Continuous Chlorophyll Fluorescence Monitoring of Greenhouse-grown Lettuce (Lactuca sativa L.)

Chlorophyll fluorescence measurements are an effective means of rapidly assessing the performance of photosystem II (PSII). Long-term chlorophyll fluorescence monitoring can provide useful information about photosynthetic efficiency under changing environmental conditions. Chlorophyll fluorescence of a greenhouse-grown crop of romaine type lettuce (Lactuca sativa L. ‘Green Towers’) consisting of 15 plants was continuously monitored in situ for 5 weeks beginning at 2 weeks after germination. Measurements were taken on a single leaf after the dark-adapted quantum efficiency of PSII (F_v/F_m) was determined. Every 2 d, the fluorometer was moved to a leaf on a different plant. The quantum yield of PSII (Φ_PSII) and photosynthetic photon flux density (PPFD) were measured every 15 min. The electron transport rate through PSII (ETR) was estimated from Φ_PSII and PPFD. The daily light integral (DLI) was calculated from the PPFD measurements and ranged from 2.3 to 28.5 mol∙m^-2∙d^-1 (13.8 ± 6.3 mol∙m^-2∙d^-). The integral of ETR over a day, or daily photochemical integral (DPI, mol∙m^-2∙d^-1), was calculated for each day. As DLI increased, DPI increased asymptotically from 0.75 to 3.39 mol∙m^-2∙d^-1 (R² = 0.84, P < 0.001), with little or no increase at DLIs greater than 15.5 mol∙m^-2∙d^-1. Over all measurement days, ETR increased asymptotically to 126 µmol∙m^-2∙s^-1 as PPFD increased, with little or no increase at PPFDs greater than 800 µmol∙m^-2∙s^-1 (R² = 0.91, P < 0.001). A corresponding decrease in Φ_PSII with increasing PPFD to a minimum of 0.13 was also observed (R² = 0.91, P < 0.001). Throughout the study F_v/F_m remained relatively constant at 0.85 ± 0.01, indicating that the maximum efficiency of PSII did not change with plant age. These results demonstrate that chlorophyll fluorescence monitoring could be effectively used to develop crop-specific lighting recommendations.