The Physiological Mechanism Of Drought-induced Seeding Mortality In Salix Psammophila And Caragana Korshinskii

Two main mortality mechanisms of trees under extreme drought are hydraulic failure and carbon starvation.It is generally accepted that isohydric plants might be more exposed to carbon starvation than hydraulic failure and anisohydric plants might be more exposed to hydraulic failure than carbon starvation.But recently some evidences did not support this view.For this reason,we selected two typical shrubs growing in water-wind erosion crisscross region of northern Shaanxi — Salix psammophila(isohydric)and Caragana korshinskii(anisohydric)as the objects and systematically studied the dynamics of their growth,hydraulic properties and the dynamics of non-structural carbohydrates contents under prolonged drought by pot experiment as well as the resprouting capacity of Caragana korshinskii under extreme drought.The major objectives were to reveal the mortality mechanisms of two shrubs succumbing to extreme drought.The results not only deepen the research on the mortality mechanisms of trees,but also are useful in predicting adaptability of the ecosystems to extreme drought in the future.The main results were as the follows:(1)The growth and hydraulic traits of two shrubs showed different response to prolonged drought.The relative growth rate of height,basal diameter and total biomass of two shrubs increased at first and decreased thereafter,and the relative growth rate of height and basal diameter of Caragana korshinskii decreased earlier and more than Salix psammophila under prolonged drought.The leaves of Salix psammophila died earlier than Caragana korshinskii.The predawn and midday leaf water potential of Caragana korshinskii decreased more than those of Salix psammophila.The transpiration rate and hydraulic conductance of Salix psammophila decreased earlier and more than Caragana korshinskii.There was no significantly difference in the degree of stem embolism between two shrubs.The hydraulic properties of Salix psammophila are more sensitive to prolonged drought than Caragana korshinskii.The relative ratio of yellow leaf area to total leaf area of two shrubs were significantly and positively correlated with the relative predawn leaf water potential,the relative midday leaf water potential and the relative percentage loss of hydraulic conductivity,and had a quadratic relationship with relative hydraulic conductance.Hydraulic constraint was an important factor leading to leaf death of two shrubs under prolonged drought.(2)The NSCs in different tissues of two shrubs showed different responses to extreme drought.The soluble sugar content decreased significantly in different tissues(roots,stems and leaves)of Salix psammophila under extreme drought and in leaves and stems of Caragana korshinskii,but increased significantly in roots of Caragana korshinskii.The ratio of soluble sugar content to starch content in leaves of two shrubs was significantly decreased under extreme drought,and did not change in stems and roots of Salix psammophila,but increased significantly in stems and roots of Caragana korshinskii.The accumulation of soluble sugar content and the increase of the ratio of soluble sugar content to starch content in roots of Caragana korshinskii are an important physiological mechanism of its adaptation to extreme drought.For both shrubs,the starch content in leaves did not change under extreme drought,but decreased significantly in stems and roots.Meanwhile,the NSCs content in different tissues of two shrubs was decreased significantly.The NSC in different tissues of Caragana korshinskii decreased earlier and larger than Salix psammophila in most cases under prolonged drought and the changes of NSCs in roots of two shrubs earlier than in stems and leaves.Both hydraulic failure and carbon starvation led to the death of two shrubs.The NSCs decreased significantly after the death of Salix psammophila leaves and prior to the death of Caragana korshinskii leaves.The conversion from hydraulic constraint to carbon constraint in Caragana korshinski was earlier than in Salix psammophila.(3)Rehydrated Caragana korshinskii seeding after extreme drought can still resprout.The resprouting ability of Caragana korshinskii was significantly and negatively correlated with the degree of stem embolism during severe drought,and significantly positively correlated with the relative water content of stems and the NSCs content in roots(mainly coarse roots),indicating that the resprouting ability of Caragana korshinskii mainly depends on the hydraulic properties of stems and carbohydrate storage of roots during drought stress.