Steven J. R. Underhill

Anthocyanin decolorisation and its role in lychee pericarp browning

Mature red lychee fruit were stored at 3 different temperature and relative humidity regimes. Total anthocyanin concentration, pigment distribution, pH of the pericarp homogenate, Hunter a values (redness index), and visual colour were measured as a function of pericarp weight loss. Pericarp colour rapidly deteriorated during both ambient and high temperature storage, resulting in a uniform browning of the pericarp surface. The degree of tissue browning was proportional to the rate of pericarp desiccation. Although anthocyanin degradation occurred concurrently with tissue browning, visual colour and Hunter a values were not consistent with total anthocyanin concentration. Instead, a more significant correlation was seen between Hunter a values and the pH of the pericarp homogenate. Pericarp colour could be altered by external pH. Acidification of whole fruit increased pericarp redness, whereas alkaline treatment caused discoloration. Both colour responses occurred independently of anthocyanin synthesis and degradation and were completely reversible. These results question the current theory that browning is due to anthocyanin degradation. No evidence of browning was observed in the anthocyanin-containing mesocarp, and acidification of already brown tissue significantly increased pericarp redness independently of anthocyanin synthesis. We believe that anthocyanin pigments were progressively decolorised during ambient storage, possibly due to changes in pericarp pH. Once colourless, independent tissue browning became visual and was enhanced.

Enzymes associated with blackheart development in pineapple fruit

The involvement of browning enzymes, polyphenoloxidase (PPO), peroxidase (POD) and phenylalanine ammonia-lyase (PAL) in blackheart development was investigated in pineapple fruit (Ananas comosus, Smooth Cayenne) following low temperature storage. An increase in PPO activity was related to the incidence of blackheart symptoms, both temporally and spatially. Fruit maturity significantly affected blackheart susceptibility; immature and over-mature fruits developed less blackheart injury than in ature fruit. The effect of maturity on blackheart susceptibility was highly correlated to the response of PPO activity to chilling. POD showed no significant change after chilling. Enhanced PAL activity was observed during chilling at 6, 13 and 18 degreesC. Chilling (6, 13 and 18 degreesC) also inhibited the increase of ascorbate peroxidase activity observed in the fruit stored continuously at 25 degreesC, but had less effect on catalase activity. The results indicate that the development of blackheart symptoms in pineapple fruit results from the disturbance of a number of metabolic processes that occur at sub-ambient temperatures

Lychee (Litchi chinensis Sonn.) pericarp desiccation and the importance of postharvest micro-cracking

Abstract Lychee fruit are prone to postharvest desiccation, resulting in browning of the pericarp. Microscopic cracks (20–100 μm wide) in the pericarp surface were observed at harvest and had significantly increasing in density 12 h later. Microcracking extended through the sub-epidermal sclerenchyma layer into the mesocarp. Pericarp desiccation occurred prior to crack development, with cracking the result of dehydration rather than the initial cause. No evidence of cuticle thinning was observed prior to micro-cracking, however, localised cuticle deterioration on the protuberance did occur. We believe that initial pericarp desiccation may be the result of high permeability of the cuticle to water vapour, cuticle damage and the presence of lenticels. Although the cracking did not increase the rate of desiccation, micro-cracks may be important in both exposing the underlying anthocyanin-containing mesocarp tissue to further desiccation, and fungal penetration into the pericarp.

PHYSIOLOGICAL, BIOCHEMICAL AND ANATOMICAL CHANGES IN LYCHEE (LITCHI-CHINENSIS SONN) PERICARP DURING STORAGE

Polyphenol oxidase activity, anthocyanin concentration, endogenous ethylene production and anatomical features of mature lychee (Litchi chinensis Sonn.) pericarp were studied to understand better post-harvest tissue browning. Polyphenol oxidase (PPO) activity decreased significantly once fruit were harvested. Although anthocyanin degradation occurred, it was far slower than tissue browning. It is considered unlikely that pericarp browning is due to direct pigment degradation by polyphenol oxidase. Pericarp browning may in fact be caused by non-PPO mediated anthocyanin decolorization, coupled with localized PPO activity affecting secondary metabolites other than anthocyanin. Pericarp browning was correlated with moisture loss. The development of pericarp micro-cracking after harvest was observed, and may be involved in facilitating rapid pericarp moisture loss.

The physiology and anatomy of lychee (Litchi chinensis Sonn.) pericarp during fruit development

Development of lychee fruit (Litchi chinensis Sonn., cv. Kway May Pink) was studied from 21 to 103 days after anthesis, with specific emphasis on the pericarp. Pericarp structure was initially folded, with subsequent expansion closely associated with aril development. Cuticle thickness decreased from 8.75 |j,m to 1.88 μm at maturity, with micro-cracking of the pericarp surface observed 62 days after anthesis. Chlorophyll degradation and flavonoid production occurred simultaneously. Pericarp pH increased during ontogeny, and its possible interaction with pigment stability is discussed. Polyphenol oxidase activity decreased during fruit maturation, with peak activity prior to anthocyanin production. The L-ascorbic acid content of the pericarp decreased throughout development.

Hot water dipping and low temperature storage of 'Eureka' lemons

Summary. Prestorage heat treatments at 47–53°C for 1–3 min were investigated as a potential control of chilling injury in cold disinfested ‘Eureka’ lemons (Citrus limon). Hot water dips significantly reduced the incidence of chilling injury in fruit stored at 1°C for 28 and 42 days. Storage at 1°C for 14 days resulted in comparatively minor chilling injury, so that hot water dips gave little additional benefit. This result is thought to reflect the low incidence of chilling injury, rather than the lack of effectiveness of heat treatments. Scald damage (surface browning) occurred after 3 min at 53°C. Heat treatments had no significant effect on the incidence of disease at all durations of storage. The rate of fruit weight loss during 7 days at 20°C after storage for 14, 28 or 42 days at 1°C was significantly lower in heat- treated fruit.

Breadfruit (Artocarpus altilis) gibberellin 2-oxidase genes in stem elongation and abiotic stress response

Breadfruit (Artocarpus altilis) is a traditional staple tree crop in the Oceania. Susceptibility to windstorm damage is a primary constraint on breadfruit cultivation. Significant tree loss due to intense tropical windstorm in the past decades has driven a widespread interest in developing breadfruit with dwarf stature. Gibberellin (GA) is one of the most important determinants of plant height. GA 2-oxidase is a key enzyme regulating the flux of GA through deactivating biologically active GAs in plants. As a first step toward understanding the molecular mechanism of growth regulation in the species, we isolated a cohort of four full-length GA2-oxidase cDNAs, AaGA2ox1- AaGA2ox4 from breadfruit. Sequence analysis indicated the deduced proteins encoded by these AaGA2oxs clustered together under the C19 GA2ox group. Transcripts of AaGA2ox1, AaGA2ox2 and AaGA2ox3 were detected in all plant organs, but exhibited highest level in source leaves and stems. In contrast, transcript of AaGA2ox4 was predominantly expressed in roots and flowers, and displayed very low expression in leaves and stems. AaGA2ox1, AaGA2ox2 and AaGA2ox3, but not AaGA2ox4 were subjected to GA feedback regulation where application of exogenous GA3 or gibberellin biosynthesis inhibitor, paclobutrazol was shown to manipulate the first internode elongation of breadfruit. Treatments of drought or high salinity increased the expression of AaGA2ox1, AaGA2ox2 and AaGA2ox4. But AaGA2ox3 was down-regulated under salt stress. The function of AaGA2oxs is discussed with particular reference to their role in stem elongation and involvement in abiotic stress response in breadfruit.

Dwarfing of Breadfruit (Artocarpus altilis) Trees: Opportunities and Challenges

Breadfruit [ Artocarpus altilis (Parkinson) Fosberg)] is a traditional staple crop grown for its starchy fruit throughout the tropics. It has long been recognized for its potential to alleviate hunger in the region. However, being a tree of 10 ‐ 30m, breadfruit is vulnerable to wind damage. Owing to the continuing trend of global climate change, the success of the species as a sustainable crop for delivering local food security is compromised by the likelihood of more intense tropical windstorms in the island nations. Tree height also forms a major constraint to disease management and fruit harvesting. These imperatives have driven an increasing interest in developing breadfruit varieties with short stature. While a great diversity of breadfruit cultivars with varying nutritional and agronomic characteristics exists, the genetic resource showing dwarfing traits is largely uncharacterised. Historically, there has been no intentional breeding for breadfruit cultivars. The long growth cycle, predominantly vegetative propagation and lack of genome information create challenge for crop improvement through traditional breeding. In this review, we highlight the current knowledge of plant dwarfism and its application in agricultural practices and genetic improvement for dwarf phenotype, and present options and tools for breadfruit dwarfing with special reference to natural genetic variability for dwarfing rootstocks, plant growth regulators, potential of mutagenesis and its combination with the currently

Breadfruit (Artocarpus altilis) gibberellin 20-oxidase genes: sequence variants, stem elongation and abiotic stress response

Breadfruit (Artocarpus altilis) is a traditional staple tree crop throughout the tropics. Susceptibility to windstorm damage is the primary constraint on breadfruit cultivation. Significant tree loss due to intense tropical windstorm in the past decades has driven an increasing interest in developing dwarf varieties of breadfruit. As a first step toward understanding the molecular mechanism of growth regulation in the species, we investigated the role of gibberellin and the regulation of GA20-oxidase genes in breadfruit. We provided first evidence that the stem elongation in breadfruit could be manipulated by exogenous gibberellin-related growth regulators. We then cloned six GA20-oxidase cDNAs, AaGA20ox1–AaGA20ox6, in full length from breadfruit. Sequence analysis showed that the predicted proteins of the AaGA20ox1–AaGA20ox3 bear all the hallmarks of functional GA20-oxidase of other species, but predicted AaGA20ox4–AaGA20ox6 as expressed, unprocessed pseudogenes closely related to AaGA20ox2. AaGA20ox1, AaGA20ox3 and AaGA20ox4 were predominantly expressed in green vegetative organs, but displayed different expression pattern in roots and reproductive organs. AaGA20ox2, AaGA20ox5 and AaGA20ox6 were expressed mainly in leaves at low level. AaGA20ox1, AaGA20ox3–AaGA20ox6 were subjected to GA feedback regulation following treatment of exogenous gibberellin and/or gibberellin biosynthesis inhibitors. AaGA20ox1 and AaGA20ox3 were down-regulated under drought and salinity stress, but AaGA20ox2 was up-regulated under salt stress. Pseudogenes AaGA20ox4 and AaGA20ox5 were up-regulated under drought or/and salt stress condition. The function of AaGA20oxs is discussed with particular reference to their role in stem elongation and involvement in abiotic stress response in breadfruit.

Characterisation and induction of 'etch' browning in the skin of mango fruit

The effects of detergents and mango sap on the development and anatomy of ‘etch’, a physiological skin disorder in mango (Mangifera indica L.) fruit, were examined. Etch was associated with a higher number of cracks in the cuticle than occurred in healthy tissue. Etch was induced on fruit dipped in the wetting agent Agral® at high concentrations (10, 50% v/v). Fruit kept in contact with pads saturated with low concentrations of detergents (Agral®, 0.1–1%; Amway LOC®, 0.1%) tended to develop etch at the perimeter of the contact area. Etch symptoms increased in severity with fruit ripening and increasing contact times with detergent. Lenticel spotting developed at lower detergent concentrations than were required for etch development. The oil, white oil/aqueous and blue lower aqueous components of mango sap were separated and applied to the skin of mangoes. No individual component resulted in more than mild perimeter etch. A 10:90 ratio of oil:aqueous fraction resulted in etch symptoms of moderate severity over the entire application area, but only when combined in wet contact with water or Agral®. The relevance of the results to mango postharvest handling systems is discussed.