Terwilliger V J, Kitajima K, Le Roux-Swarthout D J, Mulkey S, Wright S J
Department of Geography, University of Kansas, Lawrence 66045, USA.
Isotopes Environ Health Stud. 2001;37(2):133-60. doi: 10.1080/10256010108033289.
The delta13C and SD values of newly emerging to senescing tree leaves produced during a rainy season were obtained in dry seasonal and moist forest in Panamá. Newly emerging leaves had less negative delta13C values than older leaves yet instantaneous pi/pa was never lowest in the youngest leaves. Furthermore, isotopic enrichment during early growth may have a detectable influence on the delta13C values of mature leaves. The deltaD values of cellulose nitrate were only related to deltaD values of leaf water if leaf age was also considered so that, for a given deltaD of leaf water, deltaD values were highest in the youngest leaves (R2 = 98%). There was no correlation between leaf age and deltaD values of leaf water. Investment of translocated organic carbon is a factor likely to be associated with both 13C and deuterium enrichment effects in new leaves. A coarse, mass balance approach can estimate the proportional heterotrophic investment in leaf growth and improve estimates of integrated pi/pa by approximating delta13C for the most autotrophic phase of leaf growth. Delta13C values of the predominantly sucrose mobile organic fraction in new leaves were less negative than in older leaves, thereby suggesting that the enrichment did not occur at the original site of production of the substrate for new leaf growth. Although the delta values of early leaf growth must be influenced by inputs of translocated organic carbon, enrichment effects, per se, are apparently caused by other mechanisms such as, for carbon, de novo sucrose synthesis and anaplerotic replenishment. Better recognition of metabolic causes of isotopic enrichment in leaves promises to increase the power and accuracy of inferences about carbon and water use of tropical trees from delta analyses.
在巴拿马的旱季森林和湿润森林中，获取了雨季期间新长出至衰老的树叶的δ¹³C和标准差（SD）值。新长出的叶子的δ¹³C值比老叶子的负值更小，但瞬时气孔导度/大气二氧化碳浓度（pi/pa）在最嫩的叶子中从未最低。此外，早期生长期间的同位素富集可能对成熟叶子的δ¹³C值有可检测到的影响。只有在考虑叶龄的情况下，硝酸纤维素的δD值才仅与叶水的δD值相关，因此，对于给定的叶水δD值，最嫩叶子的δD值最高（R² = 98%）。叶龄与叶水的δD值之间没有相关性。转运有机碳的投入可能是与新叶中¹³C和氘富集效应都相关的一个因素。一种粗略的质量平衡方法可以估计叶片生长中异养投入的比例，并通过近似叶片生长最自养阶段的δ¹³C来改进对综合pi/pa的估计。新叶中主要为蔗糖的可移动有机部分的δ¹³C值比老叶的负值更小，从而表明富集并非发生在新叶生长底物的原始产生部位。尽管早期叶片生长的δ值必定受到转运有机碳输入的影响，但富集效应本身显然是由其他机制引起的，比如对于碳来说，是从头合成蔗糖和回补反应。更好地认识叶片中同位素富集的代谢原因有望提高从δ分析推断热带树木碳和水利用情况的能力和准确性。
