Choat Brendan, Ball Marilyn C, Luly Jon G, Donnelly Christine F, Holtum Joseph A M
Department of Tropical Plant Science, James Cook University, Townsville, Queensland 4811, Australia.
Tree Physiol. 2006 May;26(5):657-64. doi: 10.1093/treephys/26.5.657.
Diurnal and seasonal patterns of leaf gas exchange and water relations were examined in tree species of contrasting leaf phenology growing in a seasonally dry tropical rain forest in north-eastern Australia. Two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret. were studied. The deciduous species had higher specific leaf areas and maximum photosynthetic rates per leaf dry mass in the wet season than the evergreens. During the transition from wet season to dry season, total canopy area was reduced by 70-90% in the deciduous species and stomatal conductance (g(s)) and assimilation rate (A) were markedly lower in the remaining leaves. Deciduous species maintained daytime leaf water potentials (Psi(L)) at close to or above wet season values by a combination of stomatal regulation and reduction in leaf area. Thus, the timing of leaf drop in deciduous species was not associated with large negative values of daytime Psi(L) (greater than -1.6 MPa) or predawn Psi(L) (greater than -1.0 MPa). The deciduous species appeared sensitive to small perturbations in soil and leaf water status that signalled the onset of drought. The evergreen species were less sensitive to the onset of drought and g(s) values were not significantly lower during the transitional period. In the dry season, the evergreen species maintained their canopies despite increasing water-stress; however, unlike Eucalyptus species from northern Australian savannas, A and g(s) were significantly lower than wet season values.
在澳大利亚东北部季节性干燥的热带雨林中，对具有不同叶片物候的树种的叶片气体交换和水分关系的昼夜及季节模式进行了研究。研究了两种旱季落叶树种，即澳洲酒瓶树（Brachychiton australis (Schott and Endl.) A. Terracc.）和吉利弗瑞柯罗苏木（Cochlospermum gillivraei Benth.），以及两种常绿树种，即粗糠柴（Alphitonia excelsa (Fenzal) Benth.）和比氏澳香桃木（Austromyrtus bidwillii (Benth.) Burret.）。落叶树种在雨季的比叶面积和单位叶干质量的最大光合速率高于常绿树种。在从雨季向旱季过渡期间，落叶树种的总冠层面积减少了70 - 90%，其余叶片的气孔导度（g(s)）和同化率（A）显著降低。落叶树种通过气孔调节和叶面积减少相结合的方式，使白天叶片水势（Ψ(L)）维持在接近或高于雨季的值。因此，落叶树种的落叶时间与白天Ψ(L)的大幅负值（大于 -1.6 MPa）或黎明前Ψ(L)的大幅负值（大于 -1.0 MPa）无关。落叶树种似乎对土壤和叶片水分状况的微小扰动敏感，这些扰动标志着干旱的开始。常绿树种对干旱的开始不太敏感，在过渡期间g(s)值没有显著降低。在旱季，常绿树种尽管水分胁迫增加，但仍维持其冠层；然而，与澳大利亚北部稀树草原的桉树种不同，A和g(s)显著低于雨季的值。
