Photosynthetic Utilization of Radiant Energy by Temperate Lettuce Grown Under Natural Tropical Condition with Manipulation of Root-Zone Temperature

Photosynthetic utilization of radiant energy was studied by chlorophyll (Chl) fluorescence and maximum photosynthetic O₂ evolution (P _max) in temperate lettuce (Lactuca sativa L.) grown under natural tropical fluctuating ambient temperatures but with their roots exposed to two different root-zone temperatures (RZTs): a constant 20 °C-RZT (RZT₂₀) and a fluctuating ambient RZT (RZT_a) from 23 to 40 °C. On a sunny day, irrespective of RZT, F/_Fm ratio of the variable to maximal fluorescence under irradiation (the maximal photosystem 2 quantum yield with actinic light)] decreased and non-photochemical quenching (NPQ) increased parallel to the increase of photosynthetic photon flux density (PPFD). However, RZT_a plants showed lower F/F_m and higher NPQ than RZT₂₀ plants. The electron transport rate (ETR) was much higher in RZT₂₀ plants than in RZT_a plants especially during moderately sunny days. There were no significant diurnal changes in P _max although these values of RZT₂₀ plants were much higher than those of RZT_a plants. On cloudy days, no significant diurnal changes in F/F_m and NPQ occurred, but F/F_m was higher and NPQ was lower in RZT₂₀ plants than in RZT_a plants. Diurnal changes in ETR were also observed in all plants while P _max values throughout the whole cloudy days in both RZT₂₀ and RZT_a plants were constant. Again, RZT₂₀ plants had much higher values of P _max than RZT_a plants. During RZT transfer period, all Chl fluorescence parameters measured at midday fluctuated with PPFD. Impact of RZT on these parameters was observed 2–3 d after RZT transfer. ETR and P _max measured with saturating PPFD in the laboratory did not vary with the fluctuating PPFD in the greenhouse but the effects of RZT on these two parameters were observed 3–4 d after RZT transfer. Thus RZT affects photosynthetic utilization of photon energy in temperate lettuce grown under natural tropical condition.