| Rice is the most important food crop and water shortage is posing a more severe limiting factor to its production while it consumes the largest amount of water in agriculture. It is utmost important to save water consumption in rice production. As high grain yield is targeted in rice production, studies of water saving should be concentrated on increasing yield with less water consumption, by maintaining high water potential of plants under water stress. The main mechanisms for plants to maintain good water relation when subjected to water stress are as follows: one is to limit or reduce water loss, mainly happened in leaf, and another is to keep high water uptake in root. There have been some reports about the effect of leaf rolling on drought stress, but no consistent agreement has been obtained so far. In recent years, the effect of root penetration in water stress resistance has been widely studied, but there is still less research in China. Meanwhile extensive researches have been done to analyze the QTL of root, leaf and yield traits related to drought resistance in rice by using different genetic populations. However, most researches were done in a single condition (environment), and moreover the results were inconsistent. In the present research, 13 rice varieties with different ecotypes were used to investigate the functions of leaf rolling and root penetration on drought resistance. In addition, a recombinant inbred line (RIL) population developed from the cross Zhenshan 97B and Miyang 46 was used to determine QTLs of leaf development and yield components under both water stress and well-irrigated conditions, so as to illustrate the physiological mechanisms of drought resistance and its effect on growth and yield formation in rice under water stress. The main results are as follows:1. The genotypic difference in leaf rolling of rice under water stress and its relation to transpiration and photosynthesisThere was a distinct difference in the change of leaf rolling across growth stages among 13 rice varieties (hybrid combinations) when they were exposed to water stress through stopping water supply at active tillering stage. Three groups could beclassified according to time and extent of leaf rolling: (1) sensitive one, which showed earlier and severe leaf rolling under water stress. Xiushui 11, Wuyunjing 7, Zhonghan 1 and Zhonghan 3 belonged to this group;(2) intermediate one, which showed slower development of leaf rolling and longer time for the maximum rolling extent, and Zhe733, Zhongzao 1, Shanyou 63, Xieyou 9308, Handao 8, Handao 502, Yunlu 29 belonged to this group. (3) insensitive one, which showed slow and less leaf rolling and slow recovery when re-watering, and IAPAR 9, Handao 227 belonged to this group.With an increase in leaf rolling of rice under water stress, stomatal conductance (SC), transpiration rate (TR) and net photosynthesis (Pn) declined greatly. However, the effect of leaf rolling on these physiological traits and the relationship between them varied with rice genotypes. For Zhonghan 1 (a sensitive variety), leaf rolling index (LRI) had significant negative effect on TR and Pn, mainly mediated by its effect on SC, and SC was significantly and positively correlated with Pn and TR. For Handao 8 (an intermediate variety), LRI also showed the significantly negative effect on TR and Pn, but SC had no correlation with TR, although it was significantly correlated with Pn, suggesting that the negative effect of LRI on TR and Pn was not mediated by its effect on SC. For IAPAR 9 (an insensitive variety), LRI had a significant and negative effect on TR, but no obvious effect on Pn was found. Moreover, there was no correlation between SC and TR.The effect of leaf rolling on water use efficiency (WUE) is dependent on its effect on TR and Pn, and also dependent on genotype. For instance, the effect was caused by the dramatic changes of both TR and Pn for Zhonghan 1, while Handao 8 and IPAR 9, only TR caused the effect. When moderate leaf rolling happened under water stress, Pn showed a slight reduction, while TR had more reduction, and WUE raised with an increased LRI. 2. The genotypic difference in root penetration ability of rice under water stressand its relation to transpiration and photosynthesisThere was a great difference among 13 rice varieties or hybrid combinations in root penetration ability under water stress, with upland rice being higher than paddy rice, hybrid rice higher than conventional variety, and japonica higher than indica.Among genotypes within a same type, as found among 7 upland rice varieties, there was also distinct difference in root penetration, and the difference became larger with the growth. Thus at 45 d after sowing, IAPAR 9, Zhonghan 3, Zhonghan 1 showed higher root penetration ability and the difference among them was small, while at 105 d after sowing, Zhonghan 3 and IAPAR 9 had significantly higher root penetration ability than other genotypes.The relationship between root penetration and some morphological characters varied with growth stage. At active tillering stage (45 d after sowing), root penetration was positively correlated with root thickness and root length, dry weight and ratio of root to shoot. At booting stage (75 d after sowing) root penetration was also correlated with root thickness, root length and dry weight, but no correlation with ratio of root to shoot was found. At maturity (105 d after sowing), root penetration was positively correlated with root thickness and root length, but no related with root dry weight and ratio of root to shoot. With the development, the effect of these root traits on root penetration became smaller.The effect of root penetration on stomatal conductance, transpiration rate and net photosynthesis and WUE also varied with genotypes. Some upland rice varieties, such as IAPAR 9, Yunlu 29 and Zhonghan 1 had higher root penetration, larger difference in SC and smaller difference in TR, while there was no consistency between Pn and WUE in response to root penetration. The two indica varieties had lower root penetration. Although their SC, TR and Pn declined greatly under water stress, WUE was less reduced. Other genotypes had middle root penetration, but they showed different SC, Pn, TR and WUE, with 2 hybrid rices being higher than the conventional varieties. 3. QTL analysis of leaf number and age development under different waterconditionsThe genetic linkage mapping with 207 molecular markers obtained from a RIL of Zhenshan 97B and Miyang 46 was used to determine QTLs of leaf number and age development over the growth under the different water conditions. QTLs related to leaf number of main culm, lasting time of leaf growth, and rate of leaf growth were mainly located on the chromosomes 6 and 9, in particular qtln6.2, qrlg6.2 located onRM197-RZ516 of the chromosome 6 controlled total leaf number and rate of leaf growth, respectively, and their additive effect value and contribution were relatively larger. In addition, a QTL controlling rate of leaf growth in main culm was also detected in the location, which showed additive effect of genotypes and environment interaction. Moreover, five QTLs were detected near to a marker RM197 on the chromosome 6 under both water conditions, and under well-irrigated condition, it was found that three additive locus qtln6.1, qltlg6.1 and qrlg6.1 on the location of RM225-RM197 on the chromosome 6 controlled total leaf number, lasting time of leaf growth, and rate of leaf growth, respectively.By analyzing QTL of leaf age development, 14 additive QTLs were detected under water stress condition. During 13 d to 29 d after sowing, one additive QTL was detected on the marker locations of RM296-RM105 of the chromosome 9 and of RM296-RM105 of the chromosome 6, respectively. However, these two QTLs were no longer found thereafter, suggesting that leaf age development under water stress was caused by the expression of the different genes. 4. QTL analysis of yield components and related traits under different waterconditionsCorrelation and QTL analysis were done on panicle weight of main shoot and nine yield-related characters by using a RIL population under two water conditions. 14 and 32 QTLs, controlling panicle weight of main shoot, plant height, panicle length, panicles per plant, shoot weight of main shoot, grains per panicle, grain-setting percentage and grain weight, were detected under water stress and well irrigated conditions, respectively, and 40 and 38 QTLs of additive X additive effect of gene interaction related to these traits were found under the two water conditions, respectively.By analyzing the performance of yield components and yield-related traits under the two water conditions, it was found that the number of additive QTLs controlling grains per panicle, grain-setting percentage, grain weight and shoot weight was obviously greater under well-irrigated condition than water stress, suggesting some genes expressed under well irrigated condition did not express when the plants were exposed to water stress. Some additive QTLs on the 6 marker locations were detectedin both water conditions., but only 3 additive QTLs expressed in both water conditions for a same trait, i.e. qgnpl.l on the marker location of RM151-RG532 of the chromosome 1, qpl2.1 on the marker location of M6-RM240 of the chromosome 2 and qtgwS.l on the marker location of RZ225-RG435 of the chromosome 5.It was found that gene pleiotropism or gene linkage was commonly present in analysis of QTL for targeting traits under the two water conditions. In analysis of QTL related to total leaf number and age development, 26 QTLs involved in 8 marker locations were detected. In analysis of QTLs related to yield characters, 31 QTLs involoved in 10 marker locations were detected.
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