| Drought is a worldwide problem. Excessive irrigation brings salinisation of soils and pollution of waters. Besides, now water resources are getting more and more scarce, which make it high time that WUE(water use efficiency) was promoted. Since agriculture expends the major portion of the water resources, water-saving agriculture has to be widely practiced to improve WUE in agriculture, which requires the comprehensive knowledge on the adaptation of plants to water deficiency. So, study on the drought stress physiology is highly significant to agriculture. Moreover, drought is a common stress frequently confronting land plants, the adaptation to which is indispensable to their survival and reproduction. Thus, drought stress physiology is one of the important fields in botany.In order to adapt to drought, land plants must has developed some mechanisms to sense and react to drought. In contrast to traditional explanation, researches during past several decades indicate that many plants' root can sense the change in soil humidity, then along xylem transmit chemical signals to leaves regulating their growth and transpiration. ABA is studied most and accepted as the most important root-sourced drought signal. During the course of transmission, ABA signal is regulated, to which sap pH make an important contribution. The results of a few experiments in literature suggest that xylem sap pH be a drought signal.To determine the effect of xylem sap pH on transpiration, several authors have performed transpiration bioassay with detached leaves, only to get contradictory results. Theoretical analysis suggests that the effect of xylem sap pH on transpiration be influenced by the sap composition and the leaf. It was verified by the results of this experiment. The leaves transpiration of one batch of tomato seedlings were sensitive to the pH change in a solution, while that of another batch were not though the solution was identical. The leaves transpiration of identical batch of tomato seedlings were sensitive to the pH change in some solutions but not to that in some other solutions. Therefore, in order to determine whether xylem sap pH can function as soil humidity signal, the transpiration bioassay should be carried out with xylem sap and leaves collected from the same seedling.Root xylem saps were collected from tomato(Lycopersicon esculentum L.) seedling at the stem base with pressure chamber after the shoot were cut off. Then saps'pH were measured and half of each was buffered to specified pH's. Compares were made between the transpiration rates of detached tomato leaflets taking up xylem saps of different pH's. A rise of 0.6-1.4 unit in sap pH, after the adding of KOH to sap, resulted in a drop of more than 14 percent in transpiration. A drop of 0.6-1.0 unit in sap pH, after the adding of HCl, resulted in a drop of more than 7 percent in transpiration. Generally speaking, the adding of Citric acid to sap did not affect transpiration. After the successive adding of KOH and HCl, pH is restored but transpiration is not. It is proposed that change in the concentration of some ion, which coincided with change in pH, affected transpiration too.Some signal, consisting in root xylem sap composition, regulates shoot xylem sap pH and leave apoplastic sap pH. Possibly it is the relative symplast absorption selectivity of anions as a whole and cations as a whole. It is proposed that the pH signal be integrated with the relative selectivity, forming a complete signal. In the end, the function of hydraulic signals is discussed. |
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