| Drought causes yield loss and quality reduction of sugarcane. Drought has become one of the major limiting factors in sugarcane industry. Numerous studies have shown that the drought resistance of sugarcane is a quantitative trait controlled by multiple genes and its genetic background is complicated. Understanding comprehensively the mechanism of drought tolerance in sugarcane is still very limited.Further study of morphological, physiological and molecular mechanisms for drought damage in sugarcane and its responses to drought has important implications for the genetic improvement of sugarcane. In this paper, the weak drought tolerance cultivar ROC 16 and another cultivar ROC22 that with strong drought tolerance used as test material, the anatomical structures of root,stem and leaf, the ultrastructure of root and leaf, photosynthetic physiology of leaf, and drought-related products and the express of theirs relevant regulation genes of leaf in the two varieties of sugarcane under persistent drought stress and re-watering were compared and analyzed in this study.The main results are as follows.1. There was an obvious difference in the organizational structures of the two varieties.①Under various water status,all parameters including the diameter of vascular cylinder and pith in the root, cuticle thickness and the density of vascular bundles in the stem. the thickness of leaf, cuticle, wreath mesophyll tissue and the ratio of the xyiem to the phloem of ROC22 were significantly or very significantly greater than that of ROC 16.② All parameters including the diameter of the root and stem, cortex thickness in the root of ROC22 were significantly or very significantly greater than that of ROC16 under drough; stress.③ The thickness of conducting tissue in the root. the ratio of the xyiem to the phroem in the stem and the thickness of epidermis in the leaf of ROC22 were significantly or ver significantly greater than that of ROC16 under the mid to late drought stress.④During drought stress, the epidermis and the exodermis of the root in ROC16 fell off successively, while that of ROC22 but didn’t fall off, and the root exodermis in ROC22 even differentiated into mechanical organization.Under drought stress, the stem exodermis of ROC22 had stronger differentiation ability of forming lower epidermis than that of ROC 16. ⑤In the process of drought stress, the deformed vessel was observed earlier in stem of ROC 16 and the number was more than that of ROC22, while single-hole vessel and polymerizative vessels of two-type and multi-type were only found in the stem of ROC22.⑥The parenchyma cells of root, stem and leaf were injured more heavily in ROC 16 than that in ROC22 under drought stress. The anatomical structures showed that ROC22 had stronger ability of absorbing and conserving of water. ROC22 also had stronger adaptability of structures changes under drought stress. Therefore, morphological structures were damaged more slightly in ROC22 and it can be recovered soon after re-watering.2. Under drought stress, the damage degree of nucleus, mitochondria and chloroplasts of the root or leaf of ROC16 were obviously heavier than that of ROC22. Both lipid balls and starch grains of chloroplast in ROC22 were induced by drought, while only lipid ball was induced in ROC 16. The numbers of lipid balls and starch grains increased along with the injury degree of chloroplasts and they disintegrated with the degradation of chloroplasts. After re-watering, the overall recovery ability of cells ultrastructures in roots or leaves of ROC 16 was weaker than that of ROC22.3. The content of Ch1a, Ch1b, Ch1 and Car, and chlorophyll fluorescenece parameters including Fm, Fm’, Fv, Fv’,Fv/Fo, Fv/Fm, Fv’/Fm’,φPSⅡ and qP decreased, while NPQ increased under continuous drought stress. The decreasing amplitude of falling inndexes in ROC16 was significantly higher than that in ROC22 in most cases. In the late period of drought stress, the increasing amplitude of NPQ in ROC] 6 was obviously less than in ROC22. After re-watering, the recovery levels of photosynthetic pigments and PSⅡ of ROC22 were significantly better than that of ROC 16. The correlation coefficients of chlorophyll fluorescence and photosynthetic pigment contents in ROC 16 were larger than in ROC22.4. Under various drought stress. proiine content was very significantly lower in ROC16 than that in ROC22. Under the late drought stress, SOD and CAT actives in ROC22 were very significantly greater than that in ROC 16 under the late drought stress. Except δ-OAT and CAT that expressed in the early stage of drought stress, the expression level of P5CS、 δ-OAT、MnSOD and CAT were induced very significantly lower by drought in ROC 16 than in ROC22.5. Because of ROC22 with morphological and structural characteristics of drought tolerance, such as less decrease of the root diameter and leaf thickness under drought stress, ROC22 was able to produce more resistance material and energy for drought resistance. The PSII system injuried more lightly and photosynthetic pigment degradated less in ROC22 because it had stronger ability of the adaptive structure change in the process of drought stress. At the late stress of extreme drought, ROC22 maintained the integrity of nucleus, chloroplasts and other subcellular organisms in certain extent, which should be one of reasons that cell structures and physiological function can be recovered quickly after re-watering. |
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