The Identity Of Dormancycontrolling Stimuli

The cessation of shoot growth and the development of bud dormancy depend on the exposure of the leaves to short photoperiods; consequently, it is assumed that

Number of photoperiodic cycles before extraction

FIG. 11.14. Inhibitor content of leaves of Acer pseudoplatanus at various times after transfer to long or short-days. The inhibitory eluates (Rp 0.55-0.88) from chromatograms of extracts of mature leaves were assayed by the wheat coleoptile test. Vince-Prue 1975 (data of Wareing, 1959).

Number of photoperiodic cycles before extraction

FIG. 11.14. Inhibitor content of leaves of Acer pseudoplatanus at various times after transfer to long or short-days. The inhibitory eluates (Rp 0.55-0.88) from chromatograms of extracts of mature leaves were assayed by the wheat coleoptile test. Vince-Prue 1975 (data of Wareing, 1959).

dormancy-controlliing stimuli are exported from leaves and that these differ in LD and SD.

The association of growth inhibitors with bud dormancy has been recognised for many years (Vince-Prue, 1985; Lavender and Silin, 1987). As plants enter dormancy, an increase in the growth-inhibiting activity of extracts (as measured by a variety of bioassays) has been recorded for a range of species. Evidence that these inhibitors originate in the leaves comes from studies with sycamore (Acer pseudoplatanus). Following transfer to SD, an increase in inhibitory activity was detected in leaf extracts after only two SD cycles (Fig. 11.14) and this was followed by an increase in the apex after five cycles, as would be expected of a dormancy-inducing stimulus. Notably, this increase in inhibitory activity in extracts from the apical buds preceded the retardation of growth (Wareing, 1959). Inhibitors have also been detected in the transport systems and there was a pronounced increase in ether-soluble inhibitors in both phloem and xylem of willow (Salix viminalis) during entry into dormancy (Bowen and Hoad, 1968).

These results raise two questions. Firstly, does the change in growth-inhibitory activity of the extract result from a change in the content of inhibitor(s) and/or of growth promoters? Secondly, are endogenous growth inhibitors involved in the photoperiodic regulation of bud dormancy?

In sycamore, the inhibitory effect on growth in bioassays was found to be largely due to the presence of abscisic acid (ABA); indeed, this growth inhibitor was originally termed dormin because it was thought to be directly involved in the regulation of dormancy. Similarly, the inhibitor in the sap of willow was identified as ABA, which was the main inhibitory component present (Bowen and Hoad, 1968; Alvim et al., 1976). Abscisic acid has also been identified in other dormant tissues.

DORMANCY-CONTROLLING STIMULI 'Young'

'Mature'

FIG. 11.15. Effect of daylength on levels of free abscisic acid in leaves of different ages from 7 week old seedlings of Acer pseudoplatanus. The experimental treatments consisted of 3 cycles of 18 h light/6 h dark (LD) or 8 h light/16 h dark (SD) and the extracts were made at the end of the final dark period. Prior to this, seedlings were grown in long photoperiods for 7 weeks. Leaves were small, not fully expanded (Young), fully expanded with no visible senescence (Mature), or the four lowermost yellowing leaves (Old). After Phillips et al.. 1980.

However, the induction of dormancy in willow was not correlated with maximum ABA levels in either buds, leaves or xylem sap (Barros and Neill, 1989). Moreover, when chemical methods rather than bioassays have been used, the ABA content of leaves and buds have not generally been found to increase when plants are transferred to SD (see Appendix II). In Salix viminalis, for example, no differences could be detected in the ABA content of young leaves from plants growing in short- or long photoperiods (Alvim et al., 1979). Other workers have also failed to detect increases in endogenous ABA following transfer to SD (in, for example, Betula pubescens, Acer pseudoplatanus, Lenton et al., 1972; Salix pentandra, Johansen et al., 1986). These results are consistent with those obtained for a number of herbaceous plants. In Acer pseudoplatanus, photoperception occurs in mature leaves and so ABA would be expected initially to increase in these if it is associated with the transmissible photoperiodic signal. However, after three cycles, the level of ABA in these leaves was higher in LD than in SD (Fig. 11.15). Nevertheless, a relationship with dormancy and the cessation of extension growth is perhaps indicated by the fact that the ABA content of the young leaves near the apex was somewhat higher after 3 SD. As always, the large increases in ABA content which result from water stress may have caused a problem with some determinations of the effect of daylength on the content of ABA; however, experiments in which the water stress factor was eliminated confirmed that transfer to SD did not increase the content of ABA in strawberry and sycamore (Plancher and Naumann, 1978; Phillips et al., 1980). It seems likely, therefore, that the increase in the growth-inhibitory activity of extracts from shoots exposed to SD resulted from changes in inhibitors other than ABA or from a decrease in growth-promoting substances which co-chromatograph with ABA in some solvent systems.

In contrast to the above results, there are a few recent investigations in which SD-

150-

150-

Days from germination

FIG. 11.16. Effects of daylength on shoot growth and the content of free abscisic acid in seedlings of Pinus sylvestris. Plants were grown for 35 days in LD (18 h) before they were transferred to SD (8 h) conditions or remained in LD. The irradiance was 470 (lmol m~2 s~\ and the day and night temperatures were 25°C and 15°C respectively. The abscisic acid extracts were made after plants had received 1 week in SD. After Od6n and Dunberg, 1984.

Weeks

Days from germination

Weeks

FIG. 11.16. Effects of daylength on shoot growth and the content of free abscisic acid in seedlings of Pinus sylvestris. Plants were grown for 35 days in LD (18 h) before they were transferred to SD (8 h) conditions or remained in LD. The irradiance was 470 (lmol m~2 s~\ and the day and night temperatures were 25°C and 15°C respectively. The abscisic acid extracts were made after plants had received 1 week in SD. After Od6n and Dunberg, 1984.

dependent increases in endogenous ABA have been recorded. In birch (Betula pub-escens), the ABA content in buds increased following transfer to 12 h SD which would cause growth cessation and the onset of dormancy in this ecotype; the increase was observed in both water-stressed and well-watered plants (Rinne et al., 1994). The discrepancy between these and earlier results may be related to differences between ecotypes, since the birch was of a more northern origin than the plants used in the earlier studies. Alternatively, it may have been due to the tissue sampled, since there was no difference in the ABA content of the birch leaves, which would have been the major component in many of the earlier studies where whole shoots were analysed. A large increase in the ABA content has also been recorded in Pinus sylvestris, following transfer to SD (Odin and Dunberg, 1984). The increase occurred after one week, before shoot elongation ceased within 2 weeks of SD treatment (Fig. 11.16). There were marked additional increases when plants were subsequently exposed (sequentially) to three weeks of simulated late autumn (low light) and simulated winter (low light and low temperature) conditions in SD (Fig. 11.17). Returning plants to 'summer' conditions reduced the ABA content to a level equal to that found during the first LD period. Thus, in this conifer, a strong correlation was observed between the content of ABA in the (whole) shoot and seasonal changes in the environment; there was also a strong negative correlation between the growth increment of the seedlings and the ABA content of the shoots. However, it seems rather unlikely that the initial increase following transfer to SD was a photoperiodic effect, since plants also received a lower light integral and a lower mean temperature, both of which were found to increase the ABA content without a change in daylength (see Fig. 11.17,

Summer Early Late Winter Summer 1 autumn autumn 2

FIG. 11.17. Changes in the content of free abscisic acid in seedlings of Pinus sylvestris growing under simulated seasonal climates in controlled environment conditions. The growing sequence from germination was: five weeks in summer, three weeks in early autumn, three weeks in late autumn, three weeks in winter andjhree weeks in summer. The irradiance was 470 (imol m~2 s~\ except in winter (60 pmol m~2 s_l) and the photoperiod was 8 h, except in summer (18 h). The day/night temperatures were 25/15°C in summer and early autumn, and 15/10°C in late autumn; winter plants received a constant temperature of 4°C. After Odin and Dunberg, 1984.

Summer Early Late Winter Summer 1 autumn autumn 2

FIG. 11.17. Changes in the content of free abscisic acid in seedlings of Pinus sylvestris growing under simulated seasonal climates in controlled environment conditions. The growing sequence from germination was: five weeks in summer, three weeks in early autumn, three weeks in late autumn, three weeks in winter andjhree weeks in summer. The irradiance was 470 (imol m~2 s~\ except in winter (60 pmol m~2 s_l) and the photoperiod was 8 h, except in summer (18 h). The day/night temperatures were 25/15°C in summer and early autumn, and 15/10°C in late autumn; winter plants received a constant temperature of 4°C. After Odin and Dunberg, 1984.

early autumn compared with winter and late autumn). Moreover, increasing the duration of the SD treatment without further change in temperature or light integral had no effect. Thus, while the results are suggestive of some kind of causal relationship between the growth of the shoot and its ABA content, they do not provide any evidence to support the hypothesis that the SD induction of dormancy is achieved through changes in ABA. In another conifer, however, a transient increase in the ABA content of needles was observed when seedlings were transferred to SD conditions with no change in temperature (Fig. 11.18). Seedlings of Picea abies from two widely dispersed latitudes were grown in continuous light before being transferred to SD (16 h night) conditions at the same temperature, but at a much reduced light integral. A transient peak of free ABA in the needles occurred on the 4th SD in the northern population, but was not observed until the 8th SD in the southern one. The role of this early, transient increase in ABA is not clear, but its timing does appear to be correlated with the photoperiod response of the two populations. The same degree of both budset and budrest required exposure to fewer SD in the northern population than in the southern one, and the peak of ABA occurred earlier (see Fig. 11.18).

Even if ABA is not increased by SD, it could still be a component of the endogenous system which controls dormancy, if it is always exported from leaves to buds thus causing dormancy unless antagonised by compounds whose concentration varies with daylength. This possibility can be explored by considering the response to ABA application. The induction of dormancy in LD by the application of synthetic (±) ABA has been reported for sycamore, Betula pubescens and blackcurrant; typical bud scales (modified stipules in Betula-, modified petioles in sycamore and black-

FIG. 11.18. The effect of transfer to short-days on the cessation of elongation growth and the abscisic acid content of needles in two latitudinal ecotypes of Picea abies. Plants were raised in LD conditions at 340 nmol m~2 s~20°C, before transfer to SD (16 h dark) for the number of days indicated. Plants were then returned to LD conditions (1 h night for the northern and 5 h night for the southern population). The northern population originated at 66° 45' N and the southern population at 46° 28' N. Qamaruddin et al. 1995.

FIG. 11.18. The effect of transfer to short-days on the cessation of elongation growth and the abscisic acid content of needles in two latitudinal ecotypes of Picea abies. Plants were raised in LD conditions at 340 nmol m~2 s~20°C, before transfer to SD (16 h dark) for the number of days indicated. Plants were then returned to LD conditions (1 h night for the northern and 5 h night for the southern population). The northern population originated at 66° 45' N and the southern population at 46° 28' N. Qamaruddin et al. 1995.

currant) were formed in all cases and the buds became fully dormant (El-Antably et al., 1967). The application of ABA in LD increased low-temperature hardiness in Acer negundo and a similar effect was obtained with an inhibitor extracted from SD leaves (Irving, 1969). The formation of turions in some aquatic hydrophytes can also be induced by the application of (±) ABA; in Spirodela polyrrhiza, for example (where ABA may function in vivo, Saks et al., 1980, Smart et al., 1995) but not in Myrio-phyllum verticillatum except under marginally inductive conditions (Weber and Nooden, 1976a). However, (±) ABA applications were found to have no effect in causing dormancy in Weigela florida (a species in which dormancy appears to be induced by exposure to a long night and, therefore, to be effected by a dormancy-evoking stimulus) and Catalpa bignonioides (Cathey, 1968), nor did it induce cold hardiness in Cornus stolonifera (Fuchigami et al., 1971b). In Salix pentandra, exogenous ABA did not induced growth cessation in continuous light, although the growth rate was reduced and stomatal resistance increased (Johansen et al., 1986).

One possibility for the lack of effect is failure of sufficient ABA to reach the site of action in the shoot apex. For, example, foliar application of ABA did not induce bud dormancy in either Betula or Alnus glutinosa in LD, but only a small proportion (<10%) of the applied ABA reached the apical region unaltered (Hocking and Hill-man, 1975). Similarly, although a daily foliar spray was ineffective (except at very high concentrations of up to 1000 mgl-1), dormancy could be induced in Acer palmatum and Cornus florida by immersing the youngest expanded leaf in ABA

together with spraying the apical region of the shoot (Cathey, 1968). A foliar spray of ABA was also ineffective in causing dormancy in Betula and sycamore (Hocking and Hillman, 1975) whereas dormancy was induced in both species when ABA was applied to both the leaves and apex (El-Antably et al., 1967). However, even where it is effective in preventing elongation, (±) ABA does not always entirely mimic the effect of SD. In Acer rubrum, stem extension ceased following the application of ABA but normal winter resting buds failed to develop (Perry and Hellmers, 1973); terminal buds also failed to develop in Picea abies seedlings and normal growth was subsequently resumed. Neither did ABA application induce the abscission of leaves (Cathey, 1968) nor of the shoot apex in Ailanthus glandulosa (El-Antably et al., 1967), both of which normally accompany the entry to dormancy.

Despite much research effort, the exact role of ABA in the regulation of dormancy induction in tree species remains unclear. Seasonal changes in endogenous ABA have been observed in a number of cases but there is little evidence that these are caused by changes in daylength. However, transient increases in ABA following transfer to SD have been observed in ecotypes of Picea and appear to be correlated with their photoperiodic behaviour. The application of exogenous ABA can clearly effect some of the changes associated with dormancy, especially the cessation or reduction of shoot elongation; however, other components of the dormancy complex are not necessarily affected.

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