The Changes Of Energy Metabolism And Its Regulation In Response To Root Restriction In Fruit Vegetable Crops

With the development of modern protected horticulture in China, root restriction cultivation, such as potted culture and trough culture with limited substrate and nutrient film technique, has become prevailing in many greenhouses. Root restriction is a powerful technique for saving agricultural resources and controlling environment of root systems, and to solve the problems caused by soil sickness or soil contaminations. In addition, it has also been used to modify the growth and development of vegetative and reproductive organs. However, disadvantages such as reduction of plant growth and photosynthesis often arise from root restriction. The detailed mechanism involved, however, remains unclear. In this study, tomato (Lycopersicon esculentum Mill.) plants were grown hydroponically to investigate the changes of root energy metabolism and adaptive mechanism in response to severe root restriction condition. Factors limiting plant growth and leaf photosynthesis were also investigated accordingly. In addition, experiments were also carried out to examine the possible role of exogenous application of putrescine (Put) on the tolerance to hypoxia stress in cucumber (Cucumis sativus L.). The main results are as follows:1. The physiological process of energy synthesis and its utilization, such as respiration of isolated mitochondria and sucrose degradation were examined in response to root restriction. Root restriction significantly reduced total, cytochrome, and alternative pathway respirations of isolated mitochondria which was accompanied by simultaneous decrease in plant biomass production and resulted in a significant decrease of ATP content in the root tissue. In addition, root restriction greatly decreased activities of root plasma membrane H⁺-ATPase and vacuolar H⁺- ATPase, but increaseed activities of alcohol dehydrogenase (ADH) and lactate dehydrogenase (LDH) which brought about the degradation of root tip cell nucleolus. Ratio of Invertase/sucrose synthase activity was increased as an adaptation to the insufficient ATP production. The decreased energy synthesis under root restriction was partially compensated by the energy-conserving sucrose synthase pathway of sucrose metabolism. 2. To investigate whether O₂ deficit is involved in the depressed shoot growth under root restriction condition, tomato plants were subjected to root restriction with or without supplemental aeration. Root restriction significantly depressed root and shoot growth as early as 15 days after treatment and this depressive effect was alleviated by vigorous aeration around the restricted root zone. Growth suppression by root restriction occurred concomitantly with sharp decreases in dissolved O₂ concentration in solution together with significant decreases in root total and cytochrome pathway respiration, hydrolytic ATPase activities, and root cell viability. However, no such decreases were found in well-aerated root-restricted plants. Root restriction-induced growth suppression was independent of nutrients level in leaves and was not primarily related to the decline of leaf water potential. Root restriction resulted in an increase in xylem sap ABA concentration from day 15 to the end of the experiment but no such effect was observed in well-aerated plants. It is likely therefore, that O₂ supply was one of the main limiting factors to the reduced shoot growth under root restriction condition.3. The mechanism by which root restriction affects the photosynthetic characteristics was examined in the present study. Root restriction significantly decreased light-saturated rate of the CO₂ assimilation-(A_sat) which was accompanied by the significant decrease in leaf water potential, stomatal conductance (g_s), intercellular CO₂ concentration (C_i), increases in the stomatal limitation (l), and xylem sap ABA concentration. As the stress developed, the maximum carboxylation rate of Rubisco (Fcmax) and the capacity for ribulose-l,5-bwphosphate regeneration (J_max) also decreased, followed by substantial reductions in the quantum yield of PSII electron transport (Φ_PSII), the efficiency of energy capture by open PSII reaction centers (F_v'/F_m'), and photochemical quenching (q_p). Root restriction did not induce photoinhibition. In addition, carbohydrate levels in various tissues for the well-aerated root-restricted plants were several-fold of that in control plants while the A_sat was almost identical to the controls. It is likely that root restriction-induced depression of photosynthesis was mimicked by water stress induced by O₂ deficit.4. The previous results indicated that hypoxia stress was largely involved in the root restriction-induced depression of plant growth and photosynthesis. Both cucumber and tomato plants were low O₂ sensitive species. Since the ESTs including more than 1700 genes have been constructed in our lab, the cucumber (Cucumis salivus L.) was used in this study to examine the protective role of exogenous putrescine (Put) against hypoxia stress. Hypoxia stress brought about a reduction of plant growth accompanied by decreases in leaf gas exchange and root membrane integrity, and these were significantly alleviated by Put addition to the nutrient solution 24 h prior to the hypoxia administration. In the root, the expression levels of NR (nitrate reductase) and its structural domain genes FAD (FAD binding), CYP51G1 (Heme-iron ion binding), the activities of NR and nitrate reduction process were greatly increased both by hypoxia stress and by exogenous Put application. Put further increased these parametors in stressed roots. Additionally, Put pretreatment evidently alleviated the dramatically decrease in the NAD⁺/NADH ratio, ATP concentration, and the consequential root cell viability of hypoxia-stressed plants. There was no significant difference in LDH activity among all the treatments and ADH activity was also independent on Put level. The results suggested that Put enhanced the tolerance to hypoxia possibly by increasing the transcript levels of NR and its structural domain genes, hence stimulating the activities of .NR and nitrate reduction to maintain the redox and energy status.