Abstract
This study was conducted to determine the effects of temperature variations on photosynthetic ability, carbohydrate contents in the leaf and stem, and inflorescence initiation and the effects of exogenous sucrose treatment on the inflorescence initiation in Phalaenopsis. Twelve-month-old Phalaenopsis Queen Beer ‘Mantefon’ clones were grown at 17, 20, 23, or 28 °C. Photosystem II (PSII) operating efficiency and CO2 uptake rate were measured at 3, 7, 14, and 28 days after treatment (DAT), and carbohydrate content in the stem and leaf was analyzed at 14 and 28 DAT. Also, 0, 20, 40, and 80 g·L⁻¹ of sucrose solution were foliar-treated. Forcing treatments at 17, 20, and 23 °C induced inflorescence emergence, while treatment at 28 °C completely inhibited inflorescence initiation and also significantly increased the number of new leaves compared with other temperatures. Days to first visible inflorescence (VI, inflorescence longer than 0.5 cm) were 85.7, 44.2, and 45.8 at 17, 20, and 23 °C, respectively. PSII operating efficiency and CO2 uptake rate significantly decreased at 17 °C, and these values were similar between 20 and 23 °C throughout the treatment period. Carbohydrate content in the stem showed no difference among temperature or duration variable, while the content of soluble sugars in the leaf increased with forcing temperature. Sucrose content in the leaf at 14 DAT significantly increased at 20 and 23 °C compared with that at 28 °C, and a similar trend was observed in glucose, fructose, and sucrose at 28 DAT. Although the difference was not statistically significant, soluble sugar content at 17 °C was slightly higher than at 28 °C. Plants at 28 °C showed higher PSII operating efficiency and CO2 uptake rate but had lower carbohydrate content than in flowering-induced plants. Days to first VI showed a negative correlation with photosynthetic parameters and sucrose content in the leaf. Although exogenous sucrose treatment at 28 °C did not substitute for the forcing temperature, the treatment at 20 °C accelerated inflorescence emergence. These results indicate that inflorescence initiation by forcing temperatures induces carbohydrate accumulation in the leaves, and inflorescence emergence timing is correlated with content of soluble sugars.