Ecology. Michael Begon

membrane and is associated with an increase in calcium concentration that sets in train the activation of a wide variety of genes responsible for the biochemical changes already described (Wisniewski et al., 2014). And in insects, Zhang et al. (2011) identified cold‐responsive genes in the fruit‐fly Drosophila melanogaster associated with muscle structure and function, immune and stress responses and carbohydrate metabolism.

In a laboratory ‘selection’ experiment involving plants of alfalfa (Medicago sativa, an important animal forage species), Castonguay et al. (2011) investigated whether the plant could be selected for improved freezing tolerance. Five weeks after sowing 1500 genotypes of a particular alfalfa cultivar used in eastern Canada, the plants were moved to low‐temperature chambers for two week’s acclimation at 2°C before being transferred to –2°C for an additional fortnight to simulate ‘hardening’ conditions in frozen soil. Subsequently, temperature was progressively dropped to the expected lethal temperature for 50% of the plants (their LT₅₀ – lethal temperature for 50% of the plants in January), using a stepwise decline of temperature. After five weeks of regrowth at 20°C, genotypes that survived the original freezing cycle were intercrossed and subject to another cycle, and so on for six cycles of recurrent selection. The experiment was repeated with a second cultivar for four cycles of selection. Figure 2.14a shows that for both cultivars, several cycles of selection for freezing tolerance led to a significant decline in LT₅₀ between the first cycle and later cycles of selection: in other words, individuals in the populations subject to selection were able to tolerate lower winter temperatures. Associated biochemical (Figure 2.14b, c) and genetic patterns (Figure 2.14d) provide good evidence that recurrent selection for superior freezing tolerance in alfalfa induces marked changes in influential traits. And if deliberate selection can change the tolerance of a domesticated plant we can certainly expect that natural selection has done the same thing for plants, animals and microorganisms in nature.

Figure 2.14 Alfalfa can be selected for improved freezing tolerance. (a) Tolerance to freezing (LT₅₀, 5% confidence levels shown) of populations of two cultivars of alfalfa used for animal forage in eastern Canada, before selection (TF0) or after several cycles of recurrent selection for freezing tolerance (three, four, five or six cycles). (b, c) Starch and sucrose concentrations in crowns of alfalfa plants during autumn and winter (cultivar 1) before (0) and after five or six cycles of selection. (d) Relative expression of the cold‐induced gene cas15 before (0) and after five or six cycles of selection.

      Source: From Castonguay et al. (2011).

      APPLICATION 2.4 Selection for cold tolerance in crops to increase their productivity and geographic range

      There have been many striking cases where the geographic range of a crop species has been extended into colder regions of the world by plant breeders. Traditional crop breeding practices have generally used crossing of closely related varieties to produce new crops with desired cold‐tolerance traits.

A key challenge for plant breeders is to introgress desirable traits from wild and even quite distantly related species into important domesticated crops but at the same time retain the favourable traits of the crop. Sugar cane (Saccharum spp.) is a major crop whose tropical heritage makes it cold sensitive and generally restricted to latitudes between 30°N and 35°S. Another member of the Poaceae family of tall grasses, Miscanthus spp., on the other hand, is a temperate‐adapted species with marked cold tolerance. Głowacka et al. (2016) have shown that the chilling tolerance of Miscanthus can be transferred to sugarcane (Figure 2.15) without significant loss of overall sugarcane productivity. The chilling‐tolerant hybrid of sugarcane and Miscanthus (Miscane US87‐1019) has immediate potential for increased stock food and biofuel production, and at the same time provides the basis for extending sugarcane’s range as a crop into higher latitudes and altitudes, once we better understand the genes that confer the cold‐tolerance advantage.

Figure 2.15 The chilling tolerance of Miscanthus can be transferred to Saccharum. Comparison of cold tolerance in a laboratory experiment involving plants of sugarcane (Saccharum sp. L79‐1002), Miscanthus (Mxg ‘Illinois’) and a hybrid of Saccharum and Miscanthus, referred to as ‘Miscane’ (US87‐1019). The light‐saturated leaf net CO₂ uptake rate (A_sat in μmol m^–2 s^–1) is shown for warm conditions before chilling treatment (25ºC day, 20ºC night: dashed line), after transfer of plants to chilling (day 0: 10ºC day, 5ºC night), on day 11 of chilling treatment and one day after transfer of plants back to warm conditions (day 12: recovery) expressed as a percentage of rates observed in warm conditions before chilling (control). A plus sign indicates a significantly lower value than the control. As expected, Miscanthus was the most cold tolerant, sugarcane the most cold sensitive, while the hybrid did not differ significantly from Miscanthus after recovery.

      Source: From Głowacka et al. (2016).

      potential of genomics, transcriptomics and proteomics in crop breeding

      Future crop improvements to increase production and range in colder environments are certain to involve identification of the genes responsible for cold tolerance (both chilling and sub‐zero tolerance) and acclimatisation. Erath et al. (2017) have, for example, identified genomic regions involved in frost tolerance of winter rye (Secale cereale) by mapping of quantitative trait loci (QTLs). A QTL is a section of DNA that correlates with variation in the quantitative trait of the phenotype (cold tolerance in this case); the QTL can be expected to contain the genes that control the trait. In winter rye, a QTL on chromosome 5R harbours the Frost resistance locus 2 (Fr‐R2) and the ‘Puma’ allele at this locus was found to significantly increase frost tolerance. Discoveries of this kind can be expected to increase selection intensity for frost tolerance by preselecting plant breeding lines based on markers from the Fr‐R2 locus.

      2.3.6 Life at high temperatures

      Perhaps the most important thing about dangerously high temperatures is that, for a given organism, they usually lie only a few degrees above the metabolic optimum. This is largely an unavoidable consequence of the physicochemical properties of most enzymes (Wharton, 2002). High temperatures may be dangerous because they lead to the inactivation or even the denaturation of enzymes, but they may also have damaging indirect effects by leading to dehydration.

      high temperature and water loss in terrestrial environments

All terrestrial organisms need to conserve water, and at high temperatures the rate of water loss by evaporation can be lethal, but they are caught between two stools because evaporation is an important means of reducing body temperature. If surfaces are protected from evaporation (e.g. by closing stomata in