Acidity of Passion Fruit as Affected by Potassium Fertilizer

Passion fruit generally has high citric acid content. Less-acidic passion fruit is increasingly in demand for fresh consumption. In this study, the effect of potassium (K) fertilizer on the fruit quality, especially acidity, was examined. Two-year-old passion fruit plants were grown in 1/5000-a Wagner pots filled with river sand. Nutrient solutions containing 0, 6.3, 12.5 and 25.0 mM K were applied from June 6 to September 6. Artificial pollination was conducted from June 17 to July 6 and the number of fruits per plant was regulated to be less than six. After harvest, fruit weight and dimensions, titratable acidity (TA), total soluble solid (TSS), juice content percentage, and days after pollination to harvest (DAP) were measured. Juice K content and its relationships with TA or TSS were determined. Leaf greenness (SPAD index) was measured monthly, after which leaves were sampled to determine N, K, Ca, Mg and Fe contents. TA was lower at lower K application rate. Juice having low K content had low TA. TSS was low at zero K rate, although there was no statistical relationship between TSS and juice K content. DAP increased at lower K rates. Elongation of DAP may be one of the reasons of TA decrease. K application rate had no effect on juice content percentage, fruit weight and dimensions. Lower leaf K concentration occurred at lower K application rate. With zero K rate, leaf K content was 1.4-2.2% at harvest. Leaf Ca and Mg contents were higher at lower K rates. Leaf Fe and N contents and SPAD index were not related to K rate. Neither deficient nor excess K symptoms were observed. These results indicated that keeping leaf K content at around 2.0% might be effective to produce less-acidic passion fruit.