| Water shortage is one of the major limiting factors of agricultural production in arid and semiarid area, the issues that how to improve the water use efficiency (WUE) of plant itself is being pay more attention. The process of water transport influence directly on the plant water status, and affected on plant growth and development. Therefore, it is important to understand the water transport in plant under water stress for enhancing the WUE and exploring the approaches of biological water saving. The experiment was conducted under water stress iri aquatic and soil culture, and the issue that the tomato how to adjust its water balance by regulating the root and shoot relationship after grafting was investigated in this study. The main results as follows:1. The hydraulic conductivity (Lp) was affected remarkable by the water deficit, Lp dropped by 80% in aquatic culture (-0.3MPa), while dropped 30-50% in soil culture (soil water content was 45 percent of the soil maximum water-holding capacity), the change was related with the culturing medium and severity of water stress.2. Under wet conditions, HgCl2(50μM) treatment decreased Lp by 50%, and recovered by 30-50% after transfer to 2-mercaptoethanol (5mM). Under water stress, Lp was neither inhibited by HgCl2 and nor reversed by 2-mercaptoethanol. It is suggested that both apoplastic path and cell-to-cell path played an important role in water transport under wet conditions, but the role of apoplastic path was predominated and the aquaporins of Hg-sensitive was insignificant under water stress.3. The shoot and roots were affected significantly by water deficit. The shoot was decreased by 50% and roots was increased by 33.3% under water stress in aquatic culture; and shoot was decreased by 20% and roots area was increased by 33.3% under water stress in soil culture. The decreased shoot resulting in transpiration reducing and the increased roots causing water uptake increasing could be compensated the water uptake deficiency owing to the reduction of Lp. After rewatering, Lp recovered to 30% or more of its initial value, the amount and length of new roots was increased obviously, root weight was increased by 30%, the increasing Lp was resulted from many new roots, not from Lp recover of old roots.4. The shoot/root ratio of tomato was changed significantly after grafting. Water input and output became inconsistent because of the relative contrasting size of roots and shoot. The growth of tomato firstly regulated the size of shoot and root to optimize the root and shoot relationship, and make them acclimatized by adjusting the reallocation of photosynthesis carbon assimilation. Under water stress conditions, the root of the tomato grafted with large canopy and small root increased and its canopy decreased, while the root of the tomato grafted with small canopy and large root decreased and its canopy increased in biomass. The shoot and root could compensated slowly water uptake. Tomato has a capacity of regulating the size of canopy and root to adjust its water status.5. The photosynthesis rate of tomato was declined under water stress, and the decrease of original tomatoes was more obviously than that of grafted tomatoes. The intrinsic andinstantaneous water use efficiencies of tomato grafted are higher than that of original tomatoes under both wet and stress conditions. It is concluded that crop WUE could be improved by moderate water stress treatment and by regulating the shoot - root relationship.
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