Study On The Mechanism And Measures Of Controlling Hypocotyls Overgrowth Of Cucumber And Tomato Seedlings

In view of the hypocotyl excessive elongation which often occurred during the vegetable seedling,cucumber (Cucumis sativus L. cv. Zhongnong No.203) and tomato (Lycopersicon esculentum Mill. cv. Zhongza No.105) were selected as experimental material in this study to research the effects and mechanisms of different day and night temperature, plant growth inhibitors and high-strength nutrient solution irrigation on the control of hypocotyl overgrowth.The aim was to provide further theoretical bases and technological guide for controlling overgrowth and improving the quality of the seedlings. Test results were as follows:1. Experiments were conducted to investigate the influence of temperature on the effect of CCC controlling excessive growth of cucumber seedlings, with four different day/night temperature conditions being used. The results showed that as the day and night temperature decreases, the hypocotyls length and plant height decreased significantly, while the seedling index increased significantly of cucumber seedlings treatment with same concentration CCC. CCC treatment couldn't control the excessive growth of hypocotyls in the environment condition of day/night temperature of 35℃/25℃. Under the day/night temperature of 28℃/18℃or 25℃/15℃environment, as the temperature decreasing, the concentrations of CCC for seed soaking which is needed in strong seedling cultivation were reduced accordingly. When the day/night temperature was 20℃/10℃, the growth of seedlings were significantly inhibited by CCC treatments, and hypocotyls were shortened, while the control treatment showed no sign of overgrowth. Above results suggested that high temperature could weaken while low temperature could strengthen the inhibition effects of CCC on seedling growth.Experiments were conducted to investigate the influence of temperature on the effect of S₃₃₀₇ controlling excessive growth of tomato seedlings, with four different day/night temperature conditions being used. The results showed that under the four kinds of day and night temperatures, different concentrations of S₃₃₀₇ treatment inhibited hypocotyl elongation, while decreased anthocyanin content, promote the formation of dwarf seedlings. As the day and night temperature decreases, the time required for tomato seedling emergence was prolonged, hypocotyls length decreased significantly, chlorophyll and mineral element content decreased, anthocyanin content increased, and growth of tomato seedlings treatment with same concentration S₃₃₀₇ was reduced. Above results suggested that low day and night temperature could enhanced the effects of S₃₃₀₇ treatment on the dwarf growth of tomato hypocotyls, weaken the promoting effect of S₃₃₀₇ on chlorophyll accumulation and mineral element absorption, thereby inhibited the overgrowth of hypocotyls and dwarfed tomato seedlings.2. Experiments were conducted in order to clarify the mechanism of plant growth inhibitor controling the hypocotyls overgrowth of vegetable seedlings. Cucumber seeds soaking with 3000 mg·L^-1 CCC, and treatment with distilled water as control. The results showed that CCC treatment could induce a series of ultrastructural changes. For example, cell wall was more thick, cytoplasm density was higher, central vacuole was smaller, chloroplasts were abundant, larger number of granas were observed and stacked together with each other, and starch grains in plastids increased obviously. By changing the cellular ultrastructure, CCC soaking treatmentinhibited cell elongation, thus controlled hypocotyls overgrowth of cucumber seedling effectively.3. Experiment was conducted by supply 0.5×, 2.5×, 5.0×, 7.5×, 10.0×and 12.5×of Hoagland nutrient solution on cucumber and tomato seedlings grown in the greenhouse, increased the rhizosphere EC values linearly. By EC promotion, dry matter density was increased, but water content, growth rate and cortex parenchyma cell length were decreased in hypocotyls, thereby hypocotyls were dwarfed. Well as high EC in seedling rhizosphere, enhanced the endogenous ABA accumulation and lowed the level of endogenous GA₃ and ZA in cucumber and tomato seedling. Hypocotyls length negatively correlated with ABA concentration, and positively correlated with IAA/ABA, ZR/ABA and GA₃/ABA of hypocotyls. Above results indicated that promoted the rhizosphere EC by supply high concentration of nutrient solution, able to enhance the endogenous ABA accumulation, reverse the growth-promoting effects of IAA,GA₃ and ZR on cell elongation, and then inhibit the elongation growth of cortex parenchyma cell and hypocotyls. After recovered treatment with 0.5×Hoagland nutrient solution, shoot and root growth can return to normal of cucumber and tomato seedlings of 5×treatment, and root/ shoot ratio was significantly higher than other treatments, the quality of seedlings was best. Thus, by irrigating high concentration of nutrient solution (5×Hoagland nutrient solution) could controlled hypocotyls overgrowth of effectively , and recovered treatment with 0.5×Hoagland nutrient solution could regulate of plant type and strengthen the seedlings of cucumber or tomato seedlings.