Effect of light intensity in response to cold stress on morphological and physiological traits of stevia (Stevia rebaudiana Bertoni) medicinal plant

Cold stress is one of the main limiting factors which decrease crop production and even lead to plant death. Temperatures between 0 to 15 ºC can cause physiological injury in plants. One of these injuries is increasing the generation of reactive oxygen that adversely affect the photosynthetic pigments, protein and thylakoid membrane lipids. Also, light affects on growth, development, plant productions including primary and secondary metabolites, so that leaf area, leaf area duration and plant biomass positively response to increasing light intensity while in low light intensity respiration rate is more than photosynthesis rate. Since light intensity can effect in plant resistance to environmental stress especially cold stress and stevia is sensitive to cold stress and it cannot tolerate temperature below 9ᵒC, therefore, the purpose of this study was to determine how vegetative and physiological traits and soluble sugar of the plant response to cold stress in different light intensities.

This study was done in a factorial arrangement based on a randomized complete block design with three replicates. Treatments were five levels of cold stress (0, 4, 8, 12 and 16 days) at 6±2 ᵒC and three light intensity levels of normal light, 50 and 10 % of normal light (240, 120 and 24 µm.m-2.s-1, respectively). The seedlings of stevia plant from in vitro were planted in the plastic pots after acclimation period (40 days). The seedlings were kept in favorite growth conditions at 20-24ºC and 15 h light photoperiod for 30 days. Then, the mentioned light and temperature treatments were applied. Then chlorophyll a, b, total chlorophyll, chlorophyll a/b ratio and carotenoid were measured. Also, plants were harvested and physiological traits and vegetative dry weights were measured. Finally, plant soluble sugar was measured by dry leaf sample.

The results showed that root length, number of nodes, leaf area, leaf, root, shoot and total dry weights decreased as segmental model while stem dry weight decreased linearly when cold stress duration increased. Also traits like leaf area, plant height, vegetative organs dry weight and soluble sugar content (from 10 to 48%) were decreased by reducing light intensity from 240 to 24 µm.m-2.s-1. Moreover, 16 days after cold stress in all light intensities, chlorophyll a (between 18 to 91% ), b (between 16 to 76%), a+b (between 17 to 86%), a/b (between 1 to 60%) and carotenoid (between 16 to 91%) reduced as compared to the control in either segmental or linear models. The most reduction was recorded at low light intensity. Soluble sugar and total dry weight had the most correlation with leaf area and shoot dry weight, respectively.

the results demonstrated that physiological parameters are more sensitive to prolonged cold stress under lower light intensity.