Clark C S, Weber J A, Lee E H, Hogsett W E
ManTech Environmental Technology Inc., 200 SW 35th Street, Corvallis, OR 97333, USA.
Tree Physiol. 1995 Mar;15(3):181-9. doi: 10.1093/treephys/15.3.181.
Two-year-old ponderosa pine seedlings (Pinus ponderosa Laws.) were exposed to episodic O(3) concentrations in open-top chambers for two consecutive growing seasons (June through September of 1990 and 1991). Near the end of the second season of O(3) exposure, gas exchange was measured on needles of surviving flushes at saturating CO(2) and photosynthetic photon flux density (PPFD). Both photosynthetic capacity (A(sat)) and stomatal conductance to water vapor (g(wv)) declined linearly with needle age but differences within a flush were also found. Gas exchange rates of needles from the base of the current-year flush were significantly lower than those of needles from the top of the flush, even though age differences between needles were negligible. Although most measurements were conducted at saturating CO(2), similar patterns of gas exchange were also found at 350 micro mol mol(-1) CO(2), indicating that photosynthesis of needles at the bottom of the flush was more strongly affected by O(3) than that of needles at the top of the flush, even though the potential for O(3) uptake was probably less in needles at the bottom of the flush because of reduced stomatal conductance. Carboxylation efficiency (deltaA/deltaC(i)), the linear slope of the A/C(i) response, was highly correlated with A(sat), varying with needle age, needle position in the flush and O(3) exposure, but the magnitude of the reductions was greater than for A(sat). We conclude that susceptibility to O(3) damage among needles of an individual seedling varies not only with needle age but also with needle position, and that reductions in photosynthetic capacity may not be directly attributable to increased uptake of the pollutant. The data also indicate the need to consider within-flush variation when estimating whole-plant carbon gain and O(3) uptake.
两岁的美国黄松幼苗（Pinus ponderosa Laws.）在开顶式气室中连续两个生长季节（1990年和1991年6月至9月）暴露于间歇性臭氧浓度环境中。在臭氧暴露的第二个季节接近尾声时，在饱和二氧化碳浓度和光合光子通量密度（PPFD）条件下，对存活嫩枝上的针叶进行了气体交换测量。光合能力（A(sat)）和气孔导度（g(wv)）均随针叶年龄呈线性下降，但在同一嫩枝内也发现了差异。尽管针叶之间的年龄差异可忽略不计，但当年嫩枝基部针叶的气体交换速率显著低于嫩枝顶部针叶的气体交换速率。虽然大多数测量是在饱和二氧化碳浓度下进行的，但在350微摩尔每摩尔（-1）二氧化碳浓度下也发现了类似的气体交换模式，这表明嫩枝底部针叶的光合作用比顶部针叶受臭氧的影响更大，尽管由于气孔导度降低，嫩枝底部针叶吸收臭氧的潜力可能较小。羧化效率（deltaA/deltaC(i)），即A/C(i)响应的线性斜率，与A(sat)高度相关，随针叶年龄、嫩枝中针叶位置和臭氧暴露而变化，但降低幅度大于A(sat)。我们得出结论，单个幼苗针叶对臭氧损害的敏感性不仅随针叶年龄变化，还随针叶位置变化，而且光合能力的降低可能并非直接归因于污染物吸收的增加。数据还表明，在估计整株植物的碳增益和臭氧吸收时，需要考虑嫩枝内的变化。
