Effects Of Short-Term Light Stress On Leaf Structure And Senescence Of Watermelon Seedlings

Seedling storage is one of the key components in high-quality seedling industry. In foreign countries, seedlings have being usually stored in low temperature and weak light environment, while in China, simple darkness storage is mostly used due to the lack of advanced storage equipments. During the storage process, seedling leaves usually become yellow and shedding in the condition of low temperature, weak light and darkness, which are the typical reactions of senescence. Consequently, it is significant to to search the mechanism of leaf senescence under high light stress for optimizing the storage conditions.In our research, watermelon(Citrullus vulgaris Schrad., cv. Fairfax)seedlings were treated for 2, 4, 6 days in the following conditions of darkness in 25℃, darkness in 15℃, and weak light (15μmol·m-2·s-1) in 15℃respectively, then all treatments were transferred to normal light for 2, 4, 6 days. Leaf structure and senescence-associated indexes of seedlings were studied.The results demonstrated as follows:(1) The chloroplast swollen and became spherical in darkness, chloroplast lamellae reduced and disordered, and starch granules disappeared. In the early stage of 25℃treatment in darkness, chloroplast lamellae structure was affected, and lamellae decreased gradually in subsequent 6 days then disappeared lastly. While in darkness 15℃t reatment, the membrane of chloroplast was affected the earliest and disintegrated in 6 days, as lamellar structure of chloroplast existed still. In weak light, chloroplast structure kept well, the accumulation of starch granules, the number of grana lamellae, and the stack-degree of grana were better than dark-treated seedlings.After recovered in normal light, chloroplast lamellae stacked anew and starch granules accumulate again, which positively correlated with stress-resistance of plants. The longer darkness kept for, the more difficult chloroplast structure recovered, and the chloroplast structure in darkness 15℃treatment recovered faster than in darkness 25℃. The recovery of the chloroplast structure after being treated for 2 days in weak light recovered worse than darkness treatments, while recovered better after being treated for 4 days and 6 days. This showed that the chloroplast structure was sensitive to luminous intensity variation under short-term treatment of weak light.(2) The leaf chlorophyll content, soluble protein content, fresh weight and dry weight all decreased under two dark-treatments, and the degradation of proteins exist was earlier than the loss of chlorophyll. The above indexes of leaves in darkness 15℃declined smaller than in darkness 25℃. In weak light, leaf chlorophyll content increased firstly then decreased, soluble protein content increased, leaf fresh weight and dry weight all decreased, furthermore, all the indexes were higher than in darkness.In these three treatments, Fo, Fm, Fv all increased, which consequently induced to the inactivation or damage of PSⅡreactive center, but weak light treatment was affected lighter. In darkness 25℃t reatment, Fv/Fm, Fv/Fo andФPSⅡall decreased with the time, and these symptoms were lighter in darkness 15℃treatment. In weak light, Fv/Fm, Fv/Fo andФPSⅡdecreased in first 2 days and then continuously elevated, and thus exceeded dark-treatment. Therefore, weak light treatment can alleviate the inhibition to the photochemical reaction center of watermelon leaves in darkness .(3) In darkness treatments, SOD activity decreased, while POD activity increased then decreased. The SOD, POD activity of leaves in darkness 15℃was higher than in darkness 25℃. In above three treatments, MDA content and electrolytic leakage (EL) both increased constantly, the darkness 25℃increased the most, then the darkness 15℃and the weak light treatment. During treatment and recovery, the POD activity of leaves in weak light was higher than in darkness throughout, which demonstrated that POD was mainly responsible for the clearance of active oxygen in the plant. This result indicated that weak light treatment is helpful to keep higher protective enzyme activity, reduce the membrane lipid peroxidation, maintain the membrane permeability, and thus alleviate the damage of darkness.(4) After the recovery in normal light, the effects of darkness in 25℃on leaves which were treated for 2,4days can recover to control level slowly, however, 6d-treated seedlings can't recover which accelerated leaf senescence, but this effects can be delayed by dark-treatment in 15℃.The seedlings which were treated in weak light and darkness in 15℃for 2,4 days recovered to the same level almost.But the seedlings which were treated in weak light were better than in dark-treatment which recoverred to normal light after 6d-treatment. So, we speculated that weak light contion can delayed leaf senescence which induced by darkness, if seedlings were treated for long time.