Accentuation of gas exchange gradients in flushes of ponderosa pine exposed to ozone

Two-year-old ponderosa pine seedlings (Pinus ponderosa Laws.) were exposed to episodic O3 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 O3 exposure, gas exchange was measured on needles of surviving flushes at saturating CO2 and photosynthetic photon flux density (PPFD). Both photosynthetic capacity (Asat) 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 CO2, similar patterns of gas exchange were also found at 350 micromoles mol-1 CO2, indicating that photosynthesis of needles at the bottom of the flush was more strongly affected by O3 than that of needles at the top of the flush, even though the potential for O3 uptake was probably less in needles at the bottom of the flush because of reduced stomatal conductance. Carboxylation efficiency (delta A/delta Ci), the linear slope of the A/Ci response, was highly correlated with Asat, varying with needle age, needle position in the flush and O3 exposure, but the magnitude of the reductions was greater than for Asat. We conclude that susceptibility to O3 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 O3 uptake.