Wujeska-Klause Agnieszka, Crous Kristine Y, Ghannoum Oula, Ellsworth David S
Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales, Australia.
Translational Photosynthesis Centre of Excellence, Western Sydney University, Penrith, New South Wales, Australia.
Glob Chang Biol. 2019 Apr;25(4):1282-1295. doi: 10.1111/gcb.14555. Epub 2019 Feb 20.
Rising atmospheric CO concentrations is expected to stimulate photosynthesis and carbohydrate production, while inhibiting photorespiration. By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO (eCO ), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO : (a) A "dilution effect" caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO . This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature Eucalyptus forest exposed to Free-Air CO Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate ( ) concentrations, carbohydrates and reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO in new leaves, while the fraction and total carbohydrate concentrations remained unchanged by CO treatment for either new or mature leaves. Photorespiration decreased 31% under eCO supplying less reductant, and in situ reductase activity was concurrently reduced (-34%) in eCO , especially in new leaves during summer periods. Hence, assimilation was inhibited in leaves of E. tereticornis and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of reductase activity due to lower photorespiration in eCO can contribute to understanding how eCO -induced photosynthetic enhancement may be lower than previously expected. We suggest that large-scale vegetation models simulating effects of eCO on N biogeochemistry include both mechanisms, especially where is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.
大气中二氧化碳（CO）浓度上升预计会刺激光合作用和碳水化合物的产生，同时抑制光呼吸作用。相比之下，在高浓度CO（eCO）条件下，叶片中的氮（N）浓度通常会下降，这可能会降低光合作用增强的幅度。我们测试了关于在eCO条件下叶片氮含量降低的两个假设：（a）叶片碳水化合物浓度增加导致的“稀释效应”；（b）eCO条件下光呼吸作用中还原剂供应减少导致硝酸盐同化作用受到抑制。在此，我们首次在野外对第二个假设进行了全面测试，使用了一片成熟桉树林中的高大树木，这些树木在自由空气CO富集（EucFACE）环境中暴露了五年。对完全展开的幼叶和成熟叶进行了净光合作用、光呼吸作用、总叶片氮、硝酸盐（）浓度、碳水化合物和还原酶活性的测量，以检验这些假设。在eCO条件下，新叶中的叶片氮浓度下降了8%，而新叶或成熟叶的CO处理对硝酸盐比例和总碳水化合物浓度均无影响。在eCO条件下，由于还原剂供应减少，光呼吸作用下降了31%，同时原位还原酶活性在eCO条件下也同时降低（-34%），尤其是在夏季的新叶中。因此，尾叶桉叶片中的硝酸盐同化作用受到抑制，并且证据不支持显著的稀释效应是观察到的叶片氮浓度降低的原因。这一发现表明，eCO条件下较低的光呼吸作用导致还原酶活性降低，这有助于理解eCO诱导的光合作用增强可能低于先前预期的原因。我们建议，模拟eCO对氮生物地球化学影响的大规模植被模型应包括这两种机制，特别是在硝酸盐是优势植被的主要氮源以及在促进高光呼吸速率的温度下发生叶片萌发和出现的地方。
