Changes in photosynthetic characteristics during leaf development in apple.

A comprehensive developmental survey of leaf area, chlorophyll, photosynthetic rate, leaf resistance, transpiration ratio, CO2 compensation point and photorespiration was conducted in apple. The largest changes in each of the photosynthetic characteristics studied took place during the earliest stages of leaf development, coinciding with the period of greatest leaf expansion and chlorophyll synthesis. During early development, photosynthesis increased 5-fold, reaching a maximum rate of 40 mg CO2 dm(-2) hr(-1) at a leaf plastochron index (LPI) of 10. During this same period, leaf resistance, transpiration ratio, CO2 compensation point and mesophyll resistance decreased, while carboxylation efficiency increased. Two especially interesting aspects of the data discussed are simultaneous changes that occur at a LPI of 10 and 12 in all of the photosynthetic characteristics examined and an apparent decrease in photorespiration as leaves age. From our results it is clear that stage of leaf development is an important factor affecting the rate of photosynthesis and photorespiration.