Studies On Mechanisms Of The Effects Of Different Nitrogen Supplies On Photosynthesis And Photosynthetic Nitrogen Use Efficiency Of Rice Plants

With the ongoing desertification, erosion, and urbanization, the available land area is degenerating. With the decreasing land area, it is crucial to increase rice productivity to meet the increasing population. In order to increase rice yield, excessive fertilizers, especially nitrogenous fertilizer, are supplied. Although rice yield is increased under the application of nitrogenous fertilizer, increase in rice yield does not keep pace with the application of nitrogenous fertilizer, nitrogen use efficiency (NUE) decreases with the increasing nitrogen supplies. Photosynthesis is the main approach to product biomass, therefore, decrease in NUE must be related to the decreased photosynthetic nitrogen use efficiency (PNUE). However, it is not yet clear why PNUE decreases under high nitrogen supply, and how to increase PNUE efficiently. In this thesis, we focused on the reason for the decreased PNUE under high nitrogen supply, and on the approaches to increase PNUE.In addition, rice consumes a large amount of water utilized in agricultural production. Rice consumes more than 40% of the total utilized water, and water use efficiency is only 30%-40%. In traditional rice production system, more than 80% of the total water irrigated to the paddy field is lost through evaporation and leakage. Numerous studies have been conducted aiming at increase of water use efficiency and water-saving irrigation system in rice production system. Although soil water is controlled at 70%-90% in non-flooded cultivation, rice plants frequently suffer from drought stress, which restrains rice growth. Compared with traditional rice cultivation, nitrogen form in non-flooded cultivation is transferred from ammonium to nitrate or the mixture of nitrate and ammonium. However, it is not clear how nitrogen forms affect photosynthesis, and whether nitrogen forms can regulate the effects of water stress on rice growth.In this thesis, hydroponic experiments supplied with different nitrogen concentrations were conducted, mainly investigating rice growth, photosynthesis, leaf nitrogen content, Rubisco content and activity, CO2 conductance and chloroplast development. We mainly focused on the reasons for decreased PNUE under high nitrogen supply, and on the approaches how to improve PNUE. In addition, hydroponic experiments, supplied with different nitrogen forms and water stress (simulated with 10% PEG 6000), were conducted. We mainly focused on the effects of different nitrogen forms on photosynthesis, and the reason why ammonium enhances the tolerance of rice plants to water stress. The major results are list as follows:1. With the similar Rubisco content, chloroplast development and electron transport chain, decrease in chlorophyll within a certain degree can not restrain photosynthesis. (1) Forty percent of the chlorophyll in the mutant captured 70% of the light compared to wild type. And there is no significant difference in photosynthesis between wild type and the chl-deficient mutant; (2) The decreased light capture in the mutant was compensated for by a relatively higher quantum yield of photosystemⅡ(PSⅡ), which conferred a total electron transport rate (JT) equal to that in wild type; (3) More photons were absorbed by wild type and exhausted through thermal dissipation; (4) Gene expression analysis of the thylakoid membrane showed that the chlorophyll deficiency in the mutant did not impair the electron transport chain; (5) Chlorophyll deficiency in the mutant had no negative impact on chloroplast development in terms of size and grana stacking; moreover, the chloroplastic CO2 concentration and leaf N and Rubisco contents were comparable to those in wild type.2. Decreases in (photosynthetic) nitrogen use efficiency in plants fed high N might be resulted from the insufficient CO2 supply and decreased Rubisco activation state. (1) Shoot biomass, leaf area, and tiller numbers per plant under low N were lower than under intermediate and high N supply. No significant differences were observed between plants supplied with intermediate and high N; (2) There were increases of approximately 40% in leaf organic N and Rubisco content in plants fed high N relative to those fed low N; however, this increase in N content did not affect the actual quantum yield (a), both carboxylation efficiency (CE) and photosynthesis rate increased by about 15%; (3) The lowered gt/N ratio in high N supply resulted in decrease in Rubisco activity and photosynthetic nitrogen use efficiency.3. Chloroplast enlargement induced increases in mesophyll conductance and total conductance accounted for the increase in net photosynthetic rate under high N supply. (1) Net photosynthetic rate and CE increased with the increases of leaf N content, but there was no significant difference in a; (2) According to the classical photosynthetic models, Rubisco content and RuBP regeneration rate were sufficient for photosynthesis; (3) Mesophyll conductance increased with the increase of leaf N content, and there were closely positive relationships between net photosynthetic rate and CO2 conductance; (4) Chloroplast enlargement under high N supply decreased the distance between chloroplasts and intercellular air space and increased chloroplast area facing the intercellular air space, as a result, increased mesophyll conductance.4. Although larger chloroplast size under high nitrogen supply resulted in a higher mesophyll conductance and photosynthesis, the raito of mesophyll conductance to Rubisco decreased under high nitrogen supply, which subsequently induced relative insufficient CO2 supply and decreased Rubisco activity and PNUE. (1) There is a negative relationship between chloroplast size and PNUE regardless of plant species or growth conditions; (2) Larger chloroplasts under high nitrogen supply have a decreased CO2 mesophyll conductance (gm) to Rubisco content ratio, which leads to a relative insufficient supply of CO2 and lower Rubisco activity; (3) 1-fold increase in leaf N content will lead to about 27% decrease in PNUE, of which more than 20% decrease is induced by chloroplast enlargement.5. Relative lower stomatal opening is a major limitation for PNUE under high nitrogen supply. (1) Root water uptake rate and aquaporin function in water transportation were increased with increasing nitrogen supply, and they was higher under high nitrogen supply in Yangdao 6 than in Shanyou 63; (2) Although water uptake ability was increased under high nitrogen supply, root water uptake ability under high nitrogen supply can not meet the requirement of shoot transpiration in Shanyou 63, which as a result induced a decrease in leaf water potential under high nitrogen supply in Shanyou 63; (3) The different variation in leaf water potential between Shanyou 63 and Yangdao 6 led to a discrepancy in stomatal opening between the two cultivars. Stomatal opening was increased with the increase of leaf nitrogen in Yandao 6, while there was no significant difference in Shanyou 63.6. The increased photorespiration is not a major reason for the decreased PNUE under high nitrogen supply. (1) Photorespiration rate was increased with the increase in leaf nitrogen content. (2) PNUE in Shanyou 63 was decreased from 15.70μmol CO2 g-1N s-1 in 0.09 mg cm-2 nitrogen content leaves to 10.89μmol CO2 g-1N s-1 in 0.2 mg cm-2 nitrogen content leaves, it was decreased by about 30%. PNUE in Yangdao 6 was maintained at 11.5μmol CO2 g-1N s-1. (3) Pn2%/N in Shanyou 63 was decreased from 21.00 in 0.09 mg cm-2 nitrogen content leaves to 15.88μmol CO2 g-1N s-1 in 0.2 mg cm-2 nitrogen content leaves, it was decreased by about 24%. Pn2%/N in Yangdao 6 was maintained at about 17μmol CO2 g-1N s-1. (4) Although photorespiration rate was as high as 5-12μmol m-2 s-1, CO2 release from photorespiratory CO2 was only about 1μmol m-2 s-1.7. Ammonium nutrition can enhance the tolerance of rice plants to water stress both in hydroponic and pot experiments. (1) Under water stress conditions, the decrease in plant growth and photosynthesis under ammonium supply was less than under nitrate supply; (2) Under non-flooded cultivation, the biomass and photosynthesis in rice plants supplied with ammonium and ammonium+dicyandiamide (DCD, a nitrification inhibitor) were higher than in nitrate fertilization; (3) In hydroponic experiments, water uptake of rice seedlings under ammonium nutrition was higher than under nitrate nutrition.8. Shrinkage of chloroplasts in nitrate nutrition restrained gm and photosynthesis under drought conditions. (1) The relatively higher distribution of leaf N to Rubisco in ammonium nutrition benefited photosynthesis little, but did enhance CO2 saturated photosynthesis; (2) Water stress decreased stomatal conductance in both N forms, it decreased by 15% in ammonium nutrition compared with 36% in nitrate nutrition; (3) Water stress decreased mesophyll conductance to CO2 and chloroplast CO2 concentration in nitrate nutrition, whereas had no negative effects in ammonium nutrition; (4) Restricted supplementation of CO2 in nitrate nutrition under water stress (NNP) induced higher photorespiration, which accounted for the suppression in photosynthesis in NNP; (5) Decreased gm in NNP was related to chloroplasts shrinkage, induced by a lower leaf water potential(Ψleaf).9. Contribution of aquaporin to mesophyll conductance can be implied by the negative effects of HgCl2, the inhibitor of aquaporin activity, on photosynthesis. The decreased aquaporin contribution to mesophyll conductance partly accounted for the decreased mesophyll conductance and photosynthesis in nitrate nutrition under water stress. (1) Water stress significantly restrained OsPIP1s gene expression, gene expression level in ammonium supply was higher than in nitrate supply under both non- and water stress condition; (2) Negative effect of HgCl2 on photosynthesis in NNP treatment was lower than in other three treatments. HgCl2 decreased CE by 30.58% in AN, by 28.38% in NN, by 24.55% in ANP, and by 16.17% in NNP; (3) No negative effect on A/Cc response curves was observed in all treatments.