Paclobutrazol, Water Stress and Nitrogen Induced Flowering in ‘Khao Nam Phueng’ Pummelo

Abstract

Thailand can export pummelo for only a short period from August to September each year. However, the demand is increasing and lasts almost all year round. Hence, success in controlling the flowering time will expand the export window. In this study, flower induction of 2-year-old pummelo
trees cv. Khao Nam Phueng was investigated using five treatments: T1) daily watering (control), T2) water stress by withholding water for 12 d in December, T3) spraying with 750 mg/L paclobutrazol (PBZ) twice at an interval of 15 d, T4) spraying with 750 mg/L PBZ as in T3 combined with withholding water for 12 d and T5) urea fertilizer (46-0-0) application 4 times every 2 wk at the rate of 25 g/tree before the experiment started combined with withholding water for 12 d. The results showed that with the water stress treatments, the soil moisture in the root zone decreased from 28 to 14% and the leaf water potential (LWP) decreased from -0.66 to -2.17 MPa at the end of water stress and returned to normal after rewatering. PBZ application was highly effective in reducing shoot length and could reduce the effect of water stress by maintaining LWP. Urea application combined with water stress could induce early flowering 1 wk after rewatering, while the control trees did not flower. The water stressed trees had the greatest amount of inflorescence. The inflorescence types were classified into four types and most inflorescence had many flowers with leafy shoots. Water stress and PBZ increased the leaf total non-structural carbohydrates (TNC) and the C/N ratio at the end of water stress, which then slightly decreased until the flushing period, while total nitrogen (TN) decreased and tended to increase until flowering. The results indicated that trees treated with PBZ had comparable flowering and TNC and C/N ratio values to the water stressed trees, while urea applied before water stress stimulated early flowering. Therefore, the application of PBZ and urea combined with water stress to induce flowering of pummelo trees may be a useful development for commercial production.