Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, Ciudad de México, Mexico.
Department Territorio e Sistemi Agro-Forestali, University of Padova, Legnaro (PD), Italy.
J Exp Bot. 2019 Oct 24;70(20):5765-5772. doi: 10.1093/jxb/erz329.
As trees grow taller, hydraulic resistance can be expected to increase, causing photosynthetic productivity to decline. Yet leaves maintain productivity over vast height increases; this maintenance of productivity suggests that leaf-specific conductance remains constant as trees grow taller. Here we test the assumption of constant leaf-specific conductance with height growth and document the stem xylem anatomical adjustments involved. We measured the scaling of total leaf area, mean vessel diameter at terminal twigs and at the stem base, and total vessel number in 139 individuals of Moringa oleifera of different heights, and estimated a whole-plant conductance index from these measurements. Whole-plant conductance and total leaf area scaled at the same rate with height. Congruently, whole-plant conductance and total leaf area scaled isometrically. Constant conductance is made possible by intricate adjustments in anatomy, with conduit diameters in terminal twigs becoming wider, lowering per-vessel resistance, with a concomitant decrease in vessel number per unit leaf area with height growth. Selection maintaining constant conductance per unit leaf area with height growth (or at least minimizing drops in conductance) is likely a potent selective pressure shaping plant hydraulics, and crucially involved in the maintenance of photosynthetic productivity per leaf area across the terrestrial landscape.
随着树木的长高，人们预计液压阻力会增加，从而导致光合作用的生产力下降。然而，叶子在巨大的高度增加下仍能保持生产力；这种生产力的维持表明，随着树木的生长，叶片比导度保持不变。在这里，我们通过与高度生长的测试，检验了叶片比导度保持不变的假设，并记录了涉及的茎木质部解剖结构的调整。我们测量了不同高度的 139 株辣木的总叶面积、末端小枝和茎基部的平均导管直径以及总导管数量，并从这些测量值中估算了整株植物的导度指数。整株植物的导度和总叶面积与高度呈相同的比例增长。一致地，整株植物的导度和总叶面积呈等比增长。通过解剖结构的复杂调整，实现了恒定的导度，末端小枝中的导管直径变宽，降低了每根导管的阻力，同时随着高度的增加，单位叶面积的导管数量减少。随着高度的增长，维持单位叶面积的恒定导度（或至少最小化导度的下降）的选择，很可能是塑造植物水力的一种强大的选择压力，对维持陆地景观中每单位叶面积的光合生产力至关重要。
