Photosynthesis of Chinese cabbage and radish in response to rising leaf temperature during spring

Abstract

Photosynthesis in leaves, exposed to temperature change, was measured during spring to provide information for modeling of cropping systems for Chinese cabbage (Brassica campestris subsp. napus var. pekinensis) and radish (Raphanus sativus) in Jeju Island. The net photosynthetic rates (A) of Chinese cabbage and radish were depressed at temperatures above approximately 25°C (corresponding to the optimum temperature for photosynthesis), due in part to high rates of respiration above 25 or 30°C. Chinese cabbage was more sensitive to high temperature than radish. Above 25°C, stomatal conductance (g s) declined steeply with a sharp increase in the vapor pressure deficit (VPD) and transpiration rate (E), suggesting that stomatal closure could not be associated with transpiration rate, particularly in Chinese cabbage. Dramatic declines in water use efficiency (WUE), instantaneous transpiration efficiency (ITE), and carboxylation efficiency (CE) above 30°C indicated that the plants could be severely affected negatively by high temperatures above 30°C. However, compared to Chinese cabbage, radish had high light use efficiency and potential to assimilate CO2, judged by their high quantum yield (φ) and maximum photosynthetic rate (A max). Radish utilized high levels of photosynthetic photon flux and strongly acclimated to the light environment, considering their high light saturation point (Q sat) and low light compensation point (Q comp). These results suggested that rising temperature might substantially alter water availability of plants by itself and/or through effects on the rate of water loss from leaves of Chinese cabbage and radish under diurnal and/or seasonal fluctuations of temperature, and/or global temperature change. To maintain the high productivity and quality of these crop species, it may be necessary to harvest them until early June, when the daily maximum temperature is below 25°C in Jeju Island.