Photosynthetic utilization of radiant energy by temperate lettuce grown under natural tropical condition with manipulation of root-zone temperature

Abstract

Photosynthetic utilization of radiant energy was studied by chlorophyll (Chl) fluorescence and maximum photosynthetic O 2 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 (RZT 20) and a fluctuating ambient RZT (RZT a) from 23 to 40°C. On a sunny day, irrespective of RZT, ΔF/ 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 ΔF/F m′ and higher NPQ than RZT 20 plants. The electron transport rate (ETR) was much higher in RZT 20 plants than in RZT a plants especially during moderately sunny days. There were no significant diurnal changes in P max although these values of RZT 20 plants were much higher than those of RZT a plants. On cloudy days, no significant diurnal changes in ΔF/F m′ and NPQ occurred, but ΔF/F m′ was higher and NPQ was lower in RZT 20 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 RZT 20 and RZT a plants were constant. Again, RZT 20 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.