Evaluation of fruit growth and development over a very extended harvesting period of 'Hass', ‘Fuerte’, ‘Gem’ and ‘Ryan’ avocado fruit.

Abstract

Assessing avocado fruit growth and development by measuring fruit diameter during ontogeny may, therefore, offer clues to better understand whole plant behaviour. Plant sampling was carried out over different developmental stages from early to an extended growing season on four cultivars (‘Hass’, ‘Fuerte’, ‘Gem’ and ‘Ryan’). Mesocarp, exocarp and seed fruit tissues were used to determine internal parameters such as sugars, antioxidant, oil content, dry matter, and calcium). The sugars were extracted and analysed by isocratic HPLC. D-Mannoheptulose in mesocarp+exocarp tissues was found in significant amounts (‘Hass’ = 16.47±1.140 mg/g DM, ‘Fuerte’ = 11.92±1.780 mg/g DM, ‘Gem’ = 9.35±1.410 mg/g DM, ‘Ryan’ = 7.52±1.271 mg/g DM), with perseitol also being significant for all cultivar (‘Hass’ = 4.87±0.662 mg/g DM, ‘Fuerte’ = 5.77±0.650 mg/g DM, ‘Gem’ = 5.09±0.577 mg/g DM, ‘Ryan’ = 3.86±0.227 mg/g DM). D-Mannoheptulose was found in high levels in the mesocarp and exocarp compared to the seed. Perseitol was predominantly found in the seed for all cultivars (‘Hass’ = 7.31±0.486 mg/g DM, ‘Fuerte’ = 6.71±0.842 mg/g DM, ‘Gem’ = 6.76±0.224 mg/g DM, ‘Ryan’ = 8.62±0.473 mg/g DM). The C6 common sugars sucrose and glucose were detected in low concentrations in the mesocarp+exocarp fruit tissue, with sucrose being dominantly present in the seed. Calcium was determined by fruit ashing using HCl/HNO3 for digestion and strontium buffer solution for calcium extraction. Calcium concentration was significantly different during the ontogeny of each cultivar (‘Hass’ p = 0.007, ‘Fuerte’ p < .001, ‘Gem’ p < .001, and ‘Ryan’ p < .001). The calcium uptake peak is mostly reached during early fruit set stages of avocado fruit, followed by a decline and constant continuous low concentrations as approaching maturity. When fertilizer is applied during maturity calcium uptake in the avocado fruit tends to increase. Maturity indicators such as oil content, dry matter and fruit are significantly different across all fruit developmental stages. Oil content percentage (p < .001 all cultivars), dry matter (p < .001 all cultivars) and fruit size for both low and high tree fruit load (p < .001 all cultivars, except ‘Hass’ with p = 0.812 for high tree load fruits). During the extended hanging period maturity indices accumulation had a continually increased per cultivar, Oil% (‘Hass’ = 18.1%, ‘Fuerte’ = 12.74%, ‘Gem’ = 13.41%, and ‘Ryan’ = 17.41%), dry matter (‘Hass’ = 40.37 mg/g DM, ‘Fuerte’ = 24.01 mg/g DM, ‘Gem’ = 44.29 mg/g DM, and ‘Ryan’ = 35.39 mg/g DM), and size (‘Hass’ = 69.73mm, ‘Fuerte’ = 68.46mm, ‘Gem’ = 75.34mm, and ‘Ryan’ = 76.75mm), all significantly increased. Overall this study revealed that avocado fruit development does not necessarily end at the commercial harvesting period, but continues on fruits still attached to the tree after the single sigmoidal growth curve. When fruit harvesting is prolonged, the internal parameter for fruit growth, and C7 sugars, content contributes significantly throughout fruit ontogeny but varies in levels between cultivars. Calcium concentration uptake is in higher demands at early fruit set, where peak accumulation is reached almost at similar period with C7 sugars per cultivar. Therefore, C7 sugars and calcium in avocado are correlated during fruit growth and development. By extending fruit harvesting it allows the avocado fruit to mature by accumulating higher concentrations of sugars and, calcium immature harvest which result in negative market outcomes. This is especially true for late maturing cultivars which are less susceptible to poor postharvest quality. Therefore, avocado fruit development does not only follow a single sigmoidal growth curve but a double sigmoidal one.