Reducing granulation in the production of Imperial mandarins

Abstract

Granulation is a physiological disorder in which juice vesicles are hardened, gelled or granular. Severely granulated fruit are an opaque white in colour, with no or minimal extractable juice. Partially granulated fruit are ‘crunchy’, with less extractable juice and with less flavour than unaffected fruit.

Granulation of Imperial mandarins is a significant problem for the Australian domestic mandarin market. Incidence varies with season and, because affected fruit cannot reliably be detected by appearance or density, a significant proportion of granulated fruit reaches the market in some years. Despite international research on this issue since the 1930s, until now little progress has been made in identifying the causes or providing solutions to this problem.

Granulation is more severe in years with high rainfall or with low crop loads. In research trials and surveys conducted as part of this project, we found granulation is associated with larger fruit size, lower acid content in juice, lighter (sandier) soils, vigorous rootstocks, and the position of fruit in the tree, with fruit inside the canopy granulating more. These factors are the basis of our hypothesis that granulation is caused by higher water potential in juice cells as the fruit develop. High water potential in juice cells is due to low soluble solid content (such as sucrose and acids) and/or higher turgor pressure. High water potential may lead to cell wall thickening and subsequent gellification or granulation of juice cells. Our hypothesis suggests that nutrition, irrigation and crop load management are key to managing this problem.

In this project, we conducted a range of on-farm trials of management techniques to give growers strategies for reducing the incidence of granulation in their crops. The most successful strategies were avoiding overwatering early in fruit development, maintaining consistently high crop loads and applying adequate nitrogen fertiliser in winter to support the spring flowering and flush.

The project included trials that attempted to answer in more detail the questions, ‘How much water should we apply?’ and ‘Which are the critical times for reducing irrigation?”. Unfortunately, frequent rainfall in several of the trial years meant these questions remain largely unanswered, although several aspects of our research suggest that the earlier stages of fruit development are more important. We tried to find management practices that might help after a wet spring, that is, strategies that can be applied later in fruit development, including foliar application of fertilisers. These were unsuccessful, emphasizing that early fruit development is the critical period. We found some influence of competition between spring flush growth and fruit quality, but our attempts to reduce this competition with the use of plant growth regulators, later pruning and branch girdling were also disappointing.

Growers are thus encouraged to focus on the three proven strategies of avoiding overwatering in early fruit development, applying sufficient nitrogen in winter and managing crop loads through thinning and timely picking of crops.