The mango (Mangifera indica L.) is a fruit crop of considerable economic and social importance. Widely renowned as the king of fruits, mangoes are produced in tropical and sub-tropical regions. In Australia, the cultivar B74 (marketed as CalypsoTM) is a recently developed hybrid cross between Kensington Pride (KP) and Sensation. Damage to lenticels, macroscopic cavities on the surface of fruit that facilitate gas exchange, limits the marketability of B74 fruit. Lenticel damage (LD) exhibits as a dark area surrounding the lenticel cavity, which reduces the visual appeal of the fruit. In the current research, the working hypothesis that high cell turgor within the lenticel cavity predisposes mango fruit to LD that is exacerbated by other stresses was tested.
The characteristics and morphology of lenticels during fruit development of B74 and four other mango cultivars were comparatively examined. Lenticels on B74 fruit formed from stomata on young fruit and, more so, from cracking of the cuticle during later stages of fruit growth. B74 fruit had 2- to 7-times higher lenticel density at commercial harvest maturity than KP, Honey Gold and R2E2 fruit. Sensation fruit had a 2-fold higher lenticel density than B74. The high lenticel density on B74 fruit, as inherited from Sensation, may explain its greater susceptibility to developing commercially significant LD. Accumulation of condensed phenolics around damaged B74 lenticels was also observed macroscopically and microscopically. A larger proportion of lenticels on B74 fruit had smaller chambers at commercial harvest as compared to KP, perhaps because more lenticels formed later from cracks. There was no spatial link in terms of close proximity between sub-cuticlar epidermal resin ducts and LD. This lack of association suggests that external stressors are key LD-causing agents.
The impact of tree irrigation was investigated towards discerning pre-disposing factors to LD. Withholding irrigation from B74 trees for 3 - 8 weeks prior to harvest reduced soil water content and leaf stomatal conductance. However, it had no appreciable effects on either fruit water status or lenticel morphology. The severity of LD on fruit at eating ripe was not reduced by irrigation treatment, although withholding irrigation for 4 weeks prior to harvest increased LD severity on fruit at eating ripe and 7 days after eating ripe. Dipping B74 fruit after harvest into water increased LD severity by ~ 1.7 - 2.0-fold after standard commercial pre-harvest irrigation. Exposure of fruit to γ-irradiation also increased LD irrespective of the irrigation treatment. Where fruit do not require irradiation, ceasing irrigation from 3 - 8 weeks before harvest might represent a cost saving.
The susceptibility of B74 fruit to developing LD in response to commercial packing house operations, specifically solution dipping and brushing, were investigated. Postharvest operations concomitantly increased LD severity and resulted in pigment accumulation in epidermal cells surrounding the lenticel cavity. LD severity increased on fruit that were agitated in water, possibly because it contributed to the breaking of air films over fruit surfaces, thereby extending wetting and facilitating water entry into lenticels. Treatment of fruit with a non-ionic surfactant solution increased LD severity, presumably by enhancing cuticle wettability and / or if the organic active components were phytotoxic to cell membranes. Treating fruit with NaCl solutions (2 - 3%) that aimed to lower solution osmotic potential to approximate cell water potential (~ -0.8 MPa) resulted in an undesired increase in LD severity. Similarly, treatment with polyethylene glycol 6000 also resulted in higher LD severity. Brushing B74 fruit with different types of brushes for varying durations did not consistently increase LD severity in the presence of water.
The efficacy of pre- and postharvest bagging and coating treatments were tested to potentially reduce LD on B74 fruit. Two treatments, on-tree bagging with paper bags at 2 months and on-tree coating with 2.5% TFC (a carnauba-based wax) at 1 day before harvest, significantly reduced LD severity. Adding a 0.1% surfactant (Maxx Organosilicone Surfactant™) to 20% TFC reduced LD severity by up to 43% until 9 days after the full yellow skin colour stage as compared to the untreated control. Postharvest coating with 30% TFC plus surfactant also reduced LD severity by up to 50% on fruit at the eating ripe stage as compared to the control. No bagging or coating treatment effects were evident on lenticel morphology. However, TFC at 15% or higher covered the fruit surface and the lenticel cavity, which appeared to minimise external stress impacts on lenticels.
Overall, the research revealed that the relatively high lenticel density on B74 fruit may explain its susceptibility to commercially significant LD during typical postharvest handling. The research also highlighted that external stressors, namely exposure of fruit to water, were key LD-causing agents. Covering fruit with a paper bag and coating fruit with TFC plus 0.1% surfactant before or after harvest effectively reduced LD. These proposed control methods may contribute to further enhancing the commercial prospects of mango cultivars such as B74 that are relatively susceptible to developing LD.