Inhibition effect of high strength nutrient solution on hypocotyl stretch in cucumber and tomato seedlings

【Objectives】Cucumber (Cucumis sativus) and tomato (Lycopersicon esculentum) are main cultivated vegetable species in China. The seedling growth in breeding processes is vulnerable to disadvantageous environments, such as high temperature, high humidity and weak light, which lead to excessive hypocotyl stretch and reduced yield at later stage. Several methods such as plant growth regulators, mechanical stimulations, supplement of light, temperature modulation and water limitation are used to prevent excessive hypocotyl stretch, but the effects are always not satisfied by complicated influence factors. To develop an effective method for controlling hypocotyl stretch in cucumber and tomato seedlings, this paper focusses on the inhibition effect of high strength nutrient solution on hypocotyl stretch in cucumber and tomato seedlings.【Methods】The concentration of complete macroelement of 1.0 strength Hoagland nutrient solution is defined as fundamental formula (1.0C). Other nutrient solution concentration gradients involved in this experiment are 0.5, 2.5, 5.0, 7.5, 10.0 and 12.5C by adding macroelement compositions proportionally without changing the microelements compositions. 0.5C nutrient solution is set as control group, and others are treatment groups. After watering cucumber and tomato seedlings by different nutrient solutions in greenhouse conditions, the growth parameters are measured and the nutrient solution effects on hypocotyl cortical cell length, growth rate and phytohrmone levels are determined at cotyledon extending stage. Then seedlings are rewatered by 0.5C nutrient solution to restore the inhibition growth of hypocotyl stretch. The growth parameters are measured at the first true leaves extending stage. The effects and mechanism of high nutrient solution on inhibiting hypocotyl stretch are determined, and the optimal strength is acquired by appropriate effect on cucumber and tomato seedlings.【Results】The effects of six strength (0.5, 2.5, 5.0, 7.5, 10.0 and 12.5C) Hoaglands nutrient solution on controlling excessive hypocotyl stretch are: (1) Electrical conductivity (EC) of substrate increases linearly (r=0.99) and growth of both cucumber and tomato are inhibited gradually from 0.5C to 12.5C nutrient solution. Compared with that under 0.5C nutrient solution, length of hypocotyl cortical cell decreases by 49% in cucumber and 48% in tomato under 10.0C nutrient solution. (2) Endogenous phytohormone homeostasis is disturbed by decreasing GA3 and ZR levels but increasing ABA levels, which significantly displays a negative correlation with hypocotyl length (P＜0.01). IAA levels shows differently tendency dependent on the concentration of nutrient solution. (3) The growth of hypocotyl stretch that has been inhibited by high strength of nutrient solution could be restored to different degree by rewatering 0.5C nutrient solution. Especially in 5.0C and 7.5C treatment, the ratio of plant height/stem diameter is better than that in other treatments, which is could be expected to effectively control excessive hypocotyl stretch. 【Conclusions】 High strength nutrient solution contributes to the inhibition effect on excessive hypocotyl stretch by changing endogenesis phytohormone homeostasis, apparently because of promoting more ABA synthesis and accumulation in hypocotyl cells, and shortening hypocotyl cortical cell length. The inhibition effect could be restored by decreasing the nutrient solution concentration. The method exhibits potential application prospect due to simple maneuverability and obvious effects, but the optimal strength and mechanism on different vegetable species still demand to be further studied and analysed.