Cold and Drought Resistance

Dormancy is an adaptation to unfavourable environmental conditions to which the dormant organ is more resistant than the non-dormant one. In species of the temperate zone, an increased resistance to below-freezing temperatures accompanies the winter dormant condition. Although they may be controlled by independent mechanisms, both the formation of winter resting buds and the development of cold resistance depend on exposure to SD conditions. This is sometimes evident in plants growing near street lamps, which may show an increased susceptibility to freezing injury. Even in species such as apple, where the time of onset of dormancy is unaffected by day length, exposure to SD increased hardiness (Howell and Weiser, 1970). Autumnal short days may thus be important for the survival of plants in which extension growth and the formation of resting buds occur early in the season before the daylength shortens. However, although visually dormant buds appear to be a pre-requisite for the ability to harden in woody plants and both are dependent on SD, the precise relationship between rest and cold acclimation has been shown to vary in different species (Dormling, 1993). In Pinus sylvestris, the buds are in a state of quiescence during the early part of SD-induced dormancy and only develop true rest late in the season, in parallel with the development of cold acclimation, which largely depends on exposure to chilling temperatures. In contrast, buds of Picea abies enter rest shortly after transfer to SD and release from rest begins 3-4 weeks later simultaneously with the build up of frost tolerance; during most of the winter, therefore, these plants are in a state of quiescence imposed by low temperature. These differences have practical implications. Long night treatments in late summer are used in Sweden to produce Picea abies with early hardiness; in Pinus sylvestris, the same procedure would not be effective.

Shedding leaves is one way of increasing resistance to winter low temperatures. Leaf fall is directly influenced by SD in many deciduous species of the temperate zone, but temperature is frequently more important. In Rhus glabra and Liriodendron tulipifera leaves are shed in SD and retained in LD but in Betula pubescens, Robinia pseudacacia, Acer pseudoplatanus, Liquidambar styraciflua and Quercus alba leaves are retained in SD provided that the temperature does not fall. An interaction with temperature is often important not only for the shedding of leaves but also for the cessation of extension growth. In SD at 10°C Robinia pseudacacia stopped growing but did not become dormant, growth being resumed following transfer to LD; at 15°C plants became dormant and the leaves were shed; while at 21°C and 27°C they became dormant but retained their leaves (Nitsch, 1962).

Shedding leaves is also an adaptation to water stress. While this may be a direct response to water shortage, daylength may also be a factor. Many species from Mediterranean climates have evolved mechanisms to reduce the leaf area during the long summer drought. In these regions of hot, dry summers and cool wet winters, the summer deciduous habit is an adaptation allowing plants to survive periods of extreme water stress. In a typical species from California (Lotus scoparius), plants lose their leaves in the summer and remain dormant until the rains begin in December (Nilsen

Time / days

FIG. 11.5. The effect of daylength on the number of abscised leaves on Lotus scoparius plants. Plants were grown in a growth chamber at 25°C day; 10°C night temperature and received either a 10 h (SD) or 14 h (LD) photoperiod at 600 pmol m~2 s-1. Plants were well-watered for 14 days and then stressed by withholding water at time zero. After Nilson and Muller 1982.

Time / days

FIG. 11.5. The effect of daylength on the number of abscised leaves on Lotus scoparius plants. Plants were grown in a growth chamber at 25°C day; 10°C night temperature and received either a 10 h (SD) or 14 h (LD) photoperiod at 600 pmol m~2 s-1. Plants were well-watered for 14 days and then stressed by withholding water at time zero. After Nilson and Muller 1982.

and Muller, 1982). When water stress was imposed under LD, leaf area decreased more rapidly and the leaf abscission rate was higher than in SD (Fig. 11.5); these effects appear to be associated with the induction of dormancy since, in LD, soluble protein synthesis was not re-established following the release from stress. Plants in longer photoperiods also showed a greater ability to adjust osmotically during stress and were able to maintain positive turgor at lower pre-dawn water potentials. This may be an important factor for leaf abscission since turgor is required for several steps in the abscission process. The inhibition of dormancy by SD is also advantageous, since it prevents the development of dormancy during the winter drought conditions that are often experienced in California.

Dormancy may also be induced by LD in geophytes of the Mediterranean region. Anemone coronaria grows actively in winter but new leaves cease to emerge during the hot dry summers; growth is then resumed following the first rains in autumn. Leaf emergence rapidly ceased in LD (15 h) or with a night-break of 4 h given in the middle of a 14 h dark period (Fig. 11.6). The critical daylength was between 11 and 12 h (Ben-Hod et al., 1988). There is, however, an interaction with temperature; dormancy is also induced by exposure to high temperature (as in other Mediterranean geophytes) and, in a 27/22°C regime, there was no difference between LD and SD (Ben-Hod et al., 1988). Many herbaceous perennial plants, including grasses, of the arid and semi-arid Mediterranean regions also become dormant in summer. This is frequently induced by LD and, in some cases (e.g. Poa bulbosa\ Ofir and Kerem, 1982) is associated with the development of bulbs or bulbils. Dormancy is not restricted to flowering plants and photoperiodic control has been recorded in Lunu-laria cruciata, a desert liverwort from Israel (Schwabe and Nachmonv-Bascombe.

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Date (1982)

FIG. 11.6. The effect of photoperiod on the onset of dormancy in a wild accession of Anemone coronaria. The long-day treatments were: a SD in sunlight followed by (0), a 4 h night-break with gro-lux lamps (1.5 nmol m~2 s_1) or (O) a day-extension for 5 h with quartz-iodide lamps (5 (imol m~2 s-'). The SD treatments were: (■) 10 h photoperiods or (1) natural daylengths (minimum 10.25 h). After Kadman-Zahavi et al„ 1984.

Date (1982)

FIG. 11.6. The effect of photoperiod on the onset of dormancy in a wild accession of Anemone coronaria. The long-day treatments were: a SD in sunlight followed by (0), a 4 h night-break with gro-lux lamps (1.5 nmol m~2 s_1) or (O) a day-extension for 5 h with quartz-iodide lamps (5 (imol m~2 s-'). The SD treatments were: (■) 10 h photoperiods or (1) natural daylengths (minimum 10.25 h). After Kadman-Zahavi et al„ 1984.

1963); in this case, dormancy was induced by exposure to LD and is clearly an adaptation to the severe water-stress conditions of the summer desert environment.

Where the induction of cold or drought resistance is influenced by photoperiod, other factors often interact and contribute to the overall level of resistance achieved. For the induction of drought resistance and leaf abscission in Lotus it is necessary to expose plants to water-stress in LD (Nilsen and Muller, 1982). Where frost resistance is induced by SD, the relationship between daylength and temperature is often crucial in determining the degree of hardiness achieved. For example, although exposure to SD alone was sufficient to develop a high degree of cold hardiness in a northern ecotype of Salix pentandra, the degree of hardiness was significantly enhanced by a short exposure to sub-zero temperatures (Junttila and Kaurin, 1990). In several species it is possible to achieve cold hardiness by exposing plants to either SD or low temperatures (e.g. Pinus sylvestris, Picea abies', Christersson, 1978; Cornus stoloni-fera\ Chen and Li, 1978). The effects of daylength and temperature appeared to be additive and it was suggested that they are probably independent physiological mechanisms for inducing frost hardiness. This is supported by the fact that, in Pinus and Picea, exposure to low temperature in LD caused hardening without inducing dormancy. In pines, the development of frost hardiness appears to occur in two stages; the first requires exposure to a photoperiod of 11 h, followed by the second stage which develops with exposure to temperatures below a threshold of 5°C (Greer and Warrington, 1982). Thus maximum cold-hardiness was found when SD preceded the exposure to low temperature, as would occur under natural conditions.

In contrast to the situation in most woody plants of the temperate zone, daylength has no effect on cold acclimation in many herbaceous species, where the development of cold hardiness depends on exposure to low temperature. However, acclimation in several clones of white clover (Trifolium repens) was significantly greater in plants maintained in 12 h photoperiods compared with those in continuous light (Junttila et al., 1990).

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