Effects Of Root Zone Temperatures On The Growth And Photosynthesis Of Hydroponically-grown Lettuce

Root zone temperature can affects the growth and metabolism of plant through mineral elements absorption and water transport.The physiological response of lettuce?Lactuca sativa L.?under different root zone temperatures was examined by chlorophyll fluorescence kinetics method,the change of gas exchange parameters and CO₂ response characteristics.The major results are as follows:1.Under five different root zone temperaturesthe plant growth and concentrations of mineral elements were examined in hydroponically-grown lettuce.When the lettuce roots were exposed to 25 ?,the highest fresh weight and dry weight of the above-ground part and total leaf area were obtained.Concentrations of mineral elements were also obviously influenced by root zone temperatures,and reached the peak at 25 ?.When the lettuce roots weresubjected to 35 ?,the lettuce growth and the absorption of mineral elements were significantly inhibited,the lowest nitrate content was found at 35 ?.Our results indicated thatthe root zone temperature of 25 ?was the most suitable root zone temperature for the growth and mineral absorption of hydroponically-grown lettuce.2.Photosynthetic fluorescence characteristics of lettuce under five different root zone temperatures were studied.Net photosynthetic rate?Pn?,maximal photochemical efficiency?Fv/Fm?,excitation energy capture efficiency?Fv'/Fm'?,actual photochemical efficiency??PSII?,photochemical quenching?qP?,photochemical fluorescence quenching coefficient?qP?and energy ratio of photochemistry reaction?P?werestrongly effected by root zone temperatures,and reached the peak at 25 ?,while the indexes of non-photochemical quenching?NPQ?and energy ratio of antenna heat dissipation?D?showed a convert tendency and hit the bottom at 25 ?.This experiment provided certain theoretical basis for selecting optimumrootzone temperature for lettuce.3.In this study,Li-COR portable photosynthesis system was used to detect the photosynthetic CO₂ response characteristics of hydroponically-grown lettuce.The CO₂ response data of lettuce were fitted and analyzed by rectangular hyperbola model.Meanwhile,the quantitative relationships between the photosynthesis and the root zone temperature were discussed.Maximum rate of net photosynthesis?Pnmax?and Carboxylation efficiency?CE?remained the highest at 25 ?.while the CO₂ saturation point?CSP?and CO₂ compensation point?CCP?stayed the lowest at 25 ?.As the CO₂ concentration rose,the net photosynthetic rate?Pn?and WUE?water using efficiency?curve of all these selected lettuce showed increase and remained stability at 800?mol·mol-1,while Gs and Tr showed the tendency of first ascending and then slowly descending after 700 ?mol·mol-1.Above all,the root zone temperature of 25 ? was the most suitable root zone temperature forthe growth and photosynthetic physiological process.This study provides some theoretical basis and practical guidance for the management of hydroponically-grown lettuce.