| Rice yield loss caused by low temperature in China could reach to 10 billion kilogram in large-loss years. Low temperature damage is more common for double-season rice with machine transplanting. In order to reduce the negative impacts of low temperature on rice production and select cold resistant double-season rice varieties for machine transplanting, studies were conducted to determine the effects of low temperature on morphological and physiological characteristics of rice varieties, which are widely grown in the middle and lower reaches of Yangtze River region, at seedling, booting and heading stages. The main results were shown as follows:(1) At seedling stage, the differences in electric conductivity, malondialdehyde (MDA) content, soluble sugar content and peroxidase (POD) activity in leaves of each tested variety were significant between low (6,8 and 10℃) and normal temperatures (25℃). Therefore, these five parameters were selected as the physiological indexes for identifying cold resistance of rice varieties at seedling stage.Based on the subordinate function and the cluster analysis,19 tested varieties were grouped into 3 categories with different levels of cold resistance:strong cold resistance varieties (ZLY505, TY167, JY706, TY15, TLY921 and ZLY120), intermediate cold resistance varieties (ZY1, JY268, ZY463, ZZ35, TLY83, BLY18, ZZ39 and TY987), and cold sensitive varieties (ZLY06, XZX42, ZLY112, ZLY706 and ZLY819).(2) During the period of low temperature treatment (10℃,5 d) at seedling stage, superoxide dismutase (SOD) and catalase (CAT) activities increased first and then decreased in leaves in each tested variety, while in roots they slowly increased; MDA content consistently increased in leaves for each tested variety, whereas in roots it slowly increased in the cold resistance varieties but sharply increased in the cold sensitive varieties; soluble protein content increased first and then decreased in leaves in each tested variety, while in roots it decreased first and then increased. During the period of returning to normal temperature (25℃,6 d), SOD and CAT activities and soluble protein content in leaves could be recovered to the levels before low temperature treatment in the cold resistance varieties, whereas they could not be recovered in the cold sensitive varieties; MDA content increased first and then decreased in leaves in each tested variety, while in roots it maintained stable in the cold resistance varieties but increased in the cold sensitive varieties.(3) Under low temperature treatment (5℃,3 d) at seedling stage, spraying of sodium nitroprusside (SNP) with suitable concentration could significantly reduce the magnitudes of increased electric conductivity and MDA content and decreased chlorophyll and soluble protein contents in rice leaves caused by the low temperature, could maintain high SOD, CAT and POD activities in leaves, and could substantially increase abscisic acid but decrease auxin, cytokinin and gibberellic adid in leaves. These indicated that spraying SNP was beneficial to improve cold resistance of rice seedlings.(4) Under low temperature treatment (16℃,5 d) at booting stage, seed setting rates of the tested varieties were decreased by 11.01%-61.14% as compared with the control; a relative seed setting rate (the seed setting rate under low temperature divided by the seed setting rate under normal condition) of more than 80% was recorded in TY821 and ZLY819, suggesting that these two varieties had strong cold resistance; a relative seed setting rate of 70%-80% was recorded in ZLY08, JY268 and ZLY30, indicating that these three varieties had intermediate cold resistance. The low temperature caused decreases in chlorophyll and soluble protein contents, an increase in MDA content, and decreases in SOD, POD and CAT activities in flag leaves. Moreover, the low temperature also had significant effects on photosynthesis and chlorophyll fluorescence parameters. Compared with the control, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), intercellular CO2 concentration (Ci), maximum photochemical efficiency of PSII (Fv/Fm), quantum yield of PSII (ΦpsII) and fraction of open PSII centers (qL) were significantly decreased while non-photochemical quenching (NPQ) was slightly but significant increased in flag leaves under the low temperature treatment.(5) Under low temperature treatment (18℃,7d) at heading stage, seed setting rates of the tested varieties were decreased by 19.48%-91.44% as compared with the control; a relative seed setting rate of more than 80% was recorded in ZLY505 and TY812, suggesting that these two varieties had strong cold resistance; a relative seed setting rate of 70%-80% was recorded in ZLY611, ZLY08, LLY21 and ZLY618, indicating that these four varieties had intermediate cold resistance. Low temperature led to significant decreases in anther length, anther dehiscence, fertile pollen rate, and chlorophyll and soluble protein contents in flag leaves in all tested varieties, however, the decrease magnitudes were lower in the cold resistance varieties than in the cold sensitive varieties. In addition, low temperature caused increases in MAD content, POD activity, NPQ but decreases in CAT activity, Pn, Gs, Ci, Tr, Fv/Fm, ΦpsII and qL in all tested varieties, while SOD activity was increased in the cold resistance varieties but decreased in the cold sensitive varieties by low temperature. Relative seed setting rate under artificial low temperature was significantly related to that under natural low temperature conditions.According to the dead seedling rate and the relative seed setting under low temperature conditions, it was concluded that TY821, ZLY505 and ZLY08 were cold resistance at both seedling and reproductive stages and therefore suitable for using in both early and late seasons with machine transplanting. |
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