| In drought stress conditions, the root of plant senses the drought and compounds some kind of combination(considered to be ABA usually)which reach the aboveground of plant through the woody part and adjust the action of leaves, such as restraining the stomatal conductance, reducing transpiration and water missing of leaves before the water condition changes ,called non-hydraulic root-sourced signal. When the drought stress condition became worse and the soil water became less, the root can hardly absorb the water and the water condition of leaves is deficient, the stomatal conductance continues decreasing and the leaves became wither, called hydraulic signal. The appearance of non-hydraulic root-sourced signal and hydraulic signal were based on the fall of stomatal conductance (the water condition of leaves not changed yet) and water potential of leaves separately. This paper is conducted to study the accumulation of ABA , Proline and SOD during the entire process of drought stress, especially from the non-hydraulic root-sourced signal to hydraulic signal under two types of drought dress caused by PEG and natural evaporation separately.There are three treatments, including control(CK), natural drought (ND), drought stress by PEG(PEG). Measuring the stomatal conductance and water potential of leaves to determine the period from non-hydraulic root-sourced signal to hydraulic signal, measuring the accumulation of ABA, Proline and SOD during the entire process of drought stress, especially from the non-hydraulic root-sourced signal to hydraulic signal under two different types of water stress, the result showed that:1 The stomatal conductance keeps decreasing under two different types of water stress. It decreases obviously (P<0.01) when soil water relative content is still high (75%) under PEG treatment, earlier than ND treatment. The non-hydraulic root-sourced signal appear earlier under PEG treatment, thought it is because of the decrease suddenly of soil water potential caused by PEG.2 The water potential of leaves keeps decreasing under two different types of water stress. It decreases obviously (P<0.01) when soil water potential is—1.14MPa under ND treatment, earlier than PEG treatment(The water potential of leaves decreases obviously (P<0.01) when soil water potential is—1.27MPa), hydraulic signal appear earlier under ND treatment, it maybe related to the forewarning effect caused by the earlier appearance of non-hydraulic root-sourced signal under PEG treatment.3 The content of ABA increases first, then decreases under two different types of water stress. It increases obviously before the appearance of non-hydraulic root-sourced signal and decreases slowly after the appearance of hydraulic signal. It offers the evidence for ABA to be the carrier of the non-hydraulic root-sourced signal, and it may lose the effect in the upper period of drought stress. The content of ABA increases obviously (P<0.01) when soil water potential decline to -0.32MPa under PEG treatment and -0.49MPa under ND treatment. The soil water potential is higher when the content of ABA increases obviously under PEG treatment.4 The content of Proline begin increasing after the appearance of non-hydraulic root-sourced signal under the two types of drought stress, and increases slowly and keeps steady in upper period. The content of Proline increases obviously (P<0.01) when soil water potential decline to -0.49MPa under two treatments, in the beginning, the speed of increasing is quicker under ND treatment, then the speed of increasing is quicker and accumulation is higher under PEG treatment, it shows the strengthening of resistance to water stress caused by additional PEG of low concentration.5 The SOD activities increases first, then decreases under the two types of drought stress, it begins increasing quickly after the appearance of the non-hydraulic root-sourced signal, then decreases quickly after the appearance of the hydraulic signal, it shows that the SOD has effect in the beginning of the period. the SOD activity increases obviously (P<0.01) when soil water potential decline to -0.49MPa under PEG treatment, earlier than it under ND treatment(increases obviously (P< 0.01) when soil water potential decline to -0.63MPa), It has relationship with the earlier decreasing of soil water potential under PEG treatment, and it is good for the resistance to water stress.Result: under PEG treatment, the appearance of non-hydraulic root-sourced signal is earlier, and the appearance of hydraulic signal is later, then the period from non-hydraulic root-sourced signal to hydraulic signal is longer, that the root-sourced signal threshold ranges is wider, this may has relationship with the forewarning effect in the beginning of this period. When the root feel the change of the soil water potential, the content of ABA in leaves begins to accumulate, the stomatal conductance decreases, resisting the water stress through reducing transpiration, then the activity of SOD increases to protect cells through sweeping ROS, and the content of Proline increases to keep the balance of water, the Lathyrus sativus protects itself and resist the water stress through a lot of ways.
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