植物抗旱性机制包括耐旱性和避旱性.耐旱性的特征主要是木质部易栓塞性和膨压损失点,而避旱性的特征则是叶片脱落和落叶性.虽然这些机制可以权衡物种连续性,但对于干旱引发的叶片脱落和持续时间的变化如何影响木质部和叶片耐受性之间的关系却知之甚少.在本研究中,我们研究两种具有不同叶片脱落叶行为的橡树(栎属)的耐旱性程度差异.我们预测了Quercus deserticola在旱季结束时落叶(晚落叶)而面临更大的空化风险,导致其比Q.laeta更耐旱,对水分的利用也更加保守.Q.laeta只在旱季中期很短的时间内落叶(短落叶).本研究在墨西哥中部进行,两种橡树各具有单个种群,它们之间相距1.58公里,海拔差为191米.Q.deserticola(晚落叶)多出现在下坡,而Q.laeta(短落叶)多出现在沿坡度海拔较高的地方.我们评估了样地内茎干的水分传导率的季节差异(雨季和旱季),并检测了两个物种间木质部对空化的脆弱性、叶片水分利用和叶片膨松损失点的差异.两种橡树在耐旱性状方面没有显著差异,主要包括木质部易栓塞性、叶片膨松损失点和气孔导度.然而,两种植物在旱季都有不同的表现:短落叶植物对木质部功能的负面影响要比晚落叶植物低.综上所述,两种橡树在植物生理性能方面的季节变化取决于冠层叶面积的减少.“,”Aims Mechanisms of plant drought resistance include both tolerance and avoidance.Xylem vulnerability to embolism and turgor loss point are considered traits that confer tolerance,while leaf abscission and deciduousness characterizes the avoidance strategy.While these mechanisms are thought to trade-off expressing a continuum among species,little is known on how variation in the timing and duration of leaf shedding in response to drought affect the relationship between xylem and leaf tolerance.In the present study,we explored the extent to which drought tolerance differs between two oak(Quercus)species that exhibit different leaf shedding behaviours.Particularly,we predicted that Q.deserticola Trel.,which loses leaves at the end of the dry season(late-deciduous)and is thus exposed to a greater risk of cavitation,would be more drought tolerant and more conservative in its water use than Q.laeta Liebm.,which loses its leaves for only a short period of time in the middle of the dry season(brevideciduous).Methods The study was conducted in central Mexico in a single population of each of the two oak species,separated from each other by a distance of 1.58 km,and by an altitudinal difference of 191 m.Quercus deserticola(late deciduous)is more frequent down slope,while Q.laeta(brevideciduous)tends to occur at higher elevations along the gradient.We assessed seasonal differences(rainy versus dry season)in native stem hydraulic conductivity,and tested for variation in xylem vulnerability to cavitation,leaf water use and leaf turgor loss point between the two species.Important Findings The two oak species did not differ in traits conferring drought tolerance,including xylem vulnerability to embolism,leaf turgor loss point,or stomatal conductance.However,both species had different performance during the dry season;the brevideciduous species had lower negative impact in the xylem function than the late-deciduous species.Overall,seasonal changes in plant physiological performance between the two oak species were determined by a reduction in the canopy leaf area.