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  • Author or Editor: Woei-Jiun Guo x
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In this study, effects of leaf age (20 to 240 days), plant age (4, 8, and 14 months after deflasking), and various day/night temperature regimes (16 to 33 °C) on photosynthesis of Phalaenopsis amabilis L. Blume var. formosa Shimadzu (Phal. TS97) leaves were investigated. The diurnal net CO2 uptake in Phal. TS97 leaves was measured and integrated to obtain total net CO2 uptake, which represents photosynthetic efficiency in plants performing crassulacean acid metabolism (CAM). Under all conditions, Phal. TS97 leaves performed typical CAM photosynthesis and reached their highest net CO2 uptake rate, ≈6 μmol·m-2·s-1, after 3 to 4 hours in the dark under a 12-hour photoperiod. When grown under 30 °C day/25 °C night temperature, the total net CO2 uptake of leaf increased with maturation and was highest at 80 days old, 20 days after full expansion. The CAM photosynthetic capacity of mature leaves remained high after maturation and began to decline at a leaf age of 240 days. The trend was consistent with malate fixation but the highest nocturnal malate concentration was observed in 100-day-old leaves. Young leaves or leaves from small juvenile plants had higher daytime CO2 fixation compared to mature leaves or large plants, suggesting that Phal. TS97 leaves progressed from C3-CAM to CAM during the course of maturation. The second newly matured leaf from the top had the highest net CO2 fixation when the newest leaf was 8 cm in length. Although plant age did not influence total CO2 uptake in the leaf, photosynthetic efficiency of leaves in small younger plants was more sensitive to high light intensity, 340 μmol·m-2·s-1 photosynthetic photon flux. The day/night temperature of 32/28 and 29/25 °C resulted in the highest total net CAM CO2 fixation in vegetative Phal. TS97 plants than higher (33/29 °C) and lower temperatures (21/16 °C).

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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.

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