Faculty of Agriculture, Ehime University, Tarumi, Matsuyama, Ehime, Japan.
Research Institute for Humanity and Nature, 457-4 Motoyama, Kamigamo, Kita-ku, Kyoto, Japan.
Tree Physiol. 2018 Dec 1;38(12):1829-1840. doi: 10.1093/treephys/tpy068.
Leaf photosynthetic and post-photosynthetic processes modulate the isotope ratios of tree-ring cellulose. Post-photosynthetic processes, such as the remobilization of stored starch in early spring, are important to understanding the mechanisms of xylem formation in tree stems; however, untangling the isotope ratio signals of photosynthetic and post-photosynthetic processes imprinted on tree rings is difficult. Portions of carbon-bound hydrogen and oxygen atoms are exchanged with medium water during post-photosynthetic processes. We investigated the δD and δ18O values of tree-ring cellulose using Quercus crispula Blume trees in two different habitats to evaluate seasonal changes in the exchange rate (f-value) of hydrogen or oxygen with medium water, and examined the associations of the post-photosynthetic processes. Theoretically, if the f-value is constant, δD and δ18O would be positively correlated due to meteorological factors, while variation in the f-value will create a discrepancy and weak correlation between δD and δ18O due to the exchange of carbon-bound hydrogen and oxygen with medium water. The values of δD decreased drastically from earlywood to latewood, while those of δ18O increased to a peak and then decreased toward the latewood. The estimated seasonal f-value was high at the beginning of earlywood and decreased toward the latewood. The post-photosynthetic processes associated with changes in the f-value were the remobilization of stored starch and triose cycling during cellulose synthesis because of the shortage of photo-assimilates in early spring. Although we did not evaluate relevant physiological parameters, the seasonal pattern of δD and δ18O in tree-ring cellulose of Q. crispula was clear, suggesting that the dual isotope (δD and δ18O) approach can be used to reveal the resource allocation mechanisms underlying seasonal xylem formation.
叶片光合作用和光合作用后过程会调节树木年轮纤维素的同位素比值。光合作用后过程，如早春储存淀粉的再利用，对于理解树干木质部形成的机制很重要；然而，要理清光合作用和光合作用后过程在树木年轮上留下的同位素比值信号是很困难的。在光合作用后过程中，与碳键结合的氢和氧原子会与介质水发生交换。我们使用两种不同生境中的栓皮栎来研究树木年轮纤维素的 δD 和 δ18O 值，以评估氢或氧与介质水交换率（f 值）的季节性变化，并检验光合作用后的过程之间的关联。理论上，如果 f 值保持不变，由于气象因素，δD 和 δ18O 将呈正相关，而 f 值的变化会由于与介质水的碳键结合的氢和氧的交换而在 δD 和 δ18O 之间产生差异和弱相关。δD 的值从早材急剧下降到晚材，而 δ18O 的值则增加到峰值，然后在晚材中下降。估计的季节性 f 值在早材开始时较高，并在晚材时逐渐降低。与 f 值变化相关的光合作用后过程是储存淀粉的再利用和三碳糖循环，这是由于早春光合同化物的短缺。尽管我们没有评估相关的生理参数，但栓皮栎树木年轮纤维素中 δD 和 δ18O 的季节性模式很明显，表明双同位素（δD 和 δ18O）方法可用于揭示季节性木质部形成的资源分配机制。
