Effect of potassium nitrate application on the flowering and fruit quality of mango varieties grown in Uganda

Abstract

The mango (Mangifera indica L.) is a tropical fruit belonging to the genus Mangifera. It is called the king of fruits because of its taste and super flashy yellow colour, and health benefits it offers. A study was conducted to evaluate the effect of potassium nitrate (KNO3) application on the flowering and nutritional fruit quality of mango varieties grown in Uganda. Specific objectives of the study were to determine the flowering response of mango varieties growing in selected agroecological zones of Uganda to the application of KNO3, and to evaluate the effect of applying KNO3 on the nutritional quality of mango fruits. The study was superimposed on existing mango orchards which were planted earlier at the same time in three locations, namely NaCRRI Namulonge, NaSARRI in Serere and BUZARDI in Hoima. It was laid out in a two factorial randomized complete block design (RCBD) in a split plot arrangement and replicated three times on each site. The first factor comprised three mango varieties, namely Bire, Tommy Atkins and Zillate, while the second factor comprised four concentrations (zero as the control, 1, 2 and 4%) of KNO3. The results of experiment 1 showed that the response to KNO3 application by the mango varieties was significantly (P<0.05) different across location, KNO3 treatments and varieties. Tress of all the varieties sprayed with KNO3 produced higher numbers of terminal buds compared to the control. Across locations, variety Bire produced a significantly (P<0.05) higher number of terminal buds (64.8) than Tommy Atkins (46.3) and Zillate (17.8). Regarding flowering, variety Bire with 28.6% performed better (P<0.05) than Tommy Atkins (20.8%) and Zillate (17.8%). The percentage flowering response of trees of all the varieties sprayed with 2% KNO3 (31.4%) was higher (P<0.05) than that of trees that received 1% KNO3 (24.7%) but was not different from that of trees that received 4% KNO3 (28.8%). The number of fruits induced per 20 panicles per tree in trees that were sprayed with 4% KNO3 (8.24) was higher (P<0.05) than that of trees that were sprayed with 1% KNO3 (4.8). Fruit yield from trees of Tommy Atkins (23.01 kg) was higher (P<0.05) than that of Bire (10.97 kg) but was similar (P>0.05) to that of Zillate (19.46 kg). Fruit yield of trees sprayed with 2% KNO3 (27.36 kg) was higher (P<0.05) than that of trees that were sprayed with 1% KNO3 (15.93 kg) but was like that of trees sprayed with 4% (26.24 kg). The results of experiment 2 showed that spraying mango trees with KNO3 significantly (P<0.05) affects the quality attributes of mango fruits, namely the acidity (pH), citric acid content (TTA), total soluble solids (TSS), 0brix/Acid ratio, total phenolic content (TPC) and total antioxidants. Across location, the acidity of fruits from Zillate (2.96) was lower (P<0.05) than that of Tommy Atkins (3.22) and Bire (3.08). Citric acid content was higher (P<0.05) in the fruits of Tommy Atkins (0.46%) than in fruits of Bire (0.30%) and Zillate (0.32%). The 0Brix/acid ratio and total phenolic content of Bire (46.8 and 28.6) were higher (P<0.05) than those of Tommy Atkins (39.75 and 22.9) and Zillate (42.4 and 22.8), respectively. Also, total antioxidants of fruits of Tommy Atkins (45.7) were higher (P<0.05) than those Bire (44.0) and Zillate (44.4). Across location, trees sprayed with 2% KNO3 produced fruits with the lowest pH (2.89) compared to those that were sprayed with 1% (3.26) and 4% KNO3 (3.03). Also, fruits from trees sprayed with 1% and 2% KNO3 contained higher (P<0.05) citric acid content (0.34%) than fruits from trees that were sprayed with 4% KNO3 (0.30%). Trees that were sprayed with 4% KNO3 produced fruits with higher (p<0.05) 0Brix/acid ratio (44.6) and total antioxidant activity (46.08) compared to those sprayed with 1 % (40 and 44.51) and 2 % (37.9 and 43.87), respectively. Trees sprayed with 2% KNO3 produced fruits with higher (P<0.05) (24.52) TPC compared to those sprayed with 4% (23.96). Based on the results of the study it was recommended that since varieties Tommy Atkins and Zillate performed equally well in fruit yield across locations, farmers can grow either of the varieties. However, they should apply either 2 or 4% KNO3 for better fruit yield. In addition, Tommy Atkins performed well in producing fruits with most of the quality attributes (high pH, total soluble solids, total phenolic content & total antioxidant activity) across locations, thus farmers can grow the variety but should apply 4% for better quality attributes.