Effects of leaf age, inflorescence, temperature, light intensity and moisture conditions on CAM photosynthesis in Phalaenopsis

Effects of some factors on CAM (Crassulacean acid metabolism) photosynthesis in Phalaenopsis were investigated by measuring diurnal CO2 exchange patterns. Following results were obtained.1. The diurnal CO2 uptake pattern of Phalaenopsis leaves is nearly typical of CAM photosynthesis. Leaf age had no influence on CAM photosynthesis; the youngest leaf absorbed slightly more CO2 during the day and less at the night than did mature leaves. Although the third and forth leaves were shaded by the upper leaves, they fixed CO2 via CAM photosynthesis at rates similar to the upper two leaves fully exposed to light.2. Formation of inflorescence did not influence the diurnal pattern of CO2 uptake of CAM photosynthesis. But the plants without inflorescence had lower activities in CO2 uptake via CAM photosynthesis than those with inflorescence.3. When the day and night temperature were kept constant, 20°C was the optimum for the night time CO2 uptake. Although the day time CO2 uptake decreased as temperature increased, total CO2 uptake during 24 hr was maximized at 20°C. Higher day and lower night temperatures (25°15°C) were more favorable than a constant 20°C with respect to CO2 assimilation of CAM photosynthesis at night or during the entire day. Low day time temperature of 10°C depressed CO2 uptake rates at night and promotes during the day so that the Phalaenopsis leaves approximated those of plants with C3-photosynthetic system.4. The day and night time CO2 uptake increased with higher light intensity up to 130μE/m2/sec. Above 130μE/m2/sec the capacity of leaves to assimilate CO2 at night was saturated and that during the day became lower. Therefore, CAM photosynthesis in Phalaenopsis has a lower light saturating point than other plants.5. Under our growing conditions, withholding water for 10 days resulted in a severe water stress which significantly depressed CO2 uptake both day and night.