There is good evidence that air pollutants may exert significant impacts on plant productivity by altering the partitioning of dry matter between plant parts (10,19,24,25). In addition to altered partitioning of carbohydrates between plant parts, the costs of repair from air pollution as well as disease-related stress may reduce carbohydrates available for growth by increasing the costs of repair of damage to cellular or metabolic systems (11). Studies with several plant species have indicated that the internal costs of maintaining leaf functions are high (11). These maintenance costs would be expected to be enhanced by exposure to pollutants and associated metabolic or cytologic injury.Several recent studies under both laboratory (8, 18, 22, 24) and field (14, 15) conditions have indicated that the processes of carbohydrate translocation may be both susceptible to exposure to air pollutants and useful as general indicators of pollutionrelated stress.In general, translocation has been reduced by exposure to air pollutants, however, with very low SO2 concentrations (0.08 ,ul I-') Milchunas et al. (15)

Seasonal patterns of change in lipids, sugars, starch, labile (ethanol soluble) constituents, holocellulose, and lignin were studied in six forest-grown white oak (Quercusalba L.) trees. Contents of metabolically active constituents in leaves, twigs, branches, boles (upper and lower), and roots (support and small lateral) were used to construct whole-tree budgets of energy allocation. [14C]Sucrose was also concurrently supplied to the study trees to follow the fate and efficiency of utilization of food reserves. Results showed that white oak rapidly mobilized and replaced food reserves during the critical period of canopy generation in the spring. Starch was more important as a reserve food than lipids or sugar. Large fluctuations in starch in roots in spring and fall suggested a bimodal belowground growth pattern. Labile constituents showed the most pronounced seasonal changes and dominated the calculated whole-tree energy flux patterns. Rapid decline in labile compounds in early spring and a parallel increase in holocellulose suggested a possible pattern of mobilization and resupply of stored reserves associated with in cell wells. This possibility was supported by a concurrent shift of labile 14C to nonlabile 14C in tissues. Canopy generation was calculated to have cost ≤17.7 kg of glucose (1.6 g glucose/g of canopy) of which 13 kg appeared to have come from within the canopy.

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The fate of photoassimilated 14C was followed by measuring 14C incorporation into leaf and branch tissues (≤ 5 years old) of two forest-grown white oak trees. Fate of 14C-labelled photosynthate was examined 7 days after 14CO2 uptake on live dates (April–October) during the growing season. Both upper and lower canopy positions were sampled. Incorporation of 14C into foliage was significant throughout the growing season. It ranged from 95% of the total 14C retained in April to 50% in October. Incorporation of 14C-labelled photosynthate into the canopy was highest in June and averaged 33% of gross photosynthetic production over the entire growing season. Higher retention of photosynthate in branches versus leaves was noted in the upper canopy than in the lower canopy during the middle and late growing season. Activity levels in tissues indicated that within-canopy sink strength was in the order acorns ≈ buds > leaves > branches. Translocation of initial 14C-labelled photosynthate from both leaves and branches was calculated based on 14C retention and estimated respiratory losses of leaf and branch tissues. These calculations indicate that leaves were still importing significant amounts of photosynthate in April when expansion was two-thirds complete. Translocation of 14C from the canopy during the remaining growing season amounted to 25–45% of gross photosynthesis. Our data indicate that seasonal demands of growth and maintenance of the forest canopy constitute a substantial sink for photosynthate allocation by white oak.

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