2018 ASHS Annual Conference

The objective this experiment was to study the influence of various ammonium (NH₄⁺) to nitrate (NO₃^-) ratios on growth, photosynthetic pigment composition, and photosynthetic apparatus associated with photosystem II (PSII) photochemistry and thermal energy dissipation in hydroponically grown lettuce (Lactuca sativa L.). Two lettuce cultivars (Rex RZ, Black Seeded Simpson) were grown with hydroponic nutrient solutions containing 5 different NH₄⁺:NO₃^- ratios (100:0, 75:25, 50:50, 25:75, 0:100, adjusted to total 12 meq L^-1 N) and evaluated during the first (T1) and second (T2) week of culture. Plants grown under higher NH₄⁺/NO₃^- ratios exhibited significantly suppressed shoot growth with a poorly developed root system, dark-green leaves, and less water and nitrate uptake. On the other hand, plants grown under higher NO₃^-/ NH₄⁺ ratios exhibited vigorous shoot growth with well-developed root systems and high water and nitrate uptake. There were strong correlations between growth parameters and photosynthetic pigment stoichiometry as well as ratios. Total chlorophyll content increased, but the ratios of Chl a/b and carotenoid x+c/Chl a+b decreased progressively with increasing NH₄⁺/NO₃^- ratios which are associated with a greater increase in Chl b. There were significant changes in PSII efficiency under different NH₄⁺/NO₃^- ratios for cultivar Rex RZ during T1 period, however these changes are likely to be temporal and cultivar-dependent. In this study, the steady-state chlorophyll fluorescence (Fs) was directly correlated with total biomass and maximum efficiency of PSII photochemistry (F'_v/F'_m) and inversely correlated with non-photochemical quenching (NPQ) under different NH₄⁺/NO₃^- ratios, but no clear correlation with quantum yield of PSII electron transport (F_PSII) and photochemical quenching coefficient (q_P). Values of parameters F_PSII and q_P were strongly influenced by Fs, leading these values to peak at 25% NO₃^- ratio and then decrease with increasing NO₃^- ratios. Leaves under high NH₄⁺/NO₃^- ratios exhibited increases in F_PSII (actual PSII efficiency) but maintained F_NPQ and F_NO unchanged. However, leaves under high NO₃^-/NH₄⁺ ratios kept F_PSII and F_NPQ unchanged but significantly increased F_NO, enhancing thermal dissipation of the light absorbed by PSII within the antenna bed.