Correlation and Quantitative Trait Loci Analyses of Total Chlorophyll Content and Photosynthetic Rate of Rice(Oryza sativa) under Water Stress and Well-watered Conditions

In order to explore the relevant molecular genetic mechanisms of photosynthetic rate （PR） and chlorophyll content （CC） in rice （Oryza sativa L.）, we conducted a series of related experiments using a population of recombinant inbred lines （Zhenshan97B x IRAT109）. We found a significant correlation between CC and PR （R = 0.19＊＊） in well-watered conditions, but no significant correlation during water stress （r = 0.08）. We detected 13 main quantitative trait loci （QTLs） located on chromosomes 1, 2, 3, 4, 5, 6, and 10, which were associated with CC, including six QTLs located on chromosomes 1, 2, 3, 4, and 5 during water stress, and seven QTLs located on chromosomes 2, 3, 4, 6, and 10 in well-watered conditions. These QTLs explained 47.39% of phenotypic variation during water stress and 56.19% in well-watered conditions. We detected four main QTLs associated with PR; three of them （qPR2, qPR10, qPR11） were located on chromosomes 2, 10, and 11 during water stress, and one （qPR10） was located on chromosome 10 in well-watered conditions. These QTLs explained 34.37% and 18.41% of the phenotypic variation in water stress and well-watered conditions, respectively. In total, CC was largely controlled by main QTLs, and PR was mainly controlled by epistatic QTL pairs.

Screening for Drought Resistance of Rice Recombinant Inbred Populations in the Field

In a 2-year experiment, 187 genotypes were grown under well-watered and drought stress conditions, imposed at panicle initiation stage. The relationship of genotypic variation in yield under drought conditions to potential yield, heading date and flowering delay, reduction in plant height, and to a drought response index （DRI） was detected. Grain yield under drought stress conditions was associated with yield under well-watered conditions （r= 0.47^＊＊, and r= 0.61^＊＊ during 2 years of tests）. The delay of heading date ranged from -1 （no delay） to 24days, and was negatively associated with grain yield （r =-0.40^＊）, spikelet fertility percentage （r =-0.40^＊＊）, harvest index （r =-0.58^＊＊）, but positively associated with yield reduction percentage （r = 0.60^＊＊）. The reduction in plant height was negatively associated with grain yield （r =-0.24^＊＊, and r=-0.29^＊＊）, spikelet fertility percentage （r = -0.23^＊＊, and r= -0.21^＊）, harvest index （r = -0.37^＊＊, and r= -0.54^＊＊）, and positively associated with yield reduction percentage （r = 0.58^＊＊, and r= 0.58^＊＊） in 2003 and 2004, respectively. The DRI of genotypes was strongly associated with grain yield （r = 0.87^＊＊, and r= 0.77^＊＊）, fertility percentage （r = 0.66^＊＊ and r= 0.54^＊＊）, harvest index （r= 0.67^＊＊ and r= 0.61^＊＊）, and negatively associated with grain reduction percentage （r=-0.70^＊＊, and r=-0.73^＊＊） under drought stress. The results indicate that genotypes with drought resistance can be identified by measuring yield potential, delay in flowering, reduction in plant height, or DRI under test environments of well-watered and drought stress.

Panicle Water Potential,a Physiological Trait to Identify Drought Tolerance in Rice

Two upland rice varieties （IRAT109, IAPAR9） and one lowland rice variety （Zhenshan 97B） were planted in summer and treated with both normal （full water） and drought stress in the reproductive stage. Panicle water potential （PWP） and leaf water potential （LWP） were measured every 1.0-1.5 h over 24 h on sunny days. Both PWP and LWP of upland varieties started to decrease later, maintained a higher level and recovered more quickly than that of the lowland variety. The results show that PWP can be used as an indicator of plant water status based on the parallel daily changes, and the high correlation between PWP and LWP. Similar correlations were also observed between PWP, LWP and eight traits related to plant growth and grain yield formation. PWP seemed to be more effective for distinguishing the upland rice varieties with different drought-tolerant ability. Differences in PWP and LWP between upland and lowland rice varieties were also observed at noon even under normal water conditions, implying the incorporation of the drought-tolerant mechanism to improve the photosynthesis and yield of traditional paddy rice.