Influence of leaf angle on photosynthesis and the xanthophyll cycle in the tropical tree species Acacia crassicarpa.

We examined the effects of artificially altering leaf angle of the tropical tree species Acacia crassicarpa (A. Cunn. ex Benth., Fabaceae) on light interception, leaf temperature and photosynthesis in the wet and dry seasons of tropical Australia. Reducing leaf angle from the natural near-vertical angle (90 degrees ) to 67.5, 45, 22.5 and 0 degrees greatly increased light interception and leaf temperature, and decreased photosynthetic activity. Compared with the 90 degrees phyllodes, net photosynthetic rates in the horizontal phyllodes decreased by 18 and 42% by the second day of leaf angle change in the wet and dry seasons, respectively. The corresponding values for Day 7 were 46 and 66%. Leaf angle reduction also altered the diurnal pattern of photosynthesis (from two peaks to one peak) and reduced daily CO2 fixation by 23-50% by Day 2 and by 50-75% by Day 7 in the dry season. In contrast, the xanthophyll cycle pool size in the phyllodes increased with leaf angle reduction. Thus, there are at least five major advantages to maintaining high leaf angle orientation in tropical tree species. First, it reduces excessive light interception. Second, it lowers leaf temperature. Third, it protects the photosynthetic apparatus against photodamage by excessive light. Fourth, it minimizes xanthophyll cycle activity and reduces the cost for xanthophyll biosynthesis. Finally, it enhances photosynthetic activity and helps to sustain high plant productivity.