| Drought, with no-efficient water, is one of the most serious problems confronted with in the word agriculture. It is estimated that the loss in world agriculture caused by drought is nearly same as that by the other stresses. Therefore, it is increasingly interested in the physiological response and adaptation. In order to elucidate the synergic interactions between the roots and the shoots, genotypic difference and possible mechanism in response to water stress, Azucena, an upland rice cultivar and IR1552, a lowland rice cultivar, were used to investigate the variation of the yield, nutrients contents, root system, photosynthetic and chlorophyll fluorescence parameters, and anti-oxidative enzymes activities with soil, sand or PEG-6000 treatments resulting in the different water stress conditions. The results may provide the theoretical and practical information for understanding of anti-drought mechanism and rice upland culture.The results in soil cultured rice showed that the grain yield per plant, one-thousand-grain weight, setting rate, and total dry matter of the both cultivars were lowered with the dropping of underground water. In comparison with the control rice plant grown in soil surface submerging condition, all above mentioned index in IR1552 is less decrease than those in Azucena. Under the condition of 80cm of underground water level, Azucena was infertile owing to so late development that could not head before winter, while cultivar IR1552 ripened with higher total dry matter. The nitrogen, phosphorous and potassium contents in Azucena were decreased, while those in IR1552 were increased as the lowering of underground water levels. The results in sand cultured rice exhibited the different response to the falling in underground water levels in root parameters. The number of adventitious roots per plant was significantly inhibited at low levels of underground water in Azucena but it was not in IR1552. The root/shoot ratio and maximum root length per plant, and the length, root tip, root surface area and volume of the three longest roots, reached the peak at 25 cm of underground water level in Azucena, but those did the largest at 50 cm of underground water level in IR1552.Our experiment show that, as the days of withholding water progressed, net photosynthetic rate (Pn) became lower, but PS maximum photochemical efficiency (Fv/Fm) kept stable. Although the initial fluorescence(Fo), PS II available photochemical efficiency (F'v/F'm), PS II actual photochemical efficiency and photochemical quenching (qP) declined, the decrease was less in cultivar IR 1552than in cultivar Azucena. Under drought stress, the activities of three antioxidant enzymes (SOD, POD and CAT) increased, but the increase in cultivar IR 1552 was more than in cultivar Azucena. When soil became drier and drier, pigment contents, gas exchange and fluorescence parameters became worse and worse in both cultivars, the activities of three antioxidant enzymes (SOD, POD and CAT) decreased initially, and then increased, while the content of MDA became higher. However, by comparing the cultivar Azucena with IR 1552, the higher activity of the antioxidant emzymes in the latter seemed to removing of greater ROS and lowering the levels of MDA under water deficit stress. Therefore, the biomembrane in IR1552 can be protected from peroxidation. After rewatering for 2d, all parameter in IR1552 recovered to near the level of watering control plant. These results showed that the ability of the antioxidant enzymes system to avoid cellular damages may be an important attribute linked to the drought tolerance in cultivar IR 1552. All above reflects IR1552 a lowland rice cultivar is more resistant to serious drought than Azucena, an upland rice cultivar.After rice plants were treated with various concentrations of PEG (0, 10, 15 and 20% w/v) for 16d, characteristics of gas exchange, chlorophyll fluorescence, MDA content and activities of antioxidant enzymes (SOD, POD and CAT) were measured in the leaves. The results indicated that the reason for pho...
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