Effects of Nitrogen, Phosphorus and Potassium Deficiencies on Photosynthesis and RuBP Carboxylase-Oxygenase Activities in Rice Plants

摘要

Photorespiration competes with photosynthetic CO2 fixation in C3 plant and causes the decrease of net CO2 uptake by about 5O per cent. It has been clarified that the rate of photorespiration is affected significantly by temperature and the concentrations of CO2 and O2. However, little has been known about the effects of nutrient conditions on photorespiration. In the present studies, the effects of nutrient deficiencies on photosynthesis, photorespiration and carboxylating enzyme activities were studied in rice plant (Oryza sativa L. cv. IR-8). The first experiment was conducted to survey the effects of nitrogen, phosphorus and potassium deficiencies on photosynthetic rates and carboxylating enzyme activities. Seeds were sown on the soil containing standard fertilizer. After fourth leaf had emerged, the plant was grown in water culture. Nutrient deficiencies were induced by withholding nitrogen, phosphorus and potassium from the culture solution respectively. The measurements were conducted at ripening stage when the symptons of nutrient deficiencies were clearly presented. The second experiment was conducted to confirm the effects of phosphorus deficiency on photosynthetic rates and carboxylating enzyme activities. Seedlings were grown in solutions containing different amount of phosphorus for four weeks. Thereafter phosphorus was withheld from each medium except that of control plants. The measurements were conducted at tillering stage and ripening stage. The rates of photosynthesis in terms of unit area of the fully developed uppermost leaf were measured in 3 per cent O2 and 21 per cent O2 by infrared gas analyzer. The measurements were done under the conditions of O.O3 per cent in CO2 concentration, 5O klux in light intensity and 25°C in leaf temperature. Transpiration rate was measured by electric hygrometer simultaneously. Enzymes were prepared from 0.5 g fresh leaves mortaring with l0 ml of homogenizing medium and small amount of sand. The medium contained 0.05 M HEPES buffer (pH 7.7), 1 mM EDTA and 1 mM dithiothreitol. The extracts were centrifuged at 20000 g for 15 minutes and the supernatants obtained were offered to the determinations of the enzyme activities. Each procedure mentioned above was conducted at 5°C. The activity of RuBP-carboxylase (RuBPc) was assayed by measuring the rate of 14C (NaH14CO3) incorporation into the acid-insoluble compounds in the reaction mixture containing 50 μmole tris (pH 8.5), 5 μmole MgCl2, 20 μmole (2.5 μCi) NaH14CO3, 0.5 μmole RuBP and 25 μl enzyme preparation. They made the final volume of 0.5 ml. After several minutes preincubation at 30°C, assay was conducted for 5 minutes initiating by the addition of RuBP to the mixture and stopped by adding 0.1 ml of 20 per cent acetate. The activity of RuBP-oxygenase (RuBPo) was assayed by determining the rate of O2 consumption in the reaction mixture by oxygen electrode. Reaction mixture was 100 μmole Amediole (pH 9.3), 10 μmole MgCl2, 1 μmole EDTA, 1 μmole RuBP and 200 μl enzyme preparation. They made the final volume of 1ml. Reaction was initiated by adding RuBP to the mixture at 30°C. The activity of PEP-carboxylase (PEPc) was assayed by determining the rate of 14C (NaH14CO3) incorporation into acid-insoluble compounds in the reaction mixture containing 25 μmole tris (pH 8.5), 2.5 μmole MgCl2, 0.05 μmole NADH, 0.55 IU MDH, 10 μmole (2.5 μCi) NaH14CO3, and 25 μl enzyme preparation. They made the final volume of 0.25 ml. After 10 minutes preincubation at 30°C, assay was conducted for 5 minutes initiating by the addition of PEP to the mixture and stopped by adding 0.l ml of 20 per cent acetate. [the rest omitted]