| In this research, soybean of wheat/maize/soybean relay-cropping system and sweet potato of wheat/corn/sweet potato relay-cropping system were chosen as objects under the southwestern hilly dry farms cropping systems. Pot experiments were used to simulate the natural drought conditions. The impacts of drought time on morphology, physiology and yield of soybean and sweet potato were studied in this experiment, together with the compensative growth of two crops after rewatered. The objective is to provide the theoretical basis for water-saving cultivation and the application of the two cropping systems, wheat/com/sweet potato and wheat/maize/soybean in Sichuan. The main results are as follows:1. Drought stress significantly affected the soil moisture of soybean and sweet potato, and both of them reduced fastest in the first 7 days after drought treatments. Among all treatments, the reduction of soil moisture of sweet potato was slighter than that of soybean, and the difference was about five to ten percent.2. Under drought stress, the green leaf and leaf area of soybean and sweet potato reduced within the drought time extending. The reduction of green leaves of sweet potato was 32.40%at 7 days after drought, which was 21.59%greater than that of soybean,10.81%. Thereafter, the reduction of soybean was 17%to 28%greater than that of sweet potato. In different drought treatments, the reduction about leaf area of sweet potato was greater than that of soybean and the difference reached 11%to 28%. But after rewatering, the leaf area of soybean and sweet potato was increased both.3. After the soybean and sweet potato were suffered drought stress, the relative conductivity, the content of malondialdehyde (MDA), proline and soluble protein in leaf changed greatly, showing constant increase within drought time. In particular, while compared with normal water supply, the increment of relative conductivity, MDA and /soluble protein in soybean leaf was higher than that of sweet potato at the same drought time. The amounts of relative conductivity, MDA and soluble protein reached the peak at 21 days after drought, while peaked at 28 days after drought. And the peaks of these three indexes of soybean were lower than that of sweet potato. These three indexes reduced and reverted to normal after rewatered.4. Drought stress markedly influenced the photosynthesis rate (Pn),stomatal conductance (Gs), transpiration rate (Tr) and water use efficiency. When drought for 28 days, all of these indexes were lower than that of normal water supply and the reduction in soybean was greater than that of sweet potato. Rewatering after drought for 7 to 14 days, each photosynthetic index of soybean reverted to be equivalent to, or even higher than normal level, which showed significant photosynthetic compensation effect. The increment of Pn, Gs, intercellular carbon dioxide concentration, Tr and water use efficiency in soybean was greater than that of sweet potato.5. The initial of fluorescence (Fo), PSⅡsystem andΦPSⅡsystem of soybean and sweet potato were also affecred by drought stress. Fo increased with the drought time, while the activity of PSⅡsystem andΦPSⅡdecreased. At 7th,14th,21st,28th after drought, the Fo of soybean increased by 19.36%,143.44%,121.06%arid 33.58%, respectively, and reached maximum when drought for 21 days. And the Fo of sweet potato increased lower than that of soybean. During the first 7 day, the actual photochemical efficiency of PSⅡin leaf decreased, and the decrement of sweet potato was greater than that of soybean.7th day after drought, the result inversed. The decrement of OPSII system activity in soybean was greater than that of sweet potato.6. The drought stress affected various organs dry weight as well. The dry weight of roots, stems and leaves was lower than that of normal water supply in Soybean. Within drought stress for 7 days, the reduction of root dry weight of the sweet potato was about 28.66%, while that of soybean was 13.82%, showed that reduction of sweet potato was greater than soybean. In the other treatmeants, the dry weitht of stems and leaves of sweet potato reduced more quickly than soybean. After rewatering after drought for 7 days, the dry weitht of roots and leaves in soybean increased more than that of sweet potato, howere, the change had no manifest regular pattern in other treatments.7. The yield of soybean and sweet potato was influenced by the drought strees at some extent. The results showed that short and medium term drought increased the yield of soybean while the yield of sweet potato decreased in all off the drought treatments. In the treatment of rewatering after drought for 7 days, the pod number per plant, the grain number per plant, hundred grain weight, and the grain weight increased by 23.14%,3.29%, 6.39%and 35.32%, respectively, comparing with control, but the tuber number and weight per plant were reduced by 19.19%and 3.14%comparing with control, respectively.the average of tuber fresh weight was increased by 19.92%. In the treatment of rewatering after drought for 14 days, the pod number per plant, the grain number per plant and the grain weight of soybean increased 10.97%,14.31%and 22.41%, while the hundred grain weight was reduced by 3.5%. Meanwhile the the tuber number and weight per plant were reduced by 17.17%and 5.80%, respectively and the average of tuber weight was increased by 13.77%. In the treatment of rewatering after drought for 21 days, the pod number per plant, hundred grain weight and the grain number per plant of soybean decreased 4.79%, 10.10%and 0.28%, oppositely, the grain number per pod increased 16.52%. The tuber number and weight per plant and the average of tuber weight was reduced by 9.60%, 10.78%and 1.39%. In the treatment of rewatering after drought for 28 days, the soybean died, but the sweet potato still alived, and the tuber number and weight per plant and the average of tuber weight was reduced by 6.06%,15.26%and 20.56%, respectively.
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