| China is the relatively poor country in terms of the fresh water resource, where the fresh water per head doesn’t reach a quarter of the world average level. It seems that the fresh water is more limited for its nonuniform distribution in time and space and being seriously polluted. On one hand, water is indispensable to the existence and growth of trees. And on the other hand, the water consumption capacity of trees may become an important factor that affects the sustainable supply of the fresh water in regions that lack water. In recent years, under the background of global climate change, the frequency, scope and duration time of drought all present an up trend. The losses in agriculture and forestry production caused by drought exceed that caused by other natural disasters.Eucalyptus grandis possesses many bright features such as fast growing, high yield, good adaptability and so on, which in southern China is one of the most preferred timber tree species in the construction of short-cycle plantations for industrial use, and plays an important role in meeting people’s demand for timber and promoting local economy development. However, whether the water consumption capacity of E. grandis is strong, or whether its substantial spread impacts local water balance arouses much attention. Meanwhile, under the background of frequent appearance of drought, the drought tolerance of E. grandis is directly associated with its introduction and management. Therefore, researches involving the water consumption characteristics and drought tolerance of E. grandis, and even how to strengthen its drought tolerance own profound significance in reality.Pot experiments were conducted to study the water consumption characteristics and drought tolerance of E. grandis, and the effects of nitrogen application on the drought tolerance of which from the following four aspects:(1) The comparison study on the growth and water consumption characteristics between the saplings of E. grandis and another two fast-growing tree species (bamboo willow (Salix spp.) and Alnus cremastogyne) grown under different soil water conditions,(2) The growth and physiology responses of E. grandis and A. cremastogyne saplings supplied ample water to continuous drought stress,(3) The physiology responses of E. grandis saplings grown under different soil water conditions to continuous drought stress, and (4) The effects of nitrogen application on drought tolerance of E. grandis saplings. The aim of this study is to provide some theoretic reference and technological support for the introduction of E. grandis and the water and nutrient management of its plantations. The main results are as follows,1. Under the same condition, E. grandis had relatively higher daily water consumption and total water consumption compared with bamboo willow and A. cremastogyne. Under ample water supply (Wi), the daily water consumption of E. grandis was1.82times (cloudy day) or2.06times (sunny day) that of bamboo willow, and8.11times (cloudy day) or13.35times (sunny day) that of A. cremastogyne. Under moderate drought (W2), the daily water consumption of E. grandis was4.24times (cloudy day) or6.29times (sunny day) that of bamboo willow, and11.87times (cloudy day) or16.94times (sunny day) that of A. cremastogyne. Under serious drought (W3), the daily water consumption of E. grandis was2.95times (cloudy day) or3.22times (sunny day) that of bamboo willow, and4.27times (cloudy) or5.03times (sunny day) that of A. cremastogyne. In fact, the water consumption of E. grandis was not only higher, but also more difficult to be inhibited by moderate drought. That was closely correlated with its larger total leaf area, stronger photosynthetic capacity and faster growth, even in moderate drought. So we had better to develop E. grandis rationally in areas where ample and uniform precipitation is seen, otherwise, developing E. grandis extensively is not suggested in case that local water balance is impacted greatly.2. E. grandis preferred the strategy of leaf wilt or curl so that the higher available photosynthetic area and faster growth could be maintained during drought, however, which might lead to the faster descent of its soil water content. From the responses of its osmolytes (free proline (Pro) and soluble sugar (SS), etc.), reactive oxygen species (hydrogen peroxide (H2O2), etc.) and malondialdehyde (MDA), it suffered more serious damage caused by reactive oxygen species in continuous drought stress compared with A. cremastogyne. While, A. cremastogyne decreased water consumption and provided the last emerged leaves enough water by defoliation. It was likely that A. cremastogyne would recover more quickly when drought was relieved or eased. This study also showed that the soil volumetric water content should not be lower than10%in case that E. grandis saplings were seriously impacted by drought. Additionally, in regions that own sufficient water resource and have no long-period continuous drought, it’s wiser to plant E. grandis for larger yield, while in regions where continuous drought is often seen, A. cremastogyne is more likely to be a suitable species.3. E. grandis grown in the situation that was short of water for a long period were less thirstier for water compared with those grown in ample water condition, probably because they were adaptable to water deficit. During the continuous drought, they were more active to initiate the mechanisms that help to decrease water loss and relieve oxidative damage, such as rising superoxide dismutase (SOD) activity, increasing the content of ascorbic acid (AsA), carotenoid (Car) and free proline (Pro), and regulating the stomas to reduce stomatal conductance (Gs) and transpiration rate (Tr). Therefore, the content of H2O2and MDA in their leaves increased relatively slower in continuous drought, indicating they suffered less oxidative damage. Maybe we could give a consideration to use moderate drought to make the saplings of E. grandis adaptable to water deficit for a period, so that its adaptability to drought could be strengthened after afforestation, or to reduce water supply of its young plantation moderately to help the saplings through the continuous drought.4. Nitrogen application was able to improve the water status of E. grandis (e.g., rising the leaf relative water content (LRWC) and leaf water potential (LWp)), stimulated its metabolism (e.g., enhancing the net photosynthetic rate (Pn)), avianized the wax covered on the leaf surface, promoted gas exchange, and eased the moderate drought stress it suffered. But it was for these reasons, E. grandis applicated with nitrogen in continuous drought lacked water more easily. In addition that the dry matter allocated to the root decreased accompanied with which allocated to the branch and stem increased, the burden on its root was heavier. As a result, nitrogen application made E. grandis suffer more serious oxidative damage, and the photosynthetic capacity of which decreased more sharply during drought. Obviously, it wouldn’t be conducive to its growth. Thus, nitrogen application to the young plantation of E. grandis is not suggested before the continuous drought, only if it could receive sufficient water by irrigation or other ways. |
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