Photosynthesis and Vegetative Growth of Phalaenopsis 'Hwasu 3551' and 'White-Red Lip' in Response to Low Temperature during Nursery Cultivation

We investigated the effects of low temperature on photosynthesis and vegetative growth during nursery cultivation of two Phalaenopsis hybrids, ‘Hwasu 3551’ (HS) and ‘White-Red Lip’ (WR). One-month-old plants were grown in a growth chamber at 28/26^oC and transferred to low temperature at 21/19^oC every two weeks for 28 weeks. Diurnal net CO₂ uptake pattern of HS and WR was observed into four phases; phase I (night), phase II (early morning), phase III (morning and afternoon) and phase IV (late afternoon). Regardless of transferring time, in phase I, HS and WR reached the highest net CO₂ uptake rate of ≈6.65 μmol·m^-2·s^-1, showing typical crassulacean acid metabolism (CAM) photosynthesis under both 28/26^oC and 21/19^oC. During phase II and IV, in HS and WR, CO₂ absorption rate under 28/26^oC showed C₃-CAM pattern while that under 21/19^oC showed CAM. In phase III, two cultivars released CO₂ under 21/19^oC, whereas CO₂ uptake maintained at near zero under 28/26^oC. When plants were transferred at 0 week and cultivated at 21/19^oC for 28 weeks, CO₂ uptake of HS and WR during the daytime increased by 190% and 938%, respectively, compared to that of HS and WR transferred at 16 week. At 30 weeks after the treatment, when plants were transferred at 24, 26 and 28 weeks, leaf length and leaf area of HS were averagely 3.43 cm longer and 19.74 cm² larger, respectively, than those transferred at 0, 2, 4, 6 and 8 weeks. In case of WR, leaf length and leaf area of the plants transferred at 24, 26 and 28 weeks averagely 2.89 cm longer and 14.92 cm² larger, respectively, than those transferred at 4, 10, 12, 16 and 22 weeks. These results indicated that the diurnal CO₂ uptake pattern of two Phalaenopsis hybrids changed from C₃-CAM to CAM pathway more rapidly when plants were transferred to low temperature regardless of transferring timing. When plants were transferred to low temperature after at least 24 weeks of warm temperature, low temperature does not significantly reduce the vegetative growth rate of Phalaenopsis hybrids HS and WR during nursery cultivation.