14.4 Chilling injury

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Plants may develop physiological disorders when exposed to low but non-freezing temperatures. The German plant physiologist Molisch suggested the term ‘chilling injury’ as long ago as 1897 to describe this phenomenon. Symptoms of chilling injury can differ widely between species, but usually develop rapidly in plants native to tropical and subtropical climates and almost imperceptibly slowly in plants originating in cool temperate climates. Within the range of chilling temperatures, that is, from the temperature of the freezing point of the plant tissue up to about 13°C, the rate at which chilling injury develops intensifies with decreasing temperature and increasing duration.

Throughout history, people around the world have collected plants on their travels and taken them to other countries and continents. When tropical and subtropical plants collected from low altitudes have been taken to temperate climates they have had to be housed in protective structures for all or part of the year. In the case of Citrus species introduced into northern Europe in the sixteenth and seventeenth centuries from warmer southern climates, summer temperatures in countries such as France and Germany were mild enough to allow these trees to be grown outside in the summer. At other times of the year, however, potted trees were moved into buildings known as orangeries for protection against the low temperatures. Similarly, during the late seventeenth century, a time when a great number of exotic chilling-sensitive palms, trees and foliage plants were introduced into temperate countries by plant collectors operating in the tropics and subtropics, these plants had to be protected from exposure to low temperatures at all times. Chilling-sensitive foliage plants such as Episcia spp. which are native to the Amazon basin are killed within 30 min of being exposed to 1°C. Survival of such highly sensitive plants necessitated year-round protection from cold. This requirement was met by the invention of the heated glasshouse in the 1880s. Modelled on the famous Palm House at Kew Gardens, London, the Palm House in the Adelaide Botanical Gardens is an example of the type of glasshouse that was constructed in this period.

Only when the extent of the climate change for introduced species is less severe can plants be field cultivated. In modern-day agriculture, many plants are often cultivated outside their customary microclimate. For example, avocado (Persea americana Mill.) was taken from tropical highlands in Mexico and is now grown in the temperate North Island of New Zealand and in inland Australia where nights are cold. When the new location has temperature minima below those of the region in which the plant evolved, problems of chilling injury can arise.

With annual crops, the time of greatest risk is likely to be early in a growth season, and especially during seedling establishment. Chilling injury to seedlings can show up as necrotic lesions on the young roots and shoots, with slowed growth and increased susceptibility to disease attack, and even death. Crops adversely affected by low temperatures during the establishment period take longer to mature and this in turn may mean that they are at risk to chilling temperatures towards the end of the growing season.

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Figure 14.14 Cavendish Williams bananas harvested at the hard green stage from the same banana hand were either stored at 22°C for 11 d (non-chilled) or placed at 4°C for 7 d (chilled) before transfer to 22°C for 4 d. Compared to the non-chilled bananas, which gradually turned from green to yellow as they ripened, the chilled bananas failed to yellow and instead developed extensive peel blackening due to cell death. Slight peel blackening was evident when the bananas were removed from the 4°C treatment but greatly intensified at 22°C.To maintain the postharvest quality of Williams bananas, a crop which is worth approximately $180 million per annum to the Queensland economy, marketing authorities stipulate that the produce must not be cooled below 13°C during fruit storage, and for optimal fruit condition it should be kept in the temperature range 14-21°C. During the ripening of green bananas in the commercial ripening rooms at the Brisbane fresh produce market, the lowest temperature the fruit is allowed to equilibrate to is 14.5°C (Photograph courtesy S.E. Hetherington)

Not only does chilling exposure retard the growth and maturation of crops, but chilling damage to fresh produce during postharvest storage is also of economic importance (Figure 14.14). Chilling injury is a particular problem with fresh fruit, vegetables and flowers, because storage at temperatures low enough to retard tissue respiration is still the most effective postharvest method for extending the shelf life of produce. Even in produce-handling industries, there is often insufficient appreciation of requirements and behaviour of individual crops or even specific cultivars, and losses ensue. The time taken for symptoms to develop varies greatly and is influenced by a number of factors including genotype, cultivar, stage of maturity and preharvest growth conditions. For example, with fruit stored at 1–2°C, it takes several months for chilling injury to develop in apples as a brown discolouration of the cortex, several weeks for the flesh of peaches to become mealy in texture, a number of days for avocados to show areas of grey discolouration in the flesh, and only a few hours for cucumbers to display tissue breakdown in the mesocarp. Obviously storage at 0–2°C is an excellent method for extending the storage life of apples, is moderately useful for peaches, but disastrous for cucumbers. Avocados are better kept at a higher storage temperature, the recommendation for extended storage of avocados is 6°C.

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Table 14.1

Chilling injury becomes apparent in a variety of ways (Table 14.1) that vary according to species and tissue. Visible symptoms are secondary outcomes of physiological disorders which express themselves in a number of ways. Moreover, symptomatic expression of the disorder is itself temperature dependent. Symptoms may develop slowly during the actual chilling period, to be expressed much more clearly and rapidly once the tissue is returned to warmer, non-chilling conditions.

In defining the physiological basis of chilling injury, increased cellular membrane permeability emerges as a major symptom and much research has been directed to elucidating the chemical and physical nature of lipoprotein membranes of species having different climatic origins. Dr John Raison and other scientists at CSIRO Division of Food Research in Sydney have provided evidence that physical changes occur in membranes of chilling-susceptible plants during low-temperature exposure (Figure 14.7). These scientists suggest that the molecular ordering of membrane lipids is altered in the temperature range where chilling effects become apparent. In particular, lipid composition appears to determine how membranes respond to low temperatures. Tropical species tend to have lipids with a higher proportion of saturated fatty acids (these are fatty acids such as palmitic acid which lack double bonds in their structure and therefore have higher melting points), while cool-climate plants tend to have more unsaturated fatty acids such as oleic acid. However, a consistent pattern of differences in lipid membrane composition between chilling-susceptible and chilling-resistant plants has yet to emerge and additional factors are likely to be involved. The physical nature of cell membranes remains an important point for research into chilling injury, but as yet no single physiological factor has been linked with plant susceptibility to chilling injury.

 

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