2018 ASHS Annual Conference

Understanding plant photochemical responses to photosynthetic photon flux density (PPFD) is important for developing energy-efficient supplemental lighting strategies. However, the photochemical light response varies greatly among species and cultivars, and a rapid, reliable method to describe species- and variety-specific photochemical responses is needed. Chlorophyll fluorescence measurements were used to determine the electron transport rate (ETR) of six bedding plant species: Begonia semperflorens ‘Ambassador Scarlet’ (begonia), Catharanthus roseus ‘Jams N Jellies Blackberry’ (vinca), Impatiens walleriana ‘Super Elfin Violet’ (impatiens), Pelargonium x hortorum ‘Maverick Violet’(geranium), Petunia x hybrida ‘Daddy blue’(petunia), and Salvia splendens ‘Mojave’ (salvia). Diurnal measurements were conducted in a greenhouse with fluorescence measurements taken every 15 min during the day and hourly at night with 5 measurement days per species. Additional measurements were taken in a growth chamber using a hyperbolic series of PPFDs (0, 50, 150, 300, 500, 750, 1050, 1400 μmol∙m^-2∙s^-1), with 20 min acclimation at each intensity, and 5 replications per species. For 4 species, the data collected in the growth chamber was similar to the greenhouse data, but for impatiens and petunia observed ETR was generally lower in the greenhouse. This may have been due to physical damage to the leaves induced by the fluorometer leaf clip. In all cases, an asymptotic rise to a maximum function fit the data well. This function uses only two variables: the initial slope and the asymptote of the ETR response curve: ETR= [asymptote of ETR] x [1- e^[-(initial slope of ETR/asymptote of ETR) x PPFD]. Accordingly, it was hypothesized that the photochemical light response could be adequately described by determining only the initial slope and asymptote: a rapid light response curve. This was tested in a growth chamber by measuring the ETR of each species at a very low (≈3 μmol∙m^-2∙s^-1) and very high (≈2100 μmol∙m^-2∙s^-1) light intensity for 5 min. The equation generated from this data fit the greenhouse ETRs with a mean R²=0.93 and slope of 0.89; the estimated values were generally 11% higher than the observed ETRs. Similarly, it fit the previous growth chamber data with mean R²=0.96 and slope of 0.94 (estimated ≈6% higher than observed) for all species except for impatiens, which had a much higher slope (m=1.5, R²=0.94), suggesting that the high PPFD used to determine the asymptote was photoinhibitory for impatiens. This high-throughput method accurately describes the ETR response for 5 of the 6 species.