Different Responses To Drought, High Temperature Between Drought Resistant And High Yield Wheat Cultivars

Wheat is often affected by environmental stresses.Great progress has made in the research of the adaptation of wheat to the abiotic stress conditions. However, stress conditions are traditionally studied by applying a single source of stress such as drought, salt, or heat, and fewer studies have considered the effects of combined sources of stress, which can not reflect the actual situation in the field. Because the abiotic stress condition in the field is complex, plants are often subjected to a combination of various abiotic stress conditions, e.g. low temperature and drought stresses during the booting stage of wheat development in spring, high temperature and drought stress during the flowering and filling stage in summer. The response of plants to combined stress is different from that of exposure to a single source of stress. Besides, the wheat cultivars used in research before are majorly the traditional cultivars, which is quite different from the current cultivars in both genetic and physiological properties. Shannong 16 is a new wheat cultivar (Registration Code: No. 2007047 shandong) with good adaptability to drought stress; Wei mai 8 is another wheat current cultivar with high yield characteristic. In the present study, we investigate the different responses of different wheat cultivarss to drought, high temperature stress and their combination, and analyze the physiological mechanisms of them to different stress conditions. The preparations of experimental treatments in this study were conducted by two ways: one is in the laboratory and another is in the field. In the laboratory experiments, the wheat seedlings were planted in pot, and three treatments were made, which including single drought stress (D), single high temperature (T) and their combination (D+T). While, in the field expriments, the investigations were conducted in the field under drought stress with high temperature and high light stresses occurred simultaneously under natural condition. Drought stress was made by controlling irrigation. Photosynthetic gas exchange, water status, and lipid peroxidation of the wheat leaves were examined in both experiments. The main results and conclusion were as follows:Wheat seedlings were cultivated by the routine method until the 3 rd leaf was expanded fully. Then, the seedlings were treated under single drought (D), high temperature (T) and their combination (D+T) respectively. Drought was imposed by PEG-6000 (20%); high temperature stress was applied by raising the temperature in artificial chamber at 42oC / 35oC (day time / night) for 24 hours. A combination of drought and high temperature stress was performed by subjecting the drought-stressed plants with PEG (20%) to a high temperature in artificial chamber at 42oC / 35oC (day time / night) at the same. The results were as follows:Different responses were observed between the two varieties at late growth stage under drought, high temperature and the combination of them. Growth state of wheat plants under the combination of drought and high temperature stress were obviously worse than those under single drought or high temperature stress. The growth state of shannong16 was well and has stronger adaptability than weimai8 under stress environment.Chlorophyll content and net photosynthesis rates (Pn) were decreased significantly in both wheat lines under both drought and heat stress, but the degree in shannong16 was significantly lower than that in weimai8.All stresses resulted in a significant (P<0.05) decrease in RWC of wheat leaves, but high temperature had hardly effect on it. Shannong16 can maintain well water status compared to weimai8, which may be related to the accumulation of compatible solutes, such as proline and soluble sugar. because all the compatible solutes can decrease osmotic potential of the cell and enhance water absorbing, and consequently maintain a better water status in wheat leaves.The MDA contents of the leaves increased in both weimai8 and shannong16 under adverse environmental stress, at the same time, the rate of reactive oxygen species (ROS) production increased obviously, but the oxidative damage in shannong16 was less serious than in weimai8. The results sugested that shannong16 has strong antioxidant ability. Under adverse stress, the activities of several main antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX), were higher relatively in shannong16 then in weimai8, it can clear away ROS, so that alleviate the oxidative damage in the cells.Drought, high temperature and the combination caused the decline of H~+-ATPase activity. However, the activity of H~+-ATPase in shannong16 was higher than in weimai8 under stress conditions.Stress conditions can induce the synthesis of endogenesis ABA. In this study, the ABA content was not increased under drought stress, but ABA was was induced in large amounts/in quantity under stress combination.Weimai 8 and Shannong16 were planted in field plot by 2 m×2m, setting control and dry treatments with 4 replications. Two varieties were planted in field at random, planting eight rows in each plot. Two water treatments were made, one was control (CK) with the normal irrigation, another was water stress by controlling watering. Some parameters in morphological and physiological were detected. The main results are as follow:Different responses to drought stress were observed between the two cultivars at late growth stage. Wheat flag leaves of shannong16 appear curl under drought stress, while weimai8 curl less. But the leaves of weimai 8 turn yellow, which indicated the damaged pigment phenotype.The photosynthesis and the maximum photochemical efficiency of PSII (Fv/Fm) and actual photochemical efficiency (ФPSII) of the two cultivars were significantly decreased, but shannong16 had a lesser extent. Meanwhile, drought stress led to the water lose, photosynthesis decrease. But the changes in shannong16 were lesser than that in weimai8.Drought stress deteriorated the water status of the leaves in two wheat cultivars, but shannong16 can synthesize more osmolytes under water stress condition, which keep higher RWC of flag leaves. This result was consistent with that at seedling stage.The levels of protein carbonylation and MDA were markabely increased under the drought stress. In the beginning of drought stress, several antioxidant enzyme activities in wheat leaves were increased significantly, but with stress increasing, the activities odf them began to decline. In this process, the antioxidant enzyme activity in shannong16 was always higher than that in Weimai8, but the extent of oxidative damage in shannong 16 was less than that in weimai8 .Drought stress declined of the yield of the two cultivars, but shannong16 had strong tillering ability and produced more wheat spikes than weimai8. Under drought stress, the decline of 1000-grain weight in shannong16 was less than that in weimai8, therefore, shannong16 maintained higher yield than weimai8 under water stress condition.To sum up, the drought-resistant wheat cultivar shannong16 maintained a higher net photosynthetic rate, which may be the main reason that it had a high yield under stress conditions. We suggested the reason for shannong16 got a higher photosynthetic rate than weimai8 under adverse conditions was as follow: (1) Shannong16 can quickly enough to get the signal perceived stress, synthesized some signal substances in plants, such as ABA, through signal transduction pathways, the plants can regulate the mobilization and the expression of genes contributed to drought resistance. (2) Shannong16 can synthesize more osmotic adjustment under stress conditions than weimai8, such as soluble sugar, proline and so on, which adjusted their osmotic potential inside the cells and maintained the water. (3) Shannong16 has stronger antioxidant enzyme system than weimai8, which can remove reactive oxygen species timely to reduce oxidative damage to the plants. (4) Under the environmental stress, shannong16 can protect themselves from the carbonylation of protein damage better than weimai8, thereby maintaining protein activity to ensure the normal metabolism.