| With four transgenic rice lines over-expressing maize phosphoenolpyruvate carboxylase (PC), pyruvate. othophosphate dikinase (PK). PC + PK (CK), and rice nicotinamide adenine dinucleotide phosphate-malic enzyme (ME) as materials, and an untransformed wild type Kitaake (WT) as the control, the effects of water stress imposed during grain filling on the grain yield, photosynthetic rate of leaves, and water use efficiency of the transgenic rice were investigated. The main results are as follows:1. With the decrease of soil water potentials, the grain yield decreased, with the transgenic lines decreased less and untransformed WT decreased more. Under the field-grown condition, the grain field of four transgenic rice lines of PC, CK, ME, PK was significantly higher than that of WT. In contrast with WT, the grain yield of the four transgenic lines was increased by 5.1% to 25.9%. 9.16% to 30.9%, and 12.94% to 29.1%, respectively, when the soil water potential was at 0 kPa, -25 kPa, and -50 kPa. Similar results were obtained in both cement tank and pot experiments, indicating that C4 transgenic rice lines have higher grain field than WT, especially under water stress conditions.2. When photosynthetically active radiation at the top of the canopy was between 1100 -1400 umol m'V, the photosynthetic rate of the flag leaves for C4 transgenic lines was 1.2 to 1.6 fold and 1.5 to 2.1 fold as high as that for WT, respectively, under the normal irrigation and water stress conditions. Photosynthetic rate of the transgenic lines was 2 to 3 fold as high as that of WT at 12:00-13:30 h of the day, implying that C4 transgenic rice has higher photosynthetic rate, less performance of "noon break", and stronger ability to endure water stress.3. The harvest index of all the test lines increased with the decrease of soil water potentials, indicating that water stress enhances translocation of assimilates to grains. There was no significant difference in harvest index between the transgenic lines and WT when soil water potential was the same, suggesting that increase in the grain yield for the transgenic rice was mainly attributed to an enhanced capacity of dry matter production.4. The transpiration rate decreased and water use efficiency or transpiration efficiency increased with the decrease of soil water potentials. Photosynthetic rate of WT was seriouslyreduced, but that of the transgenic lines was less or not significantly reduced, at water stress, suggesting that C4 transgenic rice increases water use efficiency without sacrifice dry matter production at a certain level of water stress.5 In contrast with WT, C4 transgenic rice showed lower malondialdehyde (MDA) concentration and higher superoxide dimutase (SOD) activity and chlorophyll content in leaves, more root exudates and stronger root activity at the same soil water potential, especially at waterstress.6 The percentage of chalky grains and chalkiness were the lowest at the moderate water stress (soil water potential at -25 kPa), the highest at the severe water stress (soil water potential at -50 kPa). and was the between at the normal irrigation (soil water potential at OkPa). At the same soil water potential, the percentage of chalky grains and chalkiness of the transgenic lines were significantly lower or less than those of WT. Amylose and copper (Cu) contents in grains increased and gel consistency decreased with the decease of soil water potentials. At the severe water stress, the content of Cu in the grains of C4 transgenic rice was higher than that of WT. The effect of water stress on protein content in grains was insignificant.The relationships of photosynthetic rate with grain yield and water use efficiency and the response of C4 transgenic rice to water stress and its mechanism were discussed in the paper.
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