Influence of fertilizers, harvest maturity, polyethylene bunch covers and postharvest treatment with 1-Methylcyclopropene on physical, physiological and biochemical quality of tissue-cultured bananas (Musa spp.)

Abstract

This study aimed at determining the effect of inorganic fertilizers on yield and postharvest quality characteristics of tissue-cultured bananas in order to establish the limiting nutrients. The study also aimed at establishing the proper maturity indices and the effect of pre-harvest polyethylene bunch covers alone and in combination with postharvest treatment with 1-Methylcyclopropene on physical, physiological and biochemical characteristics of banana fruit at harvest and during ripening. The experimental site was in Maragua Ridge, Maragua District, Agro-Ecological Zone (AEZ) upper midland zone 3 (UM3). For the experiment on effect of inorganic fertilizers on yield and postharvest quality, nutrients under investigation were, nitrogen at 400kg/ha, phosphorous at 50kg/ha, and potassium at 600kg/ha. Micronutrients were supplied as a combined treatment as follows; magnesium at 60 kg/ha, zinc at 6kg/ha, molybdenum at 0.5 kg/ha and boron at 1kg/ha. The treatments included all above nutrients applied in such a way as to omit one nutrient, where all nutrients were applied and a control where no nutrients were applied. A Randomised Complete Block Design (RCBD) with four replications was used. The other experiments consisted of studies carried out to establish clear harvest indices, the effect of pre-harvest bunch bagging on fruit quality and postharvest response to 1- MCP using a completely randomized design (CRD) with three replications. Fruits were analysed for selected postharvest quality parameters at harvest and during ripening. Data were examined for normality using R software and outliers by scatter plot using MS Excel software. Data were then subjected to analysis of variance (ANOVA) using the general linear model (GLM) procedure of SAS statistical programme (SAS, 2001). The means were compared according to Student Newman Keul’s (SNK) test and Least Significant difference (LSD) ( = 0.05) to test for significant effects. Correlations among maturity indices were tested using MS Excel software. Application of inorganic fertilizers significantly (p≤0.05) affected bunch weight, number of hands, number of fingers, grade, finger weight, finger length, pulp to peel ratio of green fruit, pulp and peel objective firmness, starch content, total soluble solids, vitamin C, pulp crude fibre, lightness (L*) of green peel, L* raw pulp, hue angle of green peel and raw and ripe pulp, pulp Ca, Mg and P content and peel P content. Green life and shelf life were not significantly (p>0.05) affected. Sensory evaluation of the fruits from all treatments showed significant (p≤0.05) differences for preference, aroma and texture but not for sweetness. Phosphorous and micronutrients were found to limit postharvest quality of tissue-cultured bananas in Maragua Ridge region. Fruits harvested at ¾ mature, light full ¾, full ¾ and full mature stages generally had similar postharvest qualities especially at the eating ripe stage, except for green life where fruits harvested at the fully mature stage had significantly (p≤0.05) short green life for both banana cultivars (cv.) Grand Nain and Williams. Shelflife was not influenced by the stage of maturity at harvest for both banana cultivars. It may be concluded here that the optimum harvest maturity stage is three quarter, light full ¾, full ¾ corresponding to 22, 24 and 26 weeks from flowering for cv. Grand Nain and 24 to 28 weeks for cv. Williams, as the fruits had acceptable grades and kept well while attaining optimum postharvest quality. Both banana cultivars showed a positive correlation (R) between bunch age and finger grade, weight, length, pulp/peel ratio, total soluble solids (TSS) and total titratable acidity (TTA). Finger grade correlated very well with such postharvest qualities as TSS, TTA and pulp/peel ratio at R=+0.85, +0.72, +0.98 for cv. Grand Nain and R=+0.75, +0.87, +0.86 for cv. Williams, respectively. The results indicate that the best maturity indices for both banana cultivars may be a combination of bunch age and grade as they correlated very well with postharvest characteristics such as TSS, pulp to peel ratios, TTA, green life and shelflife and are not destructive. Bunch covers did not influence significantly (p>0.05) the finger grade, finger length and bunch weight for banana cv. Williams. However, for cv. Grand Nain only grade was not significantly (p>0.05) affected by the bunch covers. Pulp/peel ratio for banana cv. Williams was not significantly affected while that cv. Grand Nain was affected (p≤0.05). Starch content, total soluble solids (TSS), pulp and peel moisture content, weight loss, chlorophyll content, peel and pulp firmness, peel and pulp colour, lightness (L*) and hue angle at harvest and during ripening were not influenced by bunch covers for both cultivars. Total sugar content was similar in all treatments for cv. Williams but differed in cv. Grand Nain. Bunch covers did not influence green life and shelflife of both banana cultivars. Peak ethylene production differed for cv. Grand Nain in all treatments but not for cv. Williams while respiration was influenced slightly by bagging for cv. Grand Nain during ripening but not for cv. Williams. The covered fruits were more visually appealing, cleaner and had minimal bruises compared to the unbagged fruits. However, few fingers from top hands of few bunches grown covered suffered sunburn irrespective of the bunch colour. 1-MCP application delayed and reduced the ethylene peak, respiratory