成熟马占相思水力导度对水分利用和光合响应的季节性差异

Seasonal differences in the leaf hydraulic conductance of mature Acacia mangium in response to its leaf water use and photosynthesis

通过测定成熟马占相思叶片的水势、气孔导度、蒸腾速率(Tr)、叶面积指数、边材面积等参数,研究了湿季(5月)和干季(11月)叶片的水力导度(Kl)、水分和光合特性的关系.结果表明:高大植株(平均树高20m、胸径0.26m)的边材面积与叶面积的比率(Asp/Acl)比较小植株(平均树高14.5m、胸径0.19m)高8.5%,前者的木质部水分通量大于后者,以支持冠层叶片的水分利用.对木质部易损曲线进行分析,Kl降低50%时,湿、干季的叶片水势(Ψl)分别为-1.41和-1.55MPa,且干季的木质部空穴化的易损性高于湿季.湿、干季的Kl峰值分别为5.5和4.5mmol·m-2·s-1·MPa-1,最大蒸腾速率(Trmax)分别为3.6和1.8mmol·m-2·s-1,且湿季的Kl和Trmax明显大于干季.一天中Kl和Tr的多次波动反映了木质部空穴化和修复的往复循环,叶片气孔在Kl降低超过50%或Ψl达到-1.6MPa时关闭,气孔导度在K1达到50%前仍保持较高水平.干季的水力导度与光合速率的相关性较湿季明显.季节更迭导致叶片水力导度损失是Tr和CO2交换下降的原因.

In this study, measurements were made on the leaf water potential （ψ1）, stomatal conductance （gs）, transpiration rate, leaf area index, and sapwood area of mature Acacia mangium, aimed to understand the relationships of the leaf hydraulic conductance （ K1 ） with the leaf water use and photosynthetic characteristics of the A. mangium in wet season （May） and dry season （ November）. The ratio of sapwood area to leaf area （Asp/Acl） of the larger trees with an average height of 20 m and a diameter at breast height （DBH） of 0.26 m was 8.5% higher than that of the smaller trees with an average height of 14.5 m and a DBH of 0.19 m, suggesting that the larger trees had a higher water flux in their leaf xylem, which facilitated the water use of canopy leaf. The analysis on the vulnerability curve of the xylem showed that when the K1 decreased by 50%, the ψ1 in wet season and dry season was -1.41 and -1.55 MPa, respectively, and the vulnerability of the xylem cavitation was higher in dry season than in wet season. The K1 peak value in wet season and dry season was 5.5 and 4.5 mmol ·m^-2 · s^-1 · MPa^-1, and the maximum transpiration rate （ Tr max ） was 3.6 and 1.8 mmol · m^-2 ·s^-1, respectively. Both the K1 and Tr max were obviously higher in wet season than in dry season. Within a day, the K1 and Tr fluctuated many times, reflecting the recip- rocated cycle of the xylem cavitation and refilling. The leaf stomatal closure occurred when the g1 declined over 50% or the ψ1 reached -1.6 MPa. The gs would be maintained at a high level till the K1 declined over 50%. The correlation between the hydraulic conductance and photosynthetic rate was more significant in dry season than in wet season. The loss of leaf hydraulic conductance induced by seasonal change could be the causes of the decrease of Tr and CO2 gas exchange.