Physiology, anatomy, and cell membrane thermostability selection of leafy radish (Raphanus sativus var. oleiformis Pers.) with different tolerance under heat stress

Abstract

Leafy radish (Raphanus sativus L. var. oleiformis Pers.), a temperate crop, is usually heat-sensitive. Line 9911-9 is a heat-tolerant line bred for summer vegetables in Taiwan. Understanding the heat tolerance mechanism of Line 9911-9 may facilitate future breeding programs. We conducted a comparative study on growing plants of heat-tolerant Line 9911-9, moderately tolerant Line 9611, and heat-sensitive ‘Taichung No. 1’ at 40/35°C and 25/20°C to elucidate the heat tolerance mechanism of leafy radish. After 28-day of acclimation, the maintenance of net photosynthetic rate (Pn) and chlorophyll fluorescence (Fv/Fm) accompanying a double increase of stomatal conductance (gs) and transpiration rate (E) in Line 9911-9 revealed that the heat-tolerant line had adjusted physiologically to be more efficient in heat dispersing. Morphologically, Line 9911-9 developed more xylem vessels, an enlarged stomatal aperture by 5-fold, and an increased stomatal opening to 93%, which contributed 6-fold increase in the occupancy of total stomata aperture on leaf without altering stomata density under high temperature stress. Furthermore, the occupancy was correlated with gs and E with a coefficient of 0.87 and 0.98, respectively, confirming that stomatal factors play a crucial role in heat tolerance in leafy radish. Regarding the heat shock response, Line 9911-9 could have sustained over 9 days at 40/35°C, whereas Line 9611 was maintained over 3 days, with ‘Taichung No. 1’ for only 1 day. Line 9911-9 was able to maintain its cellular membrane stability by inducing the activities of superoxide dismutase (SOD) and catalase (CAT) to scavenge reactive oxygen species (ROS) within the first 24h and detoxify ROS unbalance under high temperature stress. These results indicated that Line 9911-9 was able to lower the non-stomatal limitation effect on Pn. A high correlation (R2=0.89) between cotyledon relative injury at 50°C for 30min in 21 leafy radish accessions and their yield reduction ratio under high temperature stress suggests that the CMT of cotyledon may be used as a reliable index for selecting heat-tolerant leafy radish.