Changes in soluble carbohydrates during phytochrome-regulated petiole elongation in watermelon seedlings

Changes in soluble carbohydrate composition and concentration in leaves and petioles of watermelon (Citrullus lanatus (Thunb) Matsum and Nakai cv. Sugar Baby) seedlings during early stages of phytochrome-regulated petiole elongation were investigated. Watermelon seedlings were grown in a controlled environment with 350 μmol m^-2 s^-1 photosynthetically active radiation (PAR) during a 12-h photoperiod. Low intensity end-of-day (EOD) light treatments (for 15 min) of red (R), far-red (FR) and FR followed by R (FR/R) were initiated when the seedlings were 14 days old. Seedling growth, and soluble carbohydrate concentration and composition in leaves and petioles were determined after 3 and 6 days of EOD light treatments. The EOD FR increased the petiole length and dry mass partitioned to petioles as early as 3 days into the treatment. This increased petiole dry mass in FR-treated plants was accompanied with an increase in reducing sugar (glucose and fructose) concentration in the petioles. Although both leaves and petioles showed this effect, the relative increase was greater in petioles than leaves. While the most abundant sugars in petioles were fructose and glucose, the predominant sugars in leaves were sucrose, raffinose, and stachyose. The photoreversion of FR induced changes in growth and sugar concentrations by R indicates the involvement of phytochrome in these processes.