James Shelley A., Bell David T.
Department of Botany, The University of Western Australia, Nedlands, WA 6907, Australia.
Tree Physiol. 2000 Jun;20(12):815-823. doi: 10.1093/treephys/20.12.815.
Juvenile and adult leaves of the heteroblastic species Eucalyptus globulus Labill. ssp. globulus did not show active diurnal orientation toward or away from incident radiation. Juvenile leaves of a late-maturing sapling of a Tasmanian provenance were evenly distributed in all azimuth sectors. In contrast, an early-maturing sapling of the same age from Wilsons Promontory, Victoria had a predominance of adult leaf blades facing east and west. Mid-vein and blade angles of juvenile and adult leaves were non-random with an overall vertical declination of the leaves. Both leaf types intercepted a greater irradiance during the morning than at midday. Sub-horizontal juvenile leaves intercepted 22% more irradiance than vertical adult leaves during the middle of the day. The amphistomatal and isobilateral adult leaves intercepted sunlight equally on both leaf surfaces. Stomatal conductance was variable during the period of measurement but was similar for the Tasmanian juvenile (0.4 to 0.9 cm s(-1)) leaves and Wilsons Promontory adult (0.5 to 1.2 cm s(-1)) leaves. Greater light interception by the sub-horizontal juvenile leaves would confer a growth advantage to saplings and regenerating canopies. Reduced light interception and leaf temperature of vertical adult E. globulus leaves would assist in water conservation, particularly at high solar angles.
异叶树种蓝桉（Eucalyptus globulus Labill. ssp. globulus）的幼叶和成叶并未表现出对入射辐射的主动昼夜定向。来自塔斯马尼亚种源的一棵晚熟幼树的幼叶在所有方位扇区均匀分布。相比之下，来自维多利亚州威尔逊岬同一树龄的早熟幼树，其成年叶片大多朝向东方和西方。幼叶和成叶的中脉和叶片角度并非随机分布，叶片整体呈垂直下倾。两种叶型在早晨截获的辐照度均高于中午。在中午时分，水平下倾的幼叶截获的辐照度比垂直的成年叶片多22%。两面气孔型和等面的成年叶片在叶片两面截获阳光的情况相同。在测量期间气孔导度有所变化，但塔斯马尼亚幼叶（0.4至0.9厘米·秒⁻¹）和威尔逊岬成年叶（0.5至1.2厘米·秒⁻¹）的气孔导度相似。水平下倾的幼叶对光线的更多截获将赋予幼树和正在更新的树冠生长优势。垂直的成年蓝桉叶片减少了光线截获和叶片温度，这将有助于节水，尤其是在太阳高度角较高时。
