14.4.2 Ranges of chilling tolerance

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Table 14.2

Plants are commonly reported in scientific literature as being either chilling sensitive or chilling tolerant. This can be a misleading simplification, because in practice when a range of plants are compared for chilling tolerance there is an almost continuous gradient of tolerance between the two extremes (Table 14.2). Tolerance of an individual species is likely to be related to the lowest prevailing temperature in the original habitat of that particular species. Thus marine plants which have evolved in an environment in which seasonal temperature fluctuations are generally less than those on land tend to be less tolerant of chilling than terrestrial plants. On land, annuals which only grow during warmer months are less tolerant than perennials which have to survive year round.


Figure 14.16 Increasing chilling tolerance of wild species of potato and tomato with increasing altitude implies an adaptation to that location. Each point represents a different species of Solanum (), or variant of Lycopersicum hirsutum (O), originally collected at the altitude indicated in the graph and grown under similar field conditions at sea level. Chilling tolerance determined by the chlorophyll fluorescence method. (Original data courtesy R.M. Smillie)

Progressive changes in chilling tolerance have also been documented in closely related plants naturally distributed over latitudinal or altitudinal clines. Growth at high altitudes and also at high latitudes represents a selection pressure for cold tolerance. Wild species of potato restricted to ecological niches within narrow spans of altitude in the Andean mountains of Peru and Ecuador provide a good example of how chilling tolerance changes along an altitudinal cline. Variants of wild tomato (Lycopersicum hirsutum L.) collected in the same regions as the potatoes behaved similarly with chilling tolerance increasing with altitude (Figure 14.16).

One important outcome of such research is identification of genotypes that can be used in breeding programs to improve the chilling tolerance of closely related plants thereby enabling crop expansion into new, cooler production areas. An example of where this has occurred is in New Zealand where maize can be limited in some years by low temperatures in summer. In joint studies between Australian and New Zealand physiologists and plant breeders, in vivo chlorophyll a fluorescence was used to quantify the cold tolerance of popu-lations of maize adapted for growth in different geographical regions, namely at high altitudes in Peru, in the US Corn Belt and in northern Europe (Eagles et al. 1983; Hetherington et al. 1983).