Photoperiodic control of seasonal development and dormancy in tropical stem-succulent trees

Tropical stem-succulent trees store large quantities of water in their trunks yet remain leafless during the early and mid dry season. In contrast to most other tropical trees, bud break of vegetative buds is not induced in fully hydrated stem succulents between the winter solstice and the spring equinox by leaf abscission, abnormal rain showers or irrigation. Vegetative buds of leafless trees are therefore in a state of endo-dormancy similar to that of temperate perennial plants during early winter. Highly synchronous bud break regularly occurs soon after the spring equinox, often weeks before the first rainfalls of the wet season. These observations suggested that endo-dormancy and bud break might be induced by declining and increasing photoperiods after the autumn and spring equinoxes, respectively. In phenological field observations, we confirmed highly synchronous bud break after the spring equinox in many trees of five stem-succulent species in the northern and southern hemispheres. Shoot growth of potted saplings of Plumeria rubra L. was arrested by a decline in day length below 12 h after the autumn equinox, but continued in saplings maintained in a 13-h photoperiod. Conversely, exposure to a 13-h photoperiod induced bud break of dormant apical buds in saplings and cuttings in January, whereas plants maintained in the natural day length of < 11.7 h remained dormant. Photoperiodic control of endo-dormancy of vegetative buds in stem succulents is thus supported by field observations and experimental variation of the photoperiod. At low latitudes, where annual variation of day length is less than 1 h, bud dormancy is induced and broken by variations in photoperiod of less than 30 min.     

Dormancy release and chilling requirement of buds of latitudinal ecotypes of Betula pendula and B. pubescens

Bud burst and dormancy release of latitudinal ecotypes of Betula pendula Roth and B. pubescens Ehrh. from Denmark ( approximately 56 degrees N), mid-Norway ( approximately 64 degrees N) and northern Norway ( approximately 69 degrees N) were studied in controlled environments. Dormant seedlings were chilled at 0, 5 or 10 degrees C from October 4 onward and then, at monthly intervals from mid-November to February, batches of seedlings were held at 15 degrees C in an 8-h (SD) or 24-h (LD) photoperiod to permit flushing. A decline in days to bud burst occurred with increasing chilling time in all ecotypes. In November, after 44 chilling days, time to bud burst was least in plants chilled at 0 and 5 degrees C. The difference diminished with increasing chilling time, and in February, after 136 chilling days, bud burst was earliest in plants chilled at 10 degrees C. Long photoperiods during flushing significantly reduced thermal time after short chilling periods (44 and 74 days), but had no effect when the chilling requirement was fully met after 105 or more chilling days. No significant difference in these responses was found between the two species. In both species, chilling requirement decreased significantly with increasing latitude of origin. Bud burst was normal in seedlings overwintered at 12 degrees C, but was erratic and delayed in seedlings overwintered at 15 and especially at 21 degrees C, indicating that the critical overwintering temperature is between 12 and 15 degrees C. We conclude that there is little risk of a chilling deficit in birch under Scandinavian winter conditions even with a climatic warming of 7-8 degrees C. The likely effects of a climatic warming include earlier bud burst, a longer growing season and increased risk of spring frost injury, especially in high latitude ecotypes.     

Modeling the effects of winter environment on dormancy release of Douglas-fir

Most temperate woody plants have a winter chilling requirement to prevent budburst during mid-winter periods of warm weather. The date of spring budburst is dependent on both chilling and forcing; modeling this date is an important part of predicting potential effects of global warming on trees. There is no clear evidence from the literature that the curves of chilling or forcing effectiveness differ by species so we combined our data and published information to develop new curves on the effectiveness of temperature for chilling and forcing. The new curves predict effectiveness over a wide range of temperatures and we suggest both functions may be operating at the same time. We present experimental data from 13 winter environments for 5 genotypes of Douglas-fir (Pseudotsuga menziesii var. menziesii) and use them to test various assumptions of starting and stopping dates for accumulating chilling and forcing units and the relationship between budburst and the accumulation of chilling and forcing units. Chilling started too early to be effective in one treatment but the other 12 environments resulted in budburst from many combinations of chilling and forcing. Previous reports have suggested benefits or cancellations of effects from alternating day/night or periodic temperatures. Our simple models do not include these effects but nevertheless were effective in predicting relationships between chilling and forcing for treatments with a wide range of conditions. Overall, the date of budburst changed only slightly (+1 to ?11 days) across a wide range of treatments in our colder test environment (Olympia, WA, USA) but was substantially later (+29 days) in the warmest treatment in our warmer environment (Corvallis, OR, USA). An analysis of historical climate data for both environments predicted a wide range in date to budburst could result from the same mean temperature due to the relative weightings of specific temperatures in the chilling and forcing functions. In the absence of improved understanding of the basic physiological mechanisms involved in dormancy induction and release, we suggest that simple, universal functions be considered for modeling the effectiveness of temperature for chilling and forcing. Future research should be designed to determine the exact shape of the curves; data are particularly lacking at the temperature extremes. We discuss the implications of our data and proposed functions for predicting effects of climate change. Both suggest that the trend toward earlier budburst will be reversed if winter temperatures rise substantially.     

Effects of photoperiod and temperature on the timing of bud burst in Norway spruce (Picea abies)

We examined the effects of several photoperiod and temperature regimes imposed during the winter-spring period on the timing of bud burst in rooted cuttings of Norway spruce (Picea abies (L.) Karst.) grown in a greenhouse in Finland. The treatments were initiated in November and December after the cuttings had been exposed to natural chilling and freezing events. Irrespective of the treatments applied, time to bud burst decreased with increased duration of previous exposure to natural chilling and freezing events. Fluctuating day/night temperatures and continuous lengthening of the photoperiod hastened bud burst. Shortening the photoperiod delayed bud burst, suggesting that little or no ontogenetic development toward bud burst takes place during mild periods before the winter solstice. In the case of climatic warming, this phenomenon may prevent the premature onset of growth that has been predicted by computer simulations with models that only consider temperature regulation of bud burst.     

Low temperature, but not photoperiod, controls growth cessation and dormancy induction and release in apple and pear

In contrast to most temperate woody species, apple and pear and some other woody species of the Rosaceae family are insensitive to photoperiod, and no alternative environmental seasonal signal is known to control their dormancy. We studied growth and dormancy induction in micropropagated plants of four apple (Malus pumila Mill.) and one pear (Pyrus communis L.) commercial rootstock cultivars in controlled environments. The results confirm that growth cessation and dormancy induction in apple and pear are not influenced by photoperiod, and demonstrate that low temperature (< 12 degrees C) consistently induces both processes, regardless of photoperiodic conditions. Successive stages of the autumn syndrome (growth cessation, formation of bud scales and winter buds, leaf senescence and abscission, and dormancy induction) occurred in response to low temperature. Long days increased internode length at higher temperatures, but had no significant effect on leaf production in any of the cultivars. Chilling at 6 or 9 degrees C for at least 6 weeks (about 1000 h) was required for dormancy release and growth resumption, whereas treatment at 12 degrees C was marginally effective, even after 14 weeks of exposure. We are thus faced with the paradox that the same low temperature conditions that induce dormancy are also required for dormancy release in these species.     

High autumn temperature delays spring bud burst in boreal trees, counterbalancing the effect of climatic warming

The effect of temperature during short-day (SD) dormancy induction was examined in three boreal tree species in a controlled environment. Saplings of Betula pendula Roth, B. pubescens Ehrh. and Alnus glutinosa (L.) Moench. were exposed to 5 weeks of 10-h SD induction at 9, 15 and 21 degrees C followed by chilling at 5 degrees C for 40, 70, 100 and 130 days and subsequent forcing at 15 degrees C in a 24-h photoperiod for 60 days. In all species and with all chilling periods, high temperature during SD dormancy induction significantly delayed bud burst during subsequent flushing at 15 degrees C. In A. glutinosa, high temperature during SD dormancy induction also significantly increased the chilling requirement for dormancy release. Field experiments at 60 degrees N with a range of latitudinal birch populations revealed a highly significant correlation between autumn temperature and days to bud burst in the subsequent spring. September temperature alone explained 20% of the variation between years in time of bud burst. In birch populations from 69 and 71 degrees N, which ceased growing and shed their leaves in August when the mean temperature was 15 degrees C, bud burst occurred later than expected compared with lower latitude populations (56 degrees N) in which dormancy induction took place more than 2 months later at a mean temperature of about 6 degrees C. It is concluded that this autumn temperature response may be important for counterbalancing the potentially adverse effects of higher winter temperatures on dormancy stability of boreal trees during climate warming.     

Different responses of northern and southern ecotypes of Betula pendula to exogenous ABA application

We investigated responses of northern and southern ecotypes of silver birch (Betula pendula Roth) to exogenous abscisic acid (ABA) under controlled environmental conditions to determine the role of ABA in cold acclimation and dormancy development. Abscisic acid was sprayed on the leaves and changes in freezing tolerance, determined by the electrolyte leakage test, and bud dormancy were monitored. Applied ABA induced cold acclimation but had no effect on growth cessation in seedlings grown in long day conditions (LD, 24-h photoperiod at 18 degrees C). It enhanced freezing tolerance and accelerated growth cessation in seedlings grown in short day conditions (SD, 12-h photoperiod at 18 degrees C), and slightly enhanced freezing tolerance in seedlings grown at low temperature (LT, 24-h photoperiod at 4 degrees C) in both ecotypes. There were distinct ecotypic differences in ABA-induced cold acclimation and dormancy development. The northern ecotype was more responsive to applied ABA than the southern ecotype, resulting in more rapid development of freezing tolerance in all treatments, and earlier dormancy development in SD. When plants were grown in a photoperiod just above the critical photoperiod for the ecotype (defined as the longest photoperiod that induces growth cessation), applied ABA caused growth cessation and dormancy development. Compared with ABA-treated seedlings grown in SD, dormancy development was delayed in ABA-treated seedlings exposed to a near-critical photoperiod, but even in this treatment dormancy developed faster in the northern ecotype than in the southern ecotype.     

Loblolly pine seed dormancy: constraints to germination.

Stratification by moist chilling is often used to break seed dormancy in loblolly pine (Pinus taeda L.). The role that moist chilling plays at the cellular level in preparing the embryo for germination and early seedling growth was investigated. Moist chilling did not affect the embryo's ability to mobilize seed storage proteins, which is a biochemical marker of early seedling growth. Analysis of in vivo protein synthesis profiles indicated that moist chilling has only a subtle effect on gene expression in the embryo or germinant. The means by which moist chilling affects interactions between the seed coat and the living tissues of the seed also was studied. Although the seed coat is mainly a mechanical barrier to germination, seed coat replacement studies showed that moist chilling alleviates some factor(s) in the seed coat that significantly inhibit germination.     

Effect of Temperature on the Induction of Bud Dormancy in Ecotypes of Betula pubescens and Betula pendula

The purpose of this study was to determine the influence of temperature applied during short day-induced budset on induction of dormancy in six ecotypes of Betula pubescens Ehrh. and two ecotypes of Betula pendula Roth. Seedlings were grown in a phytotron at constant temperatures of 9–21°C under a 12 h photoperiod (SD) during dormancy induction. Induction of dormancy was monitored by following bud flushing and shoot growth after transfer to long photoperiod conditions (24 h) at 18°C. Chilling requirement was studied in seedlings exposed to 10 weeks of SD. In both species induction of bud dormancy developed most rapidly at 15–18°C, and both 9–12°C and 21°C delayed the induction of dormancy. Raising the temperature (from 9 to 21°C) applied during induction of dormancy significantly increased the chilling requirement. These responses were noted for all ecotypes tested, but in general the northern ecotypes entered dormancy more quickly than the southern ones. No such trend was recorded for chilling requirement, although a B. pubescens ecotype from Iceland and another from the coast of northern Norway appeared to require a longer chilling treatment than the other ecotypes. In conclusion, induction and depth of bud dormancy in birch are significantly affected by temperature conditions and these effects may explain some of the annual variation in dormancy and chilling requirement observed in nature.     

2008年初广州常见园林植物冷害情况调查及分析

2008年2月底有针对性地对广州市常见园林植物展开冷害情况调查.调查发现,广州市中心城区不少常见的园林植物都受到不同程度的持续低温天气的影响,隶属于60个科,154种,受害等级在Ⅰ-Ⅴ之间.其中,受害程度比较严重的植物主要集中在夹竹桃科、龙舌兰科、露蔸树科、蝎尾蕉科等几个典型热带分布的科,草本植物比木本植物更容易受害.植物的受害程度还与生境、栽培历史、管理方法等因素有关.针对本次冷害园林植物受害的特点与原因,提出关于广州市园林植物引种栽培的几点建议.     

Seed dormancy and germination of Ficus lundellii and tropical forest restoration

We investigated seed dormancy and germination in Ficus lundellii Standl. (Moraceae), a native species of Mexico's Los Tuxtlas tropical rain forest. In an 8-h photoperiod at an alternating diurnal (16/8 h) temperature of 20/30 degrees C, germination was essentially complete (96%) within 28 days, whereas in darkness, all seeds remained dormant. Neither potassium nitrate (0.05-0.2%) applied continuously nor gibberellic acid applied either continuously (10-200 ppm) or as a 24 hour pretreatment (2000 ppm) induced germination in the dark. Germination in the light was not reduced by a 24-h hydrochloric acid (0.1-1%) pretreatment, but it was reduced both by a 24-h pretreatment with either H(2)O(2) (0.1-5 M) or 5% HCl, or by more than 5 days of storage at 40 degrees C (4.5% seed water content). In a study with a 2-dimensional temperature gradient plate, seeds germinated fully and rapidly in the light at a constant temperature of 30 degrees C, and fully but less rapidly in the light at alternating temperatures with low amplitudes (< 12 degrees C) about the optimal constant temperature. The base, optimal and ceiling temperatures for rate of germination were estimated as 13.8, 30.1 and 41.1 degrees C, respectively. In all temperature regimes, light was essential for the germination of F. lundellii seeds.     

银杏绿叶越冬技术

通过人工落叶及用植物生长物质对落叶前的盆栽银杏植株进行处理，可明显促进新芽萌动和生长，并可在温室条件下继续越冬生长而不落叶。通过改变盆栽银杏生长的光温条件，可推延或打破银杏的休眠，防止银杏冬季落叶。由于实现了绿叶越冬而使盆栽银杏进入冬季和早春花卉市场成为可能。     

Influence of environment, fertilizer and genotype on shoot morphology and subsequent rooting of birch cuttings

Differences in rooting ability of birch (Betula pubescens J.F. Ehrh.) cuttings were observed as a result of differences in genotype and physiology of the stock plants. The uniformity in response among cuttings from micropropagated plants compared with cuttings from seed plants confirmed the advantage of using micropropagated plants to study environmental effects. Shoot morphology of the seed stock plants was influenced by both photoperiod and thermoperiod. A day/night temperature of 15/25 degrees C reduced stem elongation compared with a day/night temperature of 25/15 degrees C regardless of photoperiod, and a continuous light regime resulted in more shoots per plant in both temperature regimes than a 16-h photoperiod. A reduction in the supply of macronutrients did not influence shoot morphology, but increased rooting substantially and seemed to override the effects of environmental factors. In cuttings of seed plants, the highest rooting percentage and number of roots were obtained in a 16-h photoperiod with a day/night temperature of 15/25 degrees C. In micropropagated stock plants, there was a positive correlation between shoot length and number of leaves per shoot and topographical distribution of light within the plants, but there was no correlation between these parameters and rooting ability of the cuttings. A rooting temperature of 16 degrees C delayed the rate of root production compared with the rate at higher temperatures, but the final rooting percentage was the same over the range from 16 to 28 degrees C. Root branching increased with temperature. At all temperatures, there was a large increase in sucrose content at the base of the cuttings during rooting, whereas the concentration of nontranslocated sugars remained constant. The carbohydrate content at the base of cuttings from micropropagated stock plants was three times higher than at the base of cuttings from seed stock plants, but the higher carbohydrate content was not correlated with a higher rooting potential.     

Influence of photoperiod and temperature on frost hardiness and free amino acid concentrations in black spruce seedlings

We investigated the effects of a 4-week exposure to an 8-h or 18-h photoperiod at 5 or 25 degrees C on the development of hardiness to -20 degrees C and the accumulation of proline (Pro), arginine (Arg) and tryptophan (Trp) in shoots of black spruce (Picea mariana (Mill.) B.S.P.) seedlings. The greatest degree of hardening to -20 degrees C occurred in seedlings exposed to an 8-h photoperiod at 25 degrees C, and some hardening occurred in seedlings exposed to 5 degrees C in either an 8-h or 18-h photoperiod. Proline accumulated in shoots in response to 5 degrees C and either an 8-h or 18-h photoperiod, whereas Trp accumulated in response to an 8-h photoperiod at either temperature, and Arg only accumulated in shoots in the 5 degrees C + 8-h photoperiod treatment. Only the accumulation of Trp was significantly related to the degree of hardiness to -20 degrees C.     

Dependence of dormancy release on temperature in different origins of Pinus sylvestris and Betula pendula seedlings

The chilling requirement of rest completion and the high temperature requirement of growth initiation were determined in three origins of silver birch (Betula pendula Roth.) seedlings and five origins of Scots pine (Pinus sylvestris L.) seedlings. In both the pine and birch the chilling requirement was highest in maritime Scottish origins and lowest in the most continental Finnish and Russian origins. The requirement for southern mountainous Spanish and Bulgarian pine origins was in between. In terms of the high temperature requirement, there were no clear differences between origins. These results suggest that owing to their high chilling requirement, which prevents the beginning of growth and the loss of frost hardiness during the frost‐exposed season, origins from a maritime climate could be the most tolerant under climatic warming.     

Physiological and biochemical responses to low non-freezing temperature of two Eucalyptus globulus clones differing in drought resistance

We have compared the metabolic responses of leaves and roots of two Eucalyptus globulus L. clones CN5 and ST51 that differ in their sensitivity to water deficits (ST51 is more drought sensitive), with regard to the effect of chilling (10/5 °C, day/night). We studied changes in growth, osmotic potential and osmotically active compounds, soluble proteins, leaf pigments, and membrane lipid composition. Our data showed that both clones have the ability to acclimatize to chilling temperatures. As a result of 10 days of acclimation, an increase of soluble sugars in leaves of treated plants of both clones was observed that disappeared later on. Differences between clones were observed in the photosynthetic pigments and soluble protein content which were more stable in CN5 under chilling. It also was apparent that CN5 presented a less negative predawn water potential (ψ_pd) and a higher leaf turgor than ST51 throughout the chilling treatment. In the case of the CN5, increased total lipids (TEA) and concomitant increase of linolenic acid (C18:3) in leaves after acclimatization may be related to a better clone performance under chilling temperatures. Moreover, a higher constitutive investment in roots in the case of CN5 as compared to ST51 may benefit new root regeneration under low temperatures favoring growth after cold Mediterranean winter.     

Tea (Camellia sinensis) clones with shorter periods of winter dormancy exhibit lower accumulation of reactive oxygen species

Tea (Camellia sinensis (L.) O. Kuntze) is a perennial crop grown throughout the world. During winter, tea undergoes a dormancy period when growth of apical buds almost ceases, severely reducing the commercial yield of tea. Low temperatures prevail during the period of winter dormancy, which alone or in combination with high solar irradiance have the potential to induce oxidative stress in plants. We studied six tea clones under field conditions to test whether a relationship exists between oxidative stress and winter dormancy. Data on the behavior of the enzymatic antioxidative system was collected for all clones during different phases of winter dormancy. There was a strong positive correlation among clones between accumulation of reactive oxygen species (ROS) and the length of the dormancy period. Clones having shorter dormancy periods exhibited higher induction of antioxidative enzymes. Results suggest that efficient scavenging of ROS is a desirable feature in tea because it leads to lower accumulations of ROS during winter months and is associated with reduced winter dormancy.     

A molecular marker associated with low-temperature induction of dormancy in red osier dogwood (Cornus sericea)

Dormancy induction in temperate deciduous plants is thought to be regulated by short photoperiods, but low temperature has been shown to eliminate the short photoperiod requirement in northern ecotypes. An F2 population (191 plants) red osier dogwood (Cornus sericea L.) derived from a polycross of an F1 population produced from reciprocal crosses of the parental clonal ecotypes, Northwest Territories (NWT, 62 degrees N) and Utah (42 degrees N), was examined to identify molecular markers of temperature-induced endodormancy. Dormancy induction curves were generated for each individual in the F2 population and a standard point prior to vegetative maturity (i-VM) was inferred from the change in slope of the dormancy acquisition curve. Under Saskatoon, Saskatchewan field conditions (52 degrees N), the NWT ecotype entered i-VM on average 5-6 weeks before the Utah ecotype. Two sub-populations of the F2 population were distinguishable based on VM acquisition on exposure to low temperature but not to short photoperiods. A sequence characterized amplified region (SCAR) marker was developed that correctly (> 92%) identified individual plants within the F2 subpopulation that were responsive to low-temperature induction of VM. Timing of bud break was strongly associated with the timing of VM in the geographical ecotypes but not in the F2 population, indicating that these are separate traits under genetic control.     

Physiological responses of three deciduous conifers (Metasequoia glyptostroboides, Taxodium distichum and Larix laricina) to continuous light: adaptive implications for the early Tertiary polar summer

Polar regions were covered with extensive forests during the Cretaceous and early Tertiary, and supported trees comparable in size and productivity to those of present-day temperate forests. With a winter of total or near darkness and a summer of continuous, low-angle illumination, these temperate, high-latitude forests were characterized by a light regime without a contemporary counterpart. Although maximum irradiances were much lower than at mid-latitudes, the 24-h photoperiod provided similar integrated light flux. Taxodium, Larix and Metasequoia, three genera of deciduous conifers that occurred in paleoarctic wet forests, have extant, closely related descendents. However, the contemporary relative abundance of these genera differs greatly from that in the paleoarctic. To provide insight into attributes that favor competitive success in a continuous-light environment, we subjected saplings of these genera to a natural photoperiod or a 24-h photoperiod and measured gas exchange, chlorophyll fluorescence, non-structural carbohydrate concentrations, biomass production and carbon allocation. Exposure to continuous light significantly decreased photosynthetic capacity and quantum efficiency of photosystem II in Taxodium and Larix, but had minimal influence in Metasequoia. In midsummer, foliar starch concentration substantially increased in both Taxodium and Larix saplings grown in continuous light, which may have contributed to end-product down-regulation of photosynthetic capacity. In contrast, Metasequoia allocated photosynthate to continuous production of new foliar biomass. This difference in carbon allocation may have provided Metasequoia with a two fold advantage in the paleoarctic by minimizing depression of photosynthetic capacity and increasing photosynthetic surface.