Osborne Colin P, Beerling David J
Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, United Kingdom.
Plant Physiol. 2003 Oct;133(2):803-12. doi: 10.1104/pp.103.026567. Epub 2003 Sep 11.
Deciduous forests covered the ice-free polar regions 280 to 40 million years ago under warm "greenhouse" climates and high atmospheric pCO2. Their deciduous habit is frequently interpreted as an adaptation for minimizing carbon losses during winter, but experiments with "living fossils" in a simulated warm polar environment refute this explanation. Measured carbon losses through leaf abscission of deciduous trees are significantly greater than losses through winter respiration in evergreens, yet annual rates of primary productivity are similar in all species. Here, we investigate mechanisms underlying this apparent paradox by measuring the seasonal patterns of leaf photosynthesis (A) under pCO2 enrichment in the same trees. During spring, A increased significantly in coastal redwood (Sequoia sempervirens), dawn redwood (Metasequoia glyptostroboides), and swamp cypress (Taxodium distichum) at an elevated pCO2 of 80 Pa compared with controls at 40 Pa. However, strong acclimation in Rubisco carboxylation capacity (Vc,max) completely offset the CO2 response of A in all species by the end of 6 weeks of continuous illumination in the simulated polar summer. Further measurements demonstrated the temporary nature of acclimation, with increases in Vc,max during autumn restoring the CO2 sensitivity of A. Contrary to expectations, the acclimation of Vc,max was not always accompanied by accumulation of leaf carbohydrates, but was associated with a decline in leaf nitrogen in summer, suggesting an alteration of the balance in plant sources and sinks for carbon and nitrogen. Preliminary calculations using A indicated that winter carbon losses through deciduous leaf abscission and respiration were recovered by 10 to 25 d of canopy carbon fixation during summer, thereby explaining the productivity paradox.
在2800万至4000万年前温暖的“温室”气候和高大气pCO₂条件下,落叶林覆盖了无冰的极地地区。它们的落叶习性通常被解释为一种在冬季将碳损失降至最低的适应方式,但在模拟温暖极地环境中对“活化石”进行的实验反驳了这一解释。通过落叶树的叶片脱落测量到的碳损失明显大于常绿树冬季呼吸造成的损失,然而所有物种的年初级生产力速率相似。在此,我们通过测量同一树木在pCO₂富集条件下叶片光合作用(A)的季节模式,研究了这一明显矛盾背后的机制。在春季,与40 Pa的对照相比,在80 Pa的升高pCO₂条件下,海岸红杉(红杉)、水杉和落羽杉的A显著增加。然而,在模拟极地夏季连续光照6周结束时,Rubisco羧化能力(Vc,max)的强烈驯化完全抵消了所有物种中A的CO₂响应。进一步的测量证明了驯化的暂时性,秋季Vc,max的增加恢复了A的CO₂敏感性。与预期相反,Vc,max的驯化并不总是伴随着叶片碳水化合物的积累,而是与夏季叶片氮含量的下降有关,这表明植物碳和氮的源库平衡发生了改变。使用A进行的初步计算表明,落叶和呼吸造成的冬季碳损失在夏季通过树冠碳固定10至25天得以恢复,从而解释了生产力悖论。