In deciduous trees, leaves not only are the organs of carbon assimilation but function as temporary stores of the recent photosynthates. During the day, carbon is exported to the rest of the plant, however starch also accumulates in the leaf so that carbon can continue to be exported during the night. Leaf carbohydrate levels should therefore be affected both by the net daily carbon uptake of the individual leaf as well as the demand for carbon from the rest of the plant. Changes in a tree's daily net assimilation may then be apparent in the pattern of leaf carbohydrate levels throughout the day and in differences between sunny and cloudy days. To test if the daily pattern of carbohydrate accumulation accurately reflected the daily pattern of photosynthesis, we repeatedly measured starch and soluble sugar levels in leaves from the top of several individuals of Quercus velutina throughout the day coupled with leaf-level ambient photosynthesis measurements.

Results/Conclusions

In general, leaf starch levels increased through most of the day and were significantly higher in the evening than at the beginning of the day. However, there was significant variation in the morning and evening starch levels between days, which corresponded well with the day's weather conditions. There was also variation in the difference between the starch levels at the end of the day and the following morning. This suggests that carbon exported from the leaves at night may partially depend upon the previous day's carbon gain. As measuring daytime carbon assimilation for large trees may be logistically difficult, diel changes in leaf starch levels may be useful as a proxy, as well as a measure of the balance between the supply of new assimilates from the leaf and the demand from the rest of the plant.