Photosynthetic light requirements and effects of low irradiance and daylength on phalaenopsis amabilis

Phalaenopsis has become one of the most important potted plants around the world. Thus, we used a key commercial Phalaenopsis amabilis cultivar, TS97, as a model to determine the light requirements for maximal carbon fixation and photosystem II (PSII) efficiency in its leaves and to investigate the effects of low irradiance and daylength on photosynthesis and flower development. In mature 'TS97' leaves, the daily total CO₂ uptake capacity and net acid fixation increased with increasing photosynthetic photon flux (PPF) and saturated at ≈200 μmol·m^-2·s^-1, whereas the fluorescence ratio values were significantly reduced to 0.68 to 0.75 above 325 μmol·m^-2·s^-1 PPF, indicating photoinhibition of PSII. Positive assimilation of the nocturnal CO₂ uptake occurred at a very low PPF (less than 5 μmol·m^-2·s^-1), suggesting highly efficient use of light energy by 'TS97' plants. Leaves developed under 30 μmol·m^-2·s^-1 PPF exhibited lower light requirement of 125 μmol·m^-2·s^-1 PPF to reach maximal CO₂ uptake, below which the daytime CO₂ uptake declined dramatically. Under a 12-hour daylength, exposing the leaves to a low PPF for 4 hours at any time during the day did not affect the photosynthetic capacity in 'TS97' leaves, suggesting that 8 hours of optimal irradiance is required for high-level photosynthesis, whereas the 12-hour daylength resulted in a higher CO₂ uptake rate and the daily total CO₂ uptake than the 8-hour daylength. Moreover, the 12-hour daylength promoted earlier flower formation and higher flower count compared with the 6- to 8-hour daylengths. Longer daylengths neither accelerated flowering formation nor enhanced total flower count. In conclusion, 8 hours of saturating PPF at 200 μmol·m^-2·s^-1 and a 12-hour daylength are sufficient for maximizing photosynthesis and flower production in 'TS97' plants.