1 of nitrogen. The greenhouse conditions averaged a daily light integral of 8.9 ± 9.6 mol·m^-2·d^-1, temperature of 23.4 ± 1.2 °C, and relative humidity of 38 ± 13% (mean ± SD). Projected canopy size (PCS), as determined using canopy imaging, differed both among cultivars and at different days after germination (DAG). Dry weights (DW) at 51 DAG were measured to identify a correlation with measured parameters. PCS at 13 DAG explained 52% of variability in final DW among cultivars. At 24, 34, and 48 DAG, the coefficient of determination was 0.12, 0.61, and 0.66, respectively. ‘Lotus’ had the highest PCS throughout the experiment and also had the highest DW. Sigmoidal regression of the cumulative photosynthetic photon flux density (PPFD) versus PCS characterized different growth patterns among cultivars. The asymptote of the sigmoidal curve was positively correlated with final DW (R² = 0.60), indicating that a large PCS is associated with greater final DW. Diurnal chlorophyll fluorescence responses to PPFD showed different photochemical efficiencies [the quantum yield of photosystem II (Φ_PSII) and electron transport rate (ETR)] among cultivars. The PCS at 13 DAG combined with the maximum ETR explained 80% of variability in DW among cultivars. Total canopy photochemistry (TCP; mol) over the cropping cycle of the cultivars was estimated from the PPFD, the ETR responses (µmol·m^-2·s^-1) to PPFD, and the PCS (m²). Because the presence of anthocyanins may lead to an overestimation of ETR, regression between estimated TCP and DW was done separately for green and purple lettuces. TCP and DW of green lettuces were highly correlated (R² = 0.96), but purple lettuces showed no such correlation. In conclusion, screening of phenotypes for greater biomass production can be accomplished based on PCS and chlorophyll fluorescence-light response curves. However, chlorophyll fluorescent measurements may not be useful for anthocyanin-rich cultivars.