Growth, yield and functional components of pummelo (Citrus maxima (Burm. ex Rumph.) Merr.) as influenced by potassium

This study aimed to determine the influence of K fertilization on the vegetative growth, flowering, fruit development and quality, yield, and functional components of 13-year old 'Magallanes' pummelo [Citrus maxima (Burm. ex Rumph.) Merr.] trees. The experiment was conducted at South Davao Corporation (SODACO) farm, Davao City from April to September 2010. Five treatments with increasing K levels were applied per tree: control, no K, 150 g K basal, 225 g K basal, and 225 g K basal + foliar application. The vegetative growth of pummelo was not affected by increasing K rates. The recommended rate (150 g K basal per tree) significantly increased canopy diameter by 76% but higher K rates did not increase tree height due to the effect of fruit load and nutrient competition. On the other hand, increasing K rates to 50% of the recommended rate (225 g K per tree) increased the number of flushes by 36-146%. Flowering, fruit set and yield of pummelo were also enhanced by higher K rates. Flowering was increased by 4 times, fruit set by 86-117%, fruit number by 7 times, weight per fruit by 22-26%, and yield up to 9 times on trees applied with 225 g K rates. The fruit size (diameter and length), however, did not increase by increasing the K level. Application of 225 g K rates also positively influenced fruit quality and functional components of pummelo. Fruit pulp diameter increased by 36% while peel thickness decreased by 29-75%. An increase of 39% juice weight per fruit was obtained in 150 g K only; however, the yield of juice per tree increased up to 11 times with 225 g K level. The 225 g K rates also increased juice pH by 8-12%, titrable acidity (TA) and total soluble solids (TSS) per tree by 9 times and 6 times, respectively. The yield of total phenol, flavonoid, vitamin C and oil per tree increased by 3-10 times with 225 g K application. The effects of basal alone and foliar + basal application of K were only significantly different from each other in terms of number of flushes, fruits that set, juice and flavonoid yield per tree. On the other hand, the application of 225 g K basal + foliar resulted to higher number of flowers, % fruit set, number of fruits, fruit weight, yield, TA, TSS, total phenol, vitamin C and oil, yield per tree in pummelo, implying a higher mobilization of K in the leaves than K uptake by the roots. The results of the study indicated the important role of K in improving the yield, quality and functional components in 'Magallanes' pummelo. The critical stage of tree growth coincided with the months of May and August. Peak leaf flushing occurred in June and peak flowering with fruit set in April. The crucial stage of fruit development was two months after fruit set. Thus, optimum fertilization must be applied during these stages for maximum growth and production in pummelo.