土壤水势对水曲柳幼苗水分生态的影响

采用根区渗灌控水技术,将土壤水势长期控制在0～-20kPa(W₁)、-20～-40kPa(W₂)、-40～-60kPa(W₃)、-60～-80kPa(W₄)、-80～-160kPa(W₅)范围内,系统地研究了不同土壤水势条件下水曲柳幼苗的蒸腾过程、吸水过程、根叶水势日动态过程及SPAC体系的水流阻力.结果表明,在亚饱和土壤水分状态下(W₁),细根水势最高,水分由土壤进入细根的阻力最小,根系吸水速率最高,从而支持了日间强烈的蒸腾作用.在田间持水量土壤水分状态下(W₂),细根吸水阻力成倍增加,吸水速率和蒸腾速率显著下降,但尚未改变蒸腾作用日动态过程的单峰模式.当土壤水分在田间持水量状态以下(W₃～W₅)时,随着土壤水势递降,细根吸水阻力急剧增加至几倍乃至几十倍,根系吸水速率过低,吸水与蒸腾矛盾加剧,叶水势降至很低,气孔关闭,蒸腾作用受到严重抑制,呈现明显的午休低谷.在实验范围内(0～-160kPa),土壤水分对水曲柳幼苗是非等效的,当土壤水分在田间持水量状态以下(<-40kPa)时,水曲柳全光苗发生显著的水分胁迫.

Effect of soil water potential on hydraulic parameters of Fraxinus mandshurica seedlings

In this paper,soil water potential was controlled in the ranges of 0～-20 kPa(W₁),-20～-40 kPa (W₂), -40～-60 kPa (W₃),-60～-80 kPa (W₄) and -80～-160 kPa (W₅) by a suite of specially designed root-sphere osmotic irrigation technique,and the dynamics of transpiration,water suction,tissue water potential and hydraulic resistance of Fraxinus mandshurica seedlings grown under these soil water conditions were studied.The results indicated that under sub-saturated soil water condition (W₁),fine root water potential fluctuated at high levels from -0.08 MPa to -0.19MPa,which brought about the lowest soil-root hydraulic resistance (35.52 MPa·s^-1·m^-2·mol^-1 in diurnal average),the highest water suction rate (4.44 mmol·m^-2·s^-1 in average from 10:00 to 14:00),and the strongest transpiration rate (5.11 mmol·m^-2·s^-1 in average from 10:00 to 14:00).Under field capacity (W₂),soil-root hydraulic resistance increased several times above that under W₁,water suction rate and transpiration rate declined significantly,while the diurnal single-peak pattern of transpiration was not altered.Under orderly increased soil water stress (W₃～W₅),soil-root hydraulic resistance (the average from 10:00 to 14:00) increased approximately 10～24 times above that under W₁,which caused the consequent low water suction rate and low fluctuation levels of leaf water potential (-1.25 MPa～-1.94 MPa for W₅),and transpiration was seriously restricted (the transpiration rate being only 0.91～1.55 mmol·m^-2·s^-1 at midday for W₃～W₅).It is concluded that soil water was not equally available to Fraxinus mandsharica seedlings in the test ranges of soil water potential,and significant water stress could occur when the soil water potential was lower than-40 kPa.