Effect of Calcium Nutrition on Yield, Jelly Seed and Postharvest Quality of Mango Fruits

Abstract

Mango (Mangifera indica L.) is an important fruit in Kenya for domestic and export markets. However, farmers attain low yields and suffer significant post-harvest losses. Physiological disorders, among them jelly seed, contribute to about 30-40% of post-harvest losses by reducing the marketability of fruits. Optimum yields, fruit quality at harvest and postharvest longevity of mango fruits require proper nutrient management. Some of the physiological disorders that contribute to postharvest losses in mango are attributed to poor calcium nutrition. A study was conducted at Karurumo in Embu County, Eastern Kenya during seasons 2017/2018 and 2018/2019 using Van Dyke cultivar with the following objectives: (1) To determine the effect of varied calcium sources, rates and timing of application on yield of mango fruits; (2) To determine the effect of varied calcium sources, rates and timing of application on jelly seed incidence, post-harvest quality and shelf life of mango fruits; (3) To determine the effect of pre and post-harvest applications of calcium chloride on the quality and shelf life of physiologically mature mango fruits. A randomised complete block design with a split-split plot arrangement replicated three times was used to achieve objectives 1 and 2. Calcium was supplied as calcium nitrate, easygro® and calcium chloride at concentrations of 1.0%, 1.5%, 2.0% or 0% (control) at three different times (fruit set, 30 days after fruit set and 30 days to physiological maturity). For objective 1, fruits were harvested at physiological maturity and fruit length, breadth, number of fruits, fruit weight, total weight, fruit retention percentage and fruit tissue calcium concentration were determined. For objective 2, a sample of physiologically mature fruits was taken and ripened in ambient conditions for determination of jelly seed occurrence, fruit weight and calcium distribution in the exocarp, mesocarp, endocarp and cotyledon. Additionally, samples of mature fruits were taken from each treatment for determination of total titratable acidity, total soluble solids, peel colour and peel firmness at harvest and after 12 days of storage under ambient conditions (25±2C, 75-80% RH), using standard protocols. Additionally, untrained panelists were used to score the ripened fruits for selected sensory quality attributes. Objective 3 was achieved by laying out a factorial experiment in a randomized complete block design with a split plot arrangement, replicated three times. Calcium chloride at 0.5%, 1.0%, 1.5% or 0% (control) was sprayed on fruits at maturity or 15 days later. Another set of mature fruits was immersed in calcium chloride (0.5%, 1.0% or 1.5% or 0%) for 10 (ten) minutes. The fruits were then stored in ambient conditions and evaluated for selected shelf life indicators and sensory quality characteristics at an interval of two days for an eight days storage period. Peel firmness, total titratable acidity, total soluble solids, flesh colour, beta carotene, and percentage change in fruit weight, carbon dioxide and ethylene evolution rates were assessed. Results indicated that calcium sprayed fruits had higher length, weight, breadth, fruit number, total weight of fruits/tree, retention percentage and calcium content than unsprayed fruits. Calcium chloride (2.0%) sprayed at fruit set was the most effective in enhancing the mean fruit weight (346.3 g and 316.4 g), and the mean total weight of fruits/tree (63,723g and 39,138g) in both seasons respectively. There was a direct correlation between calcium concentration and fruit length (r=0.56 and r=0.81), fruit weight (r=0.34 and r=0.73), fruit breadth (r=0.79 and r=0.88), number of fruits (r=0.86 and r=0.59), fruit retention percentage (r=0.52 and r=0.62) and total weight of fruits (r=0.75 and r=0.68) in both seasons respectively. Application of calcium at fruit set increased calcium content in fruits more than application at later stages of fruit development. Application of calcium chloride, 2.0%, at fruit set stage reported the lowest average jelly seed score in season 1 (1.2) and season 2 (1.2). A significant negative correlation between fruit weight (r = -0.55, r = -0.52), calcium content in the exocarp (r = -0.56, r = -0.49), mesocarp (r = -0.52, r = -0.76), endocarp (r= -0.76, r = -0.66) and jelly seed incidence occurrence was demonstrated in both seasons respectively. Fruits sprayed with calcium chloride (2.0%) at fruit set maintained higher peel firmness (4.83 N, 4.77 N), titratable acidity (1.29%, 1.27%), peel hue angle (67.9, 67.2) and total soluble solids (10.47 o Brix, 9.10 o Brix) than all other treatments in both seasons respectively. Calcium chloride application improved peel colour appearance while calcium nitrate and easygro® led to a deteriorated peel colour appearance. Further, high rates of all sources of calcium led to a deterioration in taste of fruits. Immersion of fruits in calcium chloride (1.5 %) at maturity registered a higher peel firmness (10.6 N, 10.3 N), retained flesh colour (37.45, 36.78), highest TSS (14, 13.8), TTA (0.72%, 0.70%) and higher color appearance scores than the rest of the treatments in both seasons respectively. Fruits sprayed with calcium chloride (1.5%) at maturity registered the lowest amount of carbon dioxide (30.7 ml/kg/hr) and ethylene (1.5 ml/kg/hr). Post-harvest immersion had negative effects on the pulp flavor and increased shriveling of fruits. Spraying of calcium chloride (2.0%) at fruit set gave the highest yield, suppressed jelly seed occurrence, maintained fruit shelf life and improved organoleptic attributes. Calcium applied at fruit set gave better results in the studied parameters than those sprayed later implying that calcium should be applied at early stages of fruit development. Additionally, fruits immersed in calcium chloride (1.5%) enhanced fruit shelf life. However, for good flavor and taste, rate of 1.0% are recommended when fruits are treated by immersion in calcium chloride. This study further demonstrated that calcium is available to the fruit particularly when applied by immersion rather than spraying.