| Sweet potato (pomoea batatas Lam), with the character of rich in starch and high-yield potential, is the world's seventh food crop and important feed and energy resource. Therefore, to cope with these global crises over food and energy supplies as well as environmental problems, it is urgently required to develop new industrial crop varieties. Enhancing crop drought tolerance, improving crop yield and water use efficiency is a big challenge for agricultural industry. Increasingly global water deficit crisis has severely limited the growth and yield of sweet potato. Therefore, better understanding the adaptation mechanism of sweet potato to water stress and developing new varieties of drought tolerance or improving existing by genetic engineering technology have important theoretical and practical significance on the increased food supply and improve the yield potential of sweet potato. Based on this, overexpressing of Cu/Zn superoxide dismutase (Cu/Zn SOD) and ascorbate peroxidase (APX) gene of sweet potato (TS) and non-transgenic potato (NS) were used as the materials and were comparatively analyzed the collaborative response mechanism of photosynthetic parameters, antioxidant defense system and the root hydraulic conductivity under different degrees of drought stress and rewatering conditions, which was mediated by polyethylene 2 alcohol (PEG 6000). Subsequently, potted experiment was conducted to evaluate the response of growth, yield, and harvest index and water use efficiency in transgenic and non-transgenic sweet potato under soil drought and water conditions on potato. The major results are as follows:1. Under PEG-mediated water stress conditions, the chlorophyll content (Chl), photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and maximum photochemical yield of PSII (Fv / Fm) of leaves of sweet potato were significantly lower, and with the stress time and sustained decrease. During the first 0-24 h of water stress, photosynthetic decline of sweet potato is mainly due to stomatal limitation inducing by decreased stomatal conductance; More than 24 h, the non-stomatal limitation as the main factors. The decrease of Pn, Tr, Gs in transgenic lines in all stress treatments is smaller than that in non-transgenic plants, particularly during the shorter stress, the difference of TS and NS was significant. Rewatering from 12 and 24 h water stress, Pn, Tr, Gs values with rehydration time and continues to rise. After 72 h recovery, transgenic plants can basically return to the control level (P> 0.05), whereas NS plants was not. Rewatering from 48 and 72 h water stress, the photosynthetic parameters both in transgenic and non-transgenic lines recovered slowly, and there was no significant difference between of TS NS. The results suggested that introducing antioxidant enzyme gene to sweet potato not only enhance TS plants'drought tolerance, but also improve their repair capacity.2. Under water stress conditions, leaf plasma membrane of transgenic sweet potato was much less injured than non-transgenic plants: increased MDA content, relative membrane permeability and superoxide anion content was significantly lower than the rate of NS plants, and decreased rapidly once were released from drought stress conditions, indicating that overexpression of antioxidant enzyme gene in sweet potato enhance simultaneously the capacity of oxidative stress tolerance and recovery. Under water stress, SOD and APX activity in leaves of sweet potato improved greatly, and the CAT activity was inhibited. After rehydration, SOD and APX activity showed a double bimodal trend: increased first, then declined, and increased again, while CAT showed a gradual increase trend during rewatering period. In the process of stress and rewatering, antioxidant enzyme activities in transgenic potato were significantly higher than non- transgenic plants, suggesting that TS plants could increase their drought tolerance and repair capacity by enhancing expression of SOD and APX activity. In addition, 24 h or longer water stress treatment induced "stress protein" expression in sweet potato, such as soluble protein, which involved in the regulation of cell osmotic potential of metabolic enzymes and the protective role of membrane systems. TS plants showed higher soluble protein content than NS.3. Analysis the response of sweet potato seedling root water conductivity and the antioxidant enzyme system on water stress showed that water stress significantly affected the root metabolic activity, reduced root water uptake capacity. In the early stage of water stress, hydraulic conductivity of transgenic potato decrease was significantly lower than non-transgenic plants, but after 48 and 72 h water stress, TS and NS didn't show a difference in root hydraulic conductivity: both of them were decreased to near zero. This may be due to, in a certain water stress degree, transgenic plants seedlings reduce the damage from oxidative stress by significantly enhancing the expression of antioxidant enzymes such as SOD, APX and CAT, especially SOD, APX activity, and increased osmolyte content (soluble protein). Therefore the root of the physiological function of the transgenic plants was effective protection; maintained root water uptake function. Moreover, they could were also be restored soon once was released from water stress condition. However, under severe water stress, root structure and function of both TS and NS plants were injured severely: root hydraulic conductivity rates declined to near zero and were very difficult to recovery after rehydration.4. Through potted experiment, further analysis the physiological response in transgenic and non-transgenic sweet potato under natural drought conditions. Similar as to laboratory test results, soil drought damaged to the leaves plasma membrane of sweet potato and the extent of damage is proportional to the degree of drought stress, however, the injury level of the plasma membrane in TS was lighter than NS under the water stress conditions. The response of SOD, APX and CAT antioxidant enzyme activities on water stress condition also was consistent to lab test results: was induced by water stress. However, several enzyme activities showed a difference response on different drought intensity and rewatering period. SOD activity was sustained increased with the increasing of drought stress, while APX and CAT were induced in moderate drought and then restrained by severe drought conditions. After rehydration, APX and CAT activities first increased first and then gradually decreased with the rehydration time, while SOD activities was gradually decreased after rewatering. In addition, the activity of antioxidant enzymes of in TS plants were higher than that in NS, under both of two drought stress treatments and succedent recovery period. Soil drought reduced main stem length, number of branches in sweet potato, but the length stem, branches and green leaves in TS were still higher than NS, indicating that TS can reduce the damage by drought stress to maintain the stability of the internal physiological metabolic activity, so that less restrictions on plant growth.5. Under well water conditions, the biomass and tuber yield didn't show observable difference between transgenic and non-transgenic sweet potato. Compared to well water control, moderate stress reduced aboveground biomass of TS and NS by 30.0 and 38.9%, respectively, while were down by 38.2 and 47.8% under severe drought respectively. Although the reduction in TS was less than NS, the difference was not significant. Whereas, the increase of root biomass in TS was significantly higher than rate of NS plants,under both of moderate and severe drought conditions. Soil drought treatment significantly restrained the tuber yield development, practically under sever drought condition, even some plant roots can not form a tuber enlargement. This may due to, transgenic sweet potato distributed more photosynthetic products to the roots and crown under drought stress, as a result, the tuber yield decrease was steeper. A certain degree of drought contributed to the improvement of water use efficiency of sweet potato, but too severe drought, reduced WUE. Compared to NS, TS plants showed superiority in the biological WUE, but production WUE was not. |
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