Whole-plant 14C-photosynthate allocation in Pinustaeda: seasonal patterns at ambient and elevated ozone levels

Abstract

The seasonal patterns of carbon gain and allocation were examined in Pinus taeda L. seedlings grown under field conditions. To investigate how ozone stress may influence whole-plant carbon budgets over the growing season, the seedlings were grown in either ambient air or air enriched with ozone at twice-ambient levels. On five sampling dates during the 1987 growing season, seedlings were labeled with 14CO2, and whole-plant carbon budgets were constructed. Rate of assimilation of CO2 varied by a factor of 2 during the growing season, with a late spring maximum during the first growth flush. Respiratory losses were highest in the spring and then declined sharply during the summer when photosynthate allocation to the foliage increased rapidly. A second major shift in the carbon budget occurred in the autumn when allocation to the fine roots increased at the expense of the foliage. The proportion of photosynthate allocated to coarse roots and stems varied only slightly over the growing season. Allocation to any plant component was highest when growth of that component was at a maximum. No statistically significant effects of elevated ozone on either carbon gain or photosynthate allocation were detected at any specific time during the growing season. However, seedlings grown at twice-ambient ozone levels consistently exhibited the following trends: (i) lower rates of CO2 assimilation, (ii) greater allocation of photosynthate to respiration, and (iii) corresponding reduction in photosynthate allocation to fine roots. An individual-fascicle 14C-labeling technique was found to reflect the seasonal patterns of carbon import and export by foliage and thus may serve as an acceptable surrogate for whole-tree tagging. The pronounced seasonality of the carbon budgets in P. taeda in conjunction with a pattern of ozone effects on carbon assimilation and photosynthate allocation suggests that whole-plant carbon budgets are sensitive and biologically meaningful indicators of seedlings'' responses to anthropogenic changes in atmospheric chemistry.