Anatomical, biochemical and gene expression studies in four hydrangea cultivars under low temperature stress

Hydrangea, which has large and vivid flowers, is a crucial ornamental plant that is used in landscape design. However, its limited tolerance to cold restricts its widespread application in regions with low temperatures. To investigate the effects of low-temperature stress on hydrangea, four common cultivars were examined. By comparing leaf anatomical structures and physiological parameters including soluble substances levels, antioxidant enzyme activities, and cold resistance-related gene expression levels under different low-temperature conditions (4°C, 0°C, -4°C, -8°C, and -12°C), cold resistance was comprehensively evaluated. The semilethal temperature (LT50) values of the four cultivars ranged from -2.404°C to -3.133°C, and 'Magic coral' and 'Forever summer' had the lowest values. Under low-temperature stress, the malondialdehyde (MDA) production rate in hydrangea leaves significantly increased, whereas the antioxidant enzymes activity and the soluble substances contents initially increased but subsequently decreased with decreasing temperature. Moderate low-temperature stress increased the expression of antioxidant enzymes and cold resistance-related genes (ERF, POD, CHS, and ANS), thus increasing the activity of intracellular antioxidant defense systems. This mechanism contributed to enhancing cell membrane stability and increasing overall cold resistance in hydrangeas. A comprehensive evaluation was conducted using the membership function method, which allowed the allowing for the analysis of both leaf anatomical structure indices and physiological indicators. The results revealed the cold resistance of the four selected hydrangea cultivars, indicating that the cold tolerance of 'Forever summer' and 'Magic coral' was superior to that of 'Tabei' and 'Huabao'. Therefore, 'Forever summer' and 'Magic coral' are recommended for use as priority cultivars for in landscape applications in regions with low temperatures.