Alfalfa (Medicago sativa L.) is a valuable forage legume, but its production is largely affected by high temperature. In this study, we investigated the effect of heat stress on 15 alfalfa cultivars to identify heat-tolerant and -sensitive cultivars. Seedlings were exposed to 38/35 °C day/night temperature for 7 days and various parameters were measured. Heat stress significantly reduced the biomass, relative water content (RWC), chlorophyll content, and increased the electrolyte leakage (EL) and malondialdehyde (MDA) content of heat-sensitive alfalfa cultivars. However, heat-tolerant cultivars showed higher soluble sugar (SS) and soluble protein (SP) content. The heat tolerance of each cultivar was comprehensively evaluated based on membership function value. Cultivars with higher mean membership function value of 0.86 (Bara310SC) and 0.80 (Magna995) were heat tolerant, and Gibraltar and WL712 with lower membership function value (0.24) were heat sensitive. The heat tolerance of the above four cultivars were further evaluated by chlorophyll a fluorescence analysis. Heat stress significantly affected the photosynthetic activity of heat-sensitive cultivars. The overall results indicate that Bara310SC and WL712 are heat-tolerant and heat-sensitive cultivars, respectively. This study provides basic information for understanding the effect of heat stress on growth and productivity of alfalfa.

Root tensile strength (T_r) is a fundamental root mechanical trait and serves as a key parameter for assessing the contribution of vegetation to slope stability. T_r is known to exhibit high intraspecific variability, but whether T_r varies with season remains unclear. Here, we investigated the seasonal variation in T_r in two commonly seen herbaceous species, i.e., Artemisia argyi and Cirsium setosum, both of which can be future candidates for revegetating species along roadsides in temperate and subtropical regions. We examined the T_r of their first- (closest to the stem base) and third-order lateral roots sampled in the southwest of Henan, China, in two distinct periods: September (late growing season) and December (dormant season). We found that the T_r of the thicker, first-order roots in September was significantly greater than that in December. However, such seasonal variation was not found for the thinner third-order roots. When fitting the relationship between T_r and root diameter using a two-parameter power law equation, the calibrated equation using the data collected in September led to a marked predictive bias to the data collected in December. All the above patterns were consistent for both species. Soil moisture, which exhibited strong seasonal variation in the study area, might be the key cause of variation in T_r. Our study is among the first to demonstrate seasonal variation in root mechanical traits, indicating that season potentially plays a non-negligible role in impacting soil reinforcement and slope stability by modifying roots’ mechanical quality.