| Cucumber (Cucumis sativus L.) is an important protected vegetable crop, also a global major vegetable, which plays an important role in the market supply. At present, seedlings and transplanting was mainly used in Cucurbits cultivation. The seedlings may easily spindle when environment is incorrectly control in the process of breeding. This might lead to the decline of the seedlings quality, and then affecting cucumber growth, flower bud differentiation, fruit setting and development, finally influence the product's yield and quality. Therefore, further study of the mechanism of leggy seedlings has not only important theoretical significance but also application value.Two different ecological types of cucumber cultivars 'No.4 Jinchun' and 'Deltastar' were used as the test material to study the different temperature and light treatments on growth of cucumber seedlings and determine the environmental conditions leading to leggy seedlings. Through regulating the environmental conditions, we can get leggy and normal seedlings to study the dynamic changes and differences of cucumber leggy and normal seedlings in morphology, tissue structure, cell ultrastructure, dry matter accumulation, carbon and nitrogen compound content and endogenous hormones and auxin oxidase activity, to explore the mechanism of cucumber seedlings leggy, provide theoretical basis for seedlings leggy regulation. The major results were as follows:1. Two cucumber variety 'No.4 Jinchun' and 'Deltastar' were used as test material to study the effects of normal temperature and light conditions (T1),30/25℃day and night sub-high temperature (T2) and 30/25℃day and night sub-high temperature combined with black sunshade net (T3) on cucumber seedling growth status and photosynthetic pigment content. The results showed that under the continuousness 16 day' s sub-high temperature together with shading treatment, height and hypocotyl length of the cucumber seedling increased significantly compared with control. In contrast, a significant drop in hypocotyls diameter was observed, and the same fall was found in the fresh weight and dry weight. In addition, cotyledon and the first true leaf area decreased, showing obviously leggy. Meanwhile sub-high temperature and shading make chlorophyll a and b, total chlorophyll content and carotenoid content in cucumber seedlings declined, and root-shoot ratio as well as root activity all reduced. The results demonstrate that sub-high temperature combined with shading can cause a serious leggy but single sub-high temperature only lead to slight leggy.2. 'No.4 Jinchun' and 'Deltastar' were used as test material to study the differences between normal seedlings and leggy seedlings in the morphology, leaf microstructure and cell ultrastructure. The results showed that plant height and hypocotyls length of leggy seedlings were significantly higher than the normal. Hypocotyls length had significant positive correlation at 0.01 levels with plant height; the percentage of leggy seedlings' hypocotyls length account for plant height was significantly increased. There was a significant positive correlation between hypocotyls cell length and hypocotyls length. Leggy seedlings hypocotyls length and hypocotyls cell length was significantly longer than the normal one, hypocotyls cell elongation rate was significantly higher than the normal seedlings. Leggy seedlings leaves were thin, palisade tissues became shorter, spongy tissue arranged in loose with larger intercellular space. Normal seedling leaves were thicker, palisade tissue presented slender columnar, and cells arranged in order. There were more inclusions in vacuole and less number chloroplasts in mesophyll cells in leggy seedlings; lamellar structures have been destroyed, starch grains in chloroplasts almost disappeared; Normal seedlings'central vacuole membrane was integrity; chloroplast stroma was uniform with rich layers and layers were arranged compact and orderly. Starch grains took up a large space in volume in the chloroplasts; it was observed that mitochondria appeared regular spherical or ellipsoidal. The study preliminary identified the differences of morphology, leaf tissue and subcellular structure between normal seedlings and leggy Seedlings.3. Two cucumber variety 'No.4 Jinchun' and 'Deltastar' were used as test material, we studied the dynamic changes and differences of dry matter accumulation and content of carbon and nitrogen compounds in leggy seedlings and normal seedlings. The results showed that: Compare with normal seedlings, dry matter accumulation, and fresh weight, dry weight of leggy seedlings was significantly lower. Leggy seedlings' total soluble sugar, sucrose, glucose, fructose and crude fiber contents were significantly lower. Crude protein, soluble protein and free amino acids of leggy seedlings were significantly lower than the normal seedlings. Leggy seedlings'dry matter accumulation decreased, carbohydrate content was decreased, the energy fixed less, carbon and nitrogen metabolism is slow.4. 'No.4 Jinchun' and 'Deltastar' were used as test material to study the dynamic changes and differences of leggy seedlings and normal seedlings' endogenous hormones contents and IAA oxidase activity. The results showed that compare with normal seedlings, IAA, GA3 and ZT content of leggy seedlings were increased, ABA content decreased. The enzymes that closely related with IAA catabolism IAA oxidase, IAA peroxidase, POD activity leggy one showed a significantly lower than the normal seedlings. Leggy seedlings' endogenous hormones which promotion cell elongation and division increased, while the enzymes that IAA catabolism, its activity were decreased, maybe it is one of reasons for seedlings leggy. |
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