| Rice consumes a large amount of water utilized in agricultural systems,which take more than 40% of the total utilized water. In traditional rice production system, more than 80% of the total water irrigated to the paddy field is lost through evaporation and leakage,leading to water use efficiency is only 30%-40%. Furthermore, our country is one of the areas where the water resources is lack most in the word, and its temporal-spatial distributions is imbalanced. Therefore, numerous studies have been conducted aiming to increase water use efficiency and water-saving irrigation system in rice production system,such as covering with plastic film and straw mulching. Compared with traditional flooded cultivation, nitrogen form in non-flooded cultivation is transferred from ammonium tonitrate or the mixture of nitrate and ammonium. Therefore, it is important to study the influence of different nitrogen forms on drought tolerance of rice seedlings under simulated water stress conditions.In our previous studies, we have demonstrated that ammonium (compared with nitrate)can enhance the tolerance of rice seedlings to drought stress and that this increase in tolerance is related to higher water uptake rate and photosynthesis in rice seedlings under water deficit conditions. However, it is not yet clear why water uptake rate decrease in rice seedlings fed with nitrate under water deficit conditions, and how root regulate shoot growth. In this thesis, hydroponic experiments supplied with different nitrogen forms and water deficit simulated by adding 10% (W/V) PEG 6000 were conducted, and two rice cultivars (O. sativa L., cv. 'Shanyou 63' hybrid indica China, and cv. 'Yangdao 6' indica China) were selected, in which Shanyou 63 was a drought tolerance variety,while Yangdao 6 was a drought sensitive variety. We investigated rice growth, photosynthesis, root morphology and anatomical structure and its effects on water uptake and nutrient absorption. We mainly focused on how aerenchyma formation in rice seedlings was regulated by nitrogen forms and water deficit, and the reason why ammonium enhances the tolerance of rice seedlings to water defict. The major results are list as follows:1. Water deficit significantly restrained photosynthesis and shoot growth in rice seedlings fed with nitrate, while had no negative effects on rice seedlings fed with ammonium. Compared to non-water deficit conditions, the root water uptake rates decreased 27 and 25% in Shanyou 63 and Yangdao 6 fed with ammonium, and decreased 41 and 34% fed with nitrate under water stress conditions, respectively. Under water deficit conditions, the root xylem sap rates decreased 21 and 28% in Shanyou 63 and Yangdao 6 fed with ammonium, and decreased 55 and 56% fed with nitrate, respectively. Root water uptake rate and xylem sap rate in rice seedlings fed with ammonium were higher than those in rice seedlings fed with nitrate. The higher water uptake and xylem sap rate in rice seedlings fed with ammonium are beneficial to its leaf extension and stomata opening.Water deficit increased leaf temperature in rice seedlings fed with either of two nitrogen forms, especially in Yangdao 6 fed with nitrate.2. Under water deficit conditions, there was a different effect of different nitrogen forms on root morphology and anatomical structure in rice seedlings. In the absence of water deficit, no significantly differences in lateral roots, cell death, aerenchyma formation and ligninification were observed in rice seedlings fed with either of two nitrogen forms.Water deficit promoted the development of lateral roots, especially, in rice seedlings fed with ammonium, which more closer to root tip than those in rice seedlings fed with nitrate.Water deficit increased root cell deah, ligninification and more aerenchyma formation in rice seedling fed with nitrate, while had no negative effects on rice seedlings fed with ammonium. In additional, the enhanced aerenchyma formation results in heavy lignification of root.3. In rice seedlings, water deficit promoted roots cortical cell death and more aerenchyma formation when fed with nitrate nutrition. Under water deficit conditions,the ethylene release rates of Shanyou 63 and Yangdao 6 plants fed with nitrate were 150 and 176% higher than those fed with ammonium after 4 days PGE treatment,respectively.Aerenchyma formation related genes (OsLSD1, OsLSD2, OsEDS and OsPAD4) expression were regulated by N forms and water status. The expression of OsACS5 was higher in rice seedlings fed with nitrate than ammonium in the presence or absence of water deficit indicating a higher ethylene synthesis ability in rice seedlings fed with nitrate. Exogenous ethylene stimulated aerenchyma formation,and the aerenchyam formation in rice seedlings fed with nitrate was greater than that fed with ammonium bubbling with ethylene,especially, in Yangdao 6. In additional, water deficit increased root respiration in rice seedlings fed with nitrate during early period after PEG treatment, resulting in extra aerenchyma formation. However, root respiration was decreased by aerenchyma well-development.4. Water deficit significantly decreased shoot growth and accumulations of nitrogen,phosphorus and potassium in rice seedlings fed with nitrate,while had no negative effects on rice seedlings fed with ammonium. Under water deficit conditions, roots cell death,aerenchyma formation and ligninification decreased water uptake and absorption of nitrogen, phosphorus and potassium in rice seedlings fed with nitrate. Furthermore,exogenous ethylene stimulated aerenchyma formation, especially, in rice seedlings fed with nitrate, resulting to rice biomass, the contents of nitrate and phosphorus significantly decreased in rice seedlings fed with nitrate.5. Water deficit significantly decreased leaf notrogen content, photosythesis and shoot biomass, when only half-root of rice seedlings grown in nitrate with water deficit. The leaf notrogen content, photosythesis and shoot biomass in rice seedlings supplied with amminum alone, when half-root system with water defict, were higher than those fed with nitrate alone. Water deficit significantly accelerated root cell death and more aerenchyma formation, and promoted the degree of lignification in half-root system of rice seedlings fed with nitrate. However, water deficit had no negative effects on root anatomical structure in half-root system of rice seedlings fed with ammonium,but also stimulated lateral root development. Root water uptake and xylem sap flow rate are significantly higher in half-toot system supplied with ammonium than with nitrate under water deficit conditions. |
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