Maherali Hafiz, DeLucia Evan H, Sipe Timothy W
Department of Plant Biology, University of Illinois at Urbana-Champaign, 265 Morrill Hall, 505 South Goodwin Avenue, Urbana, IL 61801-3707, USA Fax: 217-244-7246; e-mail:
Department of Biology, Franklin and Marshall University, Lancaster, PA 17601, USA, , , , , , MH.
Oecologia. 1997 Nov;112(4):472-480. doi: 10.1007/s004420050334.
The leaf-specific hydraulic conductivity (K ) of plant stems can control leaf water supply. This property is influenced by variation in leaf/sapwood area ratio (A /A ) and the specific hydraulic conductivity of xylem tissue (K ). In environments with high atmospheric vapor pressure deficit (VPD), K may increase to support higher transpiration rates. We predicted that saplings of Acerrubrum and A.pensylvanicum grown in forest canopy gaps, under high light and VPD, would have higher K and lower A /A than similar sized saplings in the understory. Leaf-specific hydraulic conductivity and K increased with sapling size for both species. In A. rubrum, K did not differ between the two environments but lower A /A (P=0.05, ANCOVA) led to higher K for gap-grown saplings (P < 0.05, ANCOVA). In A. pensylvanicum, neither K , A /A , nor K differed between environments. In a second experiment, we examined the impact of sapling size on the water relations and carbon assimilation of A.pensylvanicum. Maximum stomatal conductance for A.pensylvanicum increased with K (r =0.75, P < 0.05). A hypothetical large A. pensylvanicum sapling (2 m tall) had 2.4 times higher K and 22 times greater daily carbon assimilation than a small (1 m tall) sapling. Size-related hydraulic limitations in A.pensylvanicum caused a 68% reduction in daily carbon assimilation in small saplings. Mid-day water potential increased with A.pensylvanicum sapling size (r =0.69, P < 0.05). Calculations indicated that small A.pensylvanicum saplings (low K ) could not transpire at the rate of large saplings (high K ) without reaching theoretical thresholds for xylem embolism induction. The coordination between K and stomatal conductance in saplings may prevent xylem water potential from reaching levels that cause embolism but also limits transpiration. The K of the xylem did not vary across environments, suggesting that altering biomass allocation is the primary mechanism of increasing K . However, the ability to alter aboveground biomass allocation in response to canopy gaps is species-specific. As a result of the increase in K and K with sapling size for both species, hydraulic limitation of water flux may impose a greater restriction on daily carbon assimilation for small saplings in the gap environment.
植物茎干的叶特定水力传导率（K ）能够控制叶片的水分供应。这一特性受叶/边材面积比（A /A ）以及木质部组织的比水力传导率（K ）变化的影响。在大气蒸汽压亏缺（VPD）较高的环境中，K 可能会增加以支持更高的蒸腾速率。我们预测，在高光和高VPD条件下生长于林冠间隙的红槭和宾夕法尼亚糖槭幼树，相较于林下相似大小的幼树，会具有更高的K 和更低的A /A 。两种树种的叶特定水力传导率和K 均随幼树大小增加。在红槭中，两种环境下的K 没有差异，但间隙生长的幼树较低的A /A （P = 0.05，协方差分析）导致了更高的K （P < 0.05，协方差分析）。在宾夕法尼亚糖槭中，环境之间的K 、A /A 以及K 均无差异。在第二个实验中，我们研究了幼树大小对宾夕法尼亚糖槭水分关系和碳同化的影响。宾夕法尼亚糖槭的最大气孔导度随K 增加（r = 0.75，P < 0.05）。一棵假设的大型宾夕法尼亚糖槭幼树（高2米）的K 比小型（高1米）幼树高2.4倍，每日碳同化量则大22倍。宾夕法尼亚糖槭中与大小相关的水力限制导致小型幼树的每日碳同化量减少了68%。中午水势随宾夕法尼亚糖槭幼树大小增加（r = 0.69，P < 0.05）。计算表明，小型宾夕法尼亚糖槭幼树（低K ）若不以达到木质部栓塞诱导的理论阈值为代价，就无法以大型幼树（高K ）的速率进行蒸腾。幼树中K 和气孔导度之间的协调可能会防止木质部水势达到导致栓塞的水平，但也会限制蒸腾作用。木质部的K 在不同环境中没有变化，这表明改变生物量分配是增加K 的主要机制。然而，响应林冠间隙而改变地上生物量分配的能力具有物种特异性。由于两种树种的K 和K 均随幼树大小增加，间隙环境中水分通量的水力限制可能会对小型幼树的每日碳同化施加更大的限制。
