Abstract
Chlorophyll fluorescence is a commonly used indicator of photosynthetic capacity. Crassulacean acid metabolism (CAM) plants absorb CO 2 during the night, but nighttime gas exchange alone does not readily represent CAM. This study was conducted to determine how light levels, daytime electron transport rate (ETR), and nighttime CO 2 uptake are related. Phalaenopsis Queen Beer ‘Mantefon’ clones were treated with a combination of three light intensity levels of 50, 100, and 200 µmol∙m ⁻² ∙s ⁻¹ and three daylengths: 8 h, 12 h, 16 h, resulting in daily light integrals (DLI) between 1.44 and 11.52 mol∙m ⁻² ∙d ⁻¹ . Daytime ETR linearly increased with increasing DLI levels; however, nighttime CO 2 uptake reached saturation point around 6 mol∙m ⁻² ∙d ⁻¹ , and then decreased. As a result, daytime ETR and nighttime CO 2 uptake were linearly correlated within 1.44 to 5.76 mol∙m ⁻² ∙d ⁻¹ DLI levels in this study. Under 12 h of daylength, despite the higher DLI level and ETR values, the amounts of nighttime malic acid accumulation in 200 µmol∙m ⁻² ∙s ⁻¹ treatment were similar to those in 100 µmol∙m ⁻² ∙s ⁻¹ treatment. On the following day, when all plants were exposed to uniform light conditions of 100 µmol∙m ⁻² ∙s ⁻¹ , they exhibited identical ETR values, despite significant differences in CO 2 uptake rates observed in some plants the previous night. These results indicated that the daytime DLI level is a major factor for nighttime CO 2 uptake, but DLI levels over a specific range have minimal effects on the carbon-gaining of CAM orchid Phalaenopsis.