The Effects of Leaf Age on Photosynthesis in Rose1

Abstract

For all leaf ages of Rosa hybrida L. cv. Samantha, maximum photosynthetic rates were reached at irradiances of 450 to 500 μE m −2 sec −1 . The magnitude of this response decreased from 20.6 mg CO 2 dm −2 hr −1 in the youngest leaves studied to 15.4 mg CO 2 dm −2 hr −1 in the oldest. Maximum photosynthetic rates were reached before full leaf expansion. Mesophyll resistance, however, increased with age from 7.8 to 12.5 cm sec −1 . CO 2 compensation points and dark respiration decreased throughout most of leaf development, but increased slightly at the most advanced developmental ages studied. Photosynthetic enhancement, the percent increase in net photosynthesis at 2% O 2 compared to 21% O 2 , averaged 33% for all leaves. There was no change in the amount of photosynthetic enhancement as leaves aged, indicating that changes in photorespiration were not a major factor in photosynthetic trends over the range of leaf ages examined.