Spicer Rachel, Holbrook N Michele
Rowland Institute at Harvard, 100 Edwin H. Land Boulevard, Cambridge, MA 02142, USA.
J Exp Bot. 2007;58(6):1313-20. doi: 10.1093/jxb/erl296. Epub 2007 Feb 5.
The gaseous environment surrounding parenchyma in woody tissue is low in O2 and high in CO2, but it is not known to what extent this affects respiration or might play a role in cell death during heartwood formation. Sapwood respiration was measured in two conifers and three angiosperms following equilibration to levels of O2 and CO2 common within stems, using both inner and outer sapwood to test for an effect of age. Across all species and tissue ages, lowering the O2 level from 10% to 5% (v/v) resulted in about a 25% decrease in respiration in the absence of CO2, but a non-significant decrease at 10% CO2. The inhibitory effect of 10% CO2 was smaller and only significant at 10% O2, where it reduced respiration by about 14%. Equilibration to a wider range of gas combinations in Pinus strobus L. showed the same effect: 10% CO2 inhibited respiration by about 15% at both 20% and 10% O2, but had no net effect at 5% O2. In an extreme treatment, 1% O2+20% CO2 increased respiration by over 30% relative to 1% O2 alone, suggesting a shift in metabolic response to high CO2 as O2 decreases. Although an increase in respiration would be detrimental under limiting O2, this extreme gas combination is unlikely to exist within most stems. Instead, moderate reductions in respiration under realistic O2 and CO2 levels suggest that within-stem gas composition does not severely limit respiration and is unlikely to cause the death of xylem parenchyma during heartwood formation.
木质组织中薄壁细胞周围的气体环境氧气含量低而二氧化碳含量高，但目前尚不清楚这在多大程度上影响呼吸作用，或者在心材形成过程中的细胞死亡中可能发挥何种作用。在两种针叶树和三种被子植物中，在使氧气和二氧化碳水平达到茎内常见水平后，测量边材呼吸作用，使用边材内层和外层来测试年龄的影响。在所有物种和组织年龄中，在没有二氧化碳的情况下，将氧气水平从10%降至5%（体积/体积）会导致呼吸作用下降约25%，但在10%二氧化碳时下降不显著。10%二氧化碳的抑制作用较小，仅在10%氧气时显著，此时呼吸作用降低约14%。对北美乔松中更广泛的气体组合进行平衡处理显示了相同的效果：在20%和10%氧气时，10%二氧化碳均抑制呼吸作用约15%，但在5%氧气时没有净影响。在一种极端处理中，相对于仅1%氧气，1%氧气+20%二氧化碳使呼吸作用增加超过30%，这表明随着氧气减少，对高二氧化碳的代谢反应发生了转变。尽管在氧气有限的情况下呼吸作用增加是有害的，但这种极端气体组合在大多数茎中不太可能存在。相反，在实际的氧气和二氧化碳水平下呼吸作用适度降低表明，茎内气体组成不会严重限制呼吸作用，也不太可能在心材形成过程中导致木质部薄壁细胞死亡。
