Hikosaka Kouki, Terashima Ichiro, Katoh Sakae
Department of Botany, Faculty of Science, University of Tokyo, Hongo, Bunkyo-ku, 113, Tokyo, Japan.
Oecologia. 1994 May;97(4):451-457. doi: 10.1007/BF00325881.
Effects of leaf age, nitrogen nutrition and photon flux density (PFD) on the distribution of nitrogen among leaves were investigated in a vine, Ipomoea tricolor Cav., which had been grown horizontally so as to avoid mutual shading of leaves. The nitrogen content was highest in newly developed young leaves and decreased with age of leaves in plants grown at low nitrate concentrations and with all leaves exposed to full sunlight. Thus, a distinct gradient of leaf nitrogen content was formed along the gradient of leaf age. However, no gradient of leaf nitrogen content was formed in plants grown at a high nitrate concentration. Effects of PFD on the distribution of nitrogen were examined by shading leaves in a manner that simulated changes in the light gradient of an erect herbaceous canopy (i.e., where old leaves were placed under increasingly darker conditions with growth of the canopy). This canopy-type shading steepened the gradient of leaf nitrogen content in plants grown at a low nitrogen supply, and created a gradient in plants grown at high concentrations of nitrate. The steeper the gradient of PFD, the larger the gradient of leaf nitrogen that was formed. When the gradient of shading was inverted, that is, younger leaves were subjected to increasingly heavier shade, while keeping the oldest leaves exposed to full sunlight, an inverted gradient of leaf nitrogen content was formed at high nitrate concentrations. The gradient of leaf nitrogen content generated either by advance of leaf age at low nitrogen availability, or by canopy-type shading, was comparable to those reported for the canopies of erect herbaceous plants. It is concluded that both leaf age and PFD have potential to cause the non-uniform distribution of leaf nitrogen. It is also shown that the contribution of leaf age increases with the decrease in nitrogen nutrition level.
以水平生长以避免叶片相互遮荫的三色牵牛藤蔓植物为研究对象，研究了叶龄、氮素营养和光量子通量密度（PFD）对叶片间氮素分配的影响。在低硝酸盐浓度且所有叶片都暴露在充足阳光下生长的植株中，新发育的幼叶氮含量最高，且随叶龄增加而降低。因此，沿着叶龄梯度形成了明显的叶片氮含量梯度。然而，在高硝酸盐浓度下生长的植株中未形成叶片氮含量梯度。通过模拟直立草本植物冠层光梯度变化的方式（即随着冠层生长，老叶处于越来越暗的条件下）对叶片进行遮荫，研究了PFD对氮素分配的影响。这种冠层型遮荫使低氮供应条件下生长的植株叶片氮含量梯度变陡，并在高硝酸盐浓度下生长的植株中形成了梯度。PFD梯度越陡，形成的叶片氮梯度越大。当遮荫梯度反转时，即幼叶接受越来越重的遮荫，而老叶保持暴露在充足阳光下，在高硝酸盐浓度下形成了叶片氮含量的反转梯度。在低氮有效性下由叶龄推进或由冠层型遮荫产生的叶片氮含量梯度，与直立草本植物冠层报道的梯度相当。得出的结论是，叶龄和PFD都有可能导致叶片氮素的不均匀分布。还表明，随着氮素营养水平的降低，叶龄的作用增加。
