عنوان مقاله [English]
Introduction: Plants native to tropical and subtropical climates which grown in the temperate climate zone, suffer chilling injury when exposed to non-freezing temperatures for a certain period of time. The optimum growth temperature for cucumber (a tropical plant) is 20 to 25°C. Cucumber is sensitive to temperatures lower than 10 °C. Cucumber area of production exposes to late spring and early autumn cold weather in Khorasan-e-Razavai, Iran. Studies showed that chilling leads to an alteration in fatty acid composition of membrane lipids and its permeability, changes in photosynthetic pigments content and decrease in photosynthesis. Many researchers pointed to a possible role of polyamine compounds in plant defense against environmental stresses. Exog enous application Spd could prevent the electrolyte and amino acid leakage or recovering the plasma membrane damage in rice and cucumber in response to salinity, chilling and water stressed conditions.
Materials and methods: A factorial experiment, based on completely randomized design was conducted to investigate the effect of short-term chilling on cucumber plantlets which was earlier treated with spermidine. Factors were included two levels of temperature (6 and 12°C) and four levels of spermidine (0, 0.25, 0.5 and .0.75 mg/L). The studied cultivar was ‘Super-Dominus’. In order to determine the extent of chilling injury, plants of each treatment were rated based on visual symptoms. By assigning values of 1, 2, 3, 4, and 5 while 1: no visible symptoms 2:5% of leaf area necrotic, 3: 5-25% of leaf area necrotic, 4: 26-50% of leaf area necrotic but plant still alive, 5: lost, entire plant necrotic and collapsed. Measured traits were root and shoot length, root and shoot dry weight, root and leaf electrical leakage, and leaf chlorophyll content.
Results and discussion: Plants which exposed to low temperature showed chilling injury symptoms (5-25% leaf area necrotic). The symptoms reduced (less than 5% leaf area necrotic) by using 0.25 and 0.5 mg/L spermidine. The symptoms enhanced by 50% by applying 0.75 mg/L spermidine at 6°C. Analysis of variance showed that there was significant difference between temperature levels, spermidine levels and interaction between them in respect to root length, shoot length, shoot dry weight and root and leaf electrical leakage. Root dry weight, root to shoot ratio and chlorophyll content just affected by temperature and spermidines levels but not by interaction between them. Root and shoot length and dry weight decreased by low temperature. At cold stress condition growth decreased due to a reduction in photosynthesis and carbohydrate metabolism .Root and shoot length decreased more than 79% at 6°C compare with 12°. Root to shoot ratio increased at cold condition which was the result of lower root weight loss in response to cold temperature compared with shoot weight losses. Electrical leakage (EL) enhanced in leaf and root cells at chilling temperature, but the enhancement was significantly more at root cells. Electrical leakage enhanced more than 52% in root cells at 6°C compared with 35% in leaf cells. EL suppressed, using 0.25 and 0.5 mg/L spermidine while an increase observed in El at 0.75 mg/L spermidine. The lowest EL percentage observed for leaf samples treated with 0.25 and 0.5 mg/L spermidine at 12°C. The highest EL percentage belonged to root samples treated with 0.75 mg/L Spd at 6°C .Chlorophyll content (ChlC) decreased at cold condition. ChlC was 52% at12°C compared with 37% at 6°C. High significant correlation observed between chlorophyll content and shoot dry matter (r2= 0.96**). Root and shoot length and dry weight and leaf chlorophyll content enhanced using 0.25 and 0.5 mg/L spermidine at both chilling and control temperatures. A decrease observed in measured traits applying 0.75mg/L spermidine. There was no significant difference between 0.25 and 0.5 mg/L spermidine levels in respect of measured traits expect for shoot dry weight. Spermidine enhances chilling tolerance in cucumber by prohibiting the activity of NADPH oxidase. The capacity of PAs to enhance the tolerance of cucumber to chilling injury is attributed to the scavenging of H2O2 production under chilling condition.
Conclusion: Results showed that root and shoot length and weight, root and leaf electrical leakage and chlorophyll content of leaf adversely affected by chilling stress. Using 0.25 mg/L spermidine modulates plant responses to chilling stress. There was no significant difference between 0.25 and 0.5 mg/L spermidine in respect of measured traits. But all measured traits adversely affected using 0.75 mg/L spermidine at both 6 and 12°C.