| Currently, the impact of drought stress on cotton production has become a issue to be widespreadly concern. Improvement of the cotton drought resistance and water use efficiency has been the focus of researches in crop practices in this major crop. Because drought resistance indices are vared depending on the the materials and growth periods, multiple indices are needed to be combined for accurate evaluation of the plant tolerance to the drought stress in cotton. In this study, four cotton cultivars, including Bt cotton 33B, ZhongR16, ZhongH1717 and Hengmian 3 were selected to be subjected to the drought stress initiated by two different environments, such as indoor hydroponic culture and field with rain protection shed. The stem sap flow characterization under different water conditions, seed germination as well as the physiological and biochemical properties in plants at different development stages were investigated on which the drought resistance indexes to tolerate drought stress were determined. Our results in this study thus provide theoretical basis to direct the water-saving planting and drought resistant breeding in cotton. The main results are as follows:1. The responses on morphological, physiological and biochemical trais in cotton plants to drought stress were investigated using three kinds of cotton cultivars focusing on the seed germination property under different concentrations of PEG-6000 (0,2.5%, 5%,7.5%, and 10%). It was found that along with the intensification of water stress, the seed germination speed and the germination rate were decresed. At the concentrations of 2.5% and 5%, the germination rate was rising rapidly from the third day to the seventh day while at the concentration of 10%, the seed germination of all cultivars were severely inhibited. The radicle and hypocotyl elongations were inhibited by all the PEG-6000 treatments, and they were inhibited intensively under the more intensified water stress conditions. The Variety indexes related to seed germination were significantly and negatively correlated with the water stress degree.2. The content of soluble protein and the antioxidant enzyme activities varied largely among the cotton cultivars. The SOD and POD activities during seed germination were decreased along with the increasing drought stress in ZhongH1717, whereas the CAT activity increased firstly, then it was declined and finally maintain ed to be stable. The SOD and POD activities in ZhongR16 were greatly influenced by the drought stress and they were declined with the increasing PEG concentrations, while the CAT activity was varied slightly, and it was declined firstly and then to be kept staple with the increase of PEG concentrations. The SOD and POD activities in Hengmian3 showed a tendency of rising firstly, then to be declined and rised again with the intensification of the drought stress, while the CAT activity was influenced greatly by drought stress and varied irregularly. These results indicated that under mild drought stress (2.5% of PEG), the inner membrane system of the cotton germination seeds can repair itself, causing leass impact on the germination rate. In contrast, udenr the concentration of PEG-6000 of 7.5%, each germination index was significantly lower than the control and other PEG treatments, possibly attributing to the damage to inner membrane system of germinating seeds by the high concentration of PEG Thus, the drought stress inhibits largely the germination of cotton seeds.3. The hormone contents and the hormone balance in the germinating seeds were varied at different germination stages under the different PEG concentration treatments. At the third day after PEG treatment, the drought stress had slight and similar impact on the content of ZR and GA3 in Hengmian3 and ZhongH1717. With the increase of PEG concentration, Hengmian 3 produced more ABA than ZhongH1717 on which to inhibit the seed germination progression. At concentration 2.5% and 5% of PEG, the content of IAA was greatly influenced at the seventh day after PEG treatments. At the third day, the ratios of ZR/ABA, GA3/ABA, and IAA/ABA exhibited similar behavior patterns, indicating that ABA might have similar effect on ZR, GA3 and IAA during the hormone balancing interaction at the third day. At the seventh day after the treatment, the ratios of GA3/ABA and (ZR+GA3)/ABA showed almost the same trend each other, which indicates that GA3 might share a close relationship with ABA during the hormone balancing interaction process.4. The morphological, physiological and biochemical traits in the germinating seeds were determined nnder different concentrations of PEG (5% and 10%) treatments. With increase of the concentrations and the prolonged stress progression, the leaf wilting index was increased. The leaves were adjusted by its own ability to maintain normal growth under 5% of PEG6000 treatment, while almost all of the leaves were wilted under 10% and 15% concentrations of PEG after 72h treatments. Among the cultivasr, H1717 was the lowest. PEG-6000 stress also resulted in lower leaf water potential, chlorophyll content, and leaf Pn, Tr, Cond, which reduced the PS Ⅱ energy conversion efficiency and inhibited the energy scattered photochemical activity. Along with the PEG-6000 concentration increase, the photosynthetic performance parameters decreased swiftly.5. The contents of proline and soluble protein played an important role to tolerate the damage caused by the drought stress. These parameters were significantly or very significantly correlated with the MDA content in the antioxidant system and were also significantly correlated with the soluble protein content and the POD activity (5% and 10% of PEG) and the SOD activity (10% of PEG). Under the 5% of PEG-6000 treatment, the MDA accumulation were strongly correlated withthe photosynthetic parameters. Under the 10% of PEG-6000 treatment, the MDA content was correlated weakly with the related indices. It was found that the stress intensities influenced the antioxidant system correctly.6. The ABA content in leaves and roots acts as a critical regulator in plants. The IAA in roots at the 5% PEG-6000 treatment was significantly associated with the contents of root ZR and GA. Under 10% of the PEG-6000 treatment, the correlation among them were weakened. In addition, no significant correlations were found between hormone contents in leaves under all of the PEG-6000 treatments. The hormone contents in roots were changed with a wide range under the 5% of PEG treatments, but they were not changed largely in leaves under the 10% of PEG treatment. Therefore, roots are much more sensitive to drought stress than the leaves. Continued soil drought stress accelerated the cotton growing process in which leaf size and main stem water potential decreased extensively. Hengmian3 stepped into the reproductive growth stage earlier and the leaf size decreased significantly, whereas ZhongH1717 and ZhongR16 were shown to be similar in response to the drought stress. Under the long-term drought stress, ZhongH1717 exhibited a better recovery capability in the leaf water potential.7. The drought tolerant varieties can induce the expression of genes rapidly in order to accommodate the drought stress. The results in this study demonstrated that under long-term drought stress, the soluble protein content increased significantly, although the SOD and POD activities were increased. The protective enzyme system works in removing the free radicals that elevate the MDA content and the membrane permeability, causing the membrane system to be damaged. In addition, under the drought stress conditions, the MDA content and the SOD activity were significantly or very significantly correlated with the drought tolerance indices. Considering the leaf water potential and various physiological and biochemical indices, under drought treatments, the Cond and the POD activity had shown to be impacted on Pn, the soluble protein and the MDA content. It was observed that the continued drought led to the increase of non-stomatal limitation photosynthesis. The results showed that the primary metabolic processes (photosynthesis) and the secondary metabolism (osmotic adjustment and protective enzymes) were closely related, but different physiological and biochemical indices contributed differently to the toleance of drought in which the soluble protein content, POD acitivity, and the MDA content play large roles.8. Drought induced declined in cotton leaf photosynthetic rate and increased in respiration, which reduced the biomass accumulation, then directly affected the yield and the fiber quality by reducing the plant height, number of cotton bolls per plant and weight of cotton bolls. According to the cotton yield and drought-tolerant indices of various elements among the 3 cultivars, it can be concluded that ZhongH1717 has the strongest drought tolerance. Through the strict control of soil moisture in outdoor water-proof canopy combined with the plant physiology monitoring system, the cotton’s specific eco-environment in the middle growth period and the diurnal variation of main stem velocity sap flow were manitored. It was found that the diurnal variation rhythm of main stem petiole sap flow velocity changed under different water conditions. The peak period under high water condition appeared earlier than that under the middle and low water conditions, and the main stem petiole sap flow velocity appeared to have lunch break around 13:00 pm. Moreover, the velocity of stem sap flow was influenced by the environment factors under different water supply conditions. Under the high water conditions, RHS and ST had greater impacts on stem sap flow velocity, and ST had a significant negative correlation with the velocity of stem sap flow. RHS was relative to AT and ST, which were both significantly and negatively correlated each other. Under the middle water conditions, AT was significantly and negatively correlated with the velocity of stem sap flow while RHS was significantly and positively correlated with the velocity. Moreover, RHS was very significantly and negatively correlated with AT and ST while RHS was significantly and negatively correlated with LT. Under the low water conditions, only TIR had an impact on the velocity of main stem sap flow and they were significantly and negatively related each other. RHS had the same relationship with AT, ST and LT when the under middle water conditions. |
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