高寒山区植物根抗氧化酶系统的季节变化与抗冷冻关系

在高寒山区（海拔2900m）和选取4种多年生草本植物,即无芒雀麦（Bromus inermis）、草地早熟禾（Poa sphyondylodes）、花誉麦（Bromus sinensis）和垂重申披碱草（Elymus nutans）,测定了秋末、冬初、冬季、春季气温变化过程中其根中丙二醛（MDA）含量和抗氧酶活力（过氧化氢酶（CAT）、过氧化物酶（POD）、超氧物歧化酶（SOD））和抗坏血酸氧化酶（APX）变化,分析了抗氧酶系统在根抗冷适应中的作用,结果表明,随秋末降温植物根中MDA含量增加,尔后下降,在冬季和翌年春季保持相对稳定。从9月初到10月下旬,4种植物根中SOD、CAT、POD活力平均增加170％、130％和56％。在冬季下降,但仍远高于9月,在春季气温上升过程中酶活力上升。根能在组织结冰状况下生存与其具备完善的保护酶系统,能及时清除氧自由基抑制膜脂过氧化维持膜完整性有关,据降温过程中MDA含量和抗氧酶活力变化,可将根冷适应分为两个阶段,即第1阶段平均气温在0℃以上,抗氧酶活力增强,MDA增加阶段,第2阶段平均气温降至0℃以下,最低气温降到－15℃以下,抗氧酶活力下降,MDA无明显变化阶段。     

高寒山区牧草根中丙二醛、渗透调节物、多胺季节动态与抗冻力关系研究

温带地区的高山多年生草本植物可在－３０℃组织结冰状况下生存,然而人们并不了解其抗冻的生理机理。本研究目的拟通过测定自然生境下生长的４种高寒山区禾本科牧草（无芒雀麦（Ｂｒｏｍｕｓ ｉｎｅｒｍｉｓ Ｌｅｙｓｓ．）、花雀麦（Ｂｒｏｍｕｓ　 ｓｉｎｅｎｓｉｓ　 Ｋｅｎｇ．）、垂穗披碱草（Ｅｌｙｍｕｓ ｎｕｔａｎｓ Ｇｒｉｓｅｂ．）、草地早熟禾（Ｐｏａ ｓｐｈｙｏｎｄｙｌｏｄｅｓ　 Ｔｒｉｎ．）根中渗透调节物、膜脂过氧化产物、多胺含量季节变化,以了解他们与牧草抗冻的关系。结果表明,在晚秋（９月１～１５日）牧草根中ＭＤＡ含量增高,尔后下降,冬季保持恒定。总碳水化合物（ＴＮＣ）,可溶性糖、脯氨酸、可溶性蛋白质含量随晚秋气温下降而增加,在１１月达到最高,尔后下降,且持续到翌年春季。随晚秋气温下降从９月到１１月根中多胺含量迅速增加,４种牧草平均增加１８０％,其中亚精胺（Ｓｐｄ）占多胺含量的５３％。在１１月牧草根中多胺几乎完全消失。上述物质在晚秋入冬增加正好与牧草抗冻锻炼时间相吻合,因而是植物抗冻适应的重要生理响应和植物越冬的低温保护物质。他们在降低细胞冰点、防止细胞结冰引起的膜机械伤害,抑制膜脂过氧化保护膜稳定性方面具有重要作用。     

Nitrogen acquisition by annual and perennial grass seedlings: testing the roles of performance and plasticity to explain plant invasion

Differences in resource acquisition between native and exotic plants is one hypothesis to explain invasive plant success. Mechanisms include greater resource acquisition rates and greater plasticity in resource acquisition by invasive exotic species compared to non-invasive natives. We assess the support for these mechanisms by comparing nitrate acquisition and growth of invasive annual and perennial grass seedlings in western North America. Two invasive exotic grasses (Bromus tectorum and Taeniatherum caput-medusae) and three perennial native and exotic grasses (Pseudoroegneria spicata, Elymus elymoides, and Agropyron cristatum) were grown at various temperatures typical of autumn and springtime when resource are abundant and dominance is determined by rapid growth and acquisition of resources. Bromus tectorum and perennial grasses had similar rates of nitrate acquisition at low temperature, but acquisition by B. tectorum significantly exceeded perennial grasses at higher temperature. Consequently, B. tectorum had the highest acquisition plasticity, showcasing its ability to take advantage of transient warm periods in autumn and spring. Nitrate acquisition by perennial grasses was limited either by root production or rate of acquisition per unit root mass, suggesting a trade-off between nutrient acquisition and allocation of growth to structural tissues. Our results indicate the importance of plasticity in resource acquisition when temperatures are warm such as following autumn emergence by B. tectorum. Highly flexible and opportunistic nitrate acquisition appears to be a mechanism whereby invasive annual grasses exploit soil nitrogen that perennials cannot use.     

低温锻炼对绵头雪莲花组织培养苗抗寒性及抗氧化酶活性的影响

高山植物绵头雪莲花(Saussurea laniceps HandMazz.)组织培养苗经2℃低温锻炼后抗寒性明显增强,SOD、CAT、POD活性明显提高,可溶性蛋白质、脯氨酸含量升高,而可溶性糖含量没有明显变化.脱锻炼期间可溶性蛋白质含量仍高于低温锻炼期间的水平,脱锻炼后抗氧化酶活性略有上升或保持稳定状态.这些变化是绵头雪莲花组织培养苗适应低温逆境的生理生化基础.     

Cold acclimation in warmer extended autumns impairs freezing tolerance of perennial ryegrass (Lolium perenne) and timothy (Phleum pratense)

The effect of variable autumn temperatures in combination with decreasing irradiance and daylength on photosynthesis, growth cessation and freezing tolerance was investigated in northern‐ and southern‐adapted populations of perennial ryegrass (Lolium perenne) and timothy (Phleum pratense) intended for use in regions at northern high latitudes. Plants were subjected to three different acclimation temperatures; 12, 6 and 9/3°C (day/night) for 4 weeks, followed by 1 week of cold acclimation at 2°C under natural light conditions. This experimental setup was repeated at three different periods during autumn with decreasing sums of irradiance and daylengths. Photoacclimation, leaf elongation and freezing tolerance were studied. The results showed that plants cold acclimated during the period with lowest irradiance and shortest day had lowest freezing tolerance, lowest photosynthetic activity, longest leaves and least biomass production. Higher acclimation temperature (12°C) resulted in lower freezing tolerance, lower photosynthetic activity, faster leaf elongation rate and higher biomass compared with the other temperatures. Photochemical mechanisms were predominant in photoacclimation. The northern‐adapted populations had a better freezing tolerance than the southern‐adapted except when grown during the late autumn period and at the highest temperature; then there were no differences between the populations. Our results indicate that the projected climate change in the north may reduce freezing tolerance in grasses as acclimation will take place at higher temperatures and shorter daylengths with lower irradiance.     

Interspecific diversity in root antioxidative enzyme activities reflect root turnover strategies and preferred habitats in wetland graminoids

Antioxidant enzymes protect cells against oxidative stress and are associated with stress tolerance and longevity. In animals, variation in their activities has been shown to relate to species ecology, but in plants, comparative studies with wild species are rare. We investigated activities of five antioxidant enzymes – ascorbate peroxidase (APX), catalase (CAT), glutathione reductase (GR), peroxidase (POX), and superoxide dismutase (SOD) – in roots of four perennial graminoid wetland species over a growing season to find out whether differences in root turnover or habitat preferences would be associated with variation in seasonal patterns of antioxidant enzyme activities. The investigated species differ in their root turnover strategies (fine roots senesce in the fall or fine roots survive the winter) and habitat preferences (nutrient‐poor vs. productive wetlands). Roots were collected both in the field and from garden‐grown plants. Antioxidant enzyme activities were higher and lipid peroxidation rates lower in species with annual root systems, and for species of the nutrient‐poor wetland, compared with perennial roots and species of productive wetlands, respectively. There was variation in the activities of individual antioxidant enzymes, but discriminant analyses with all enzymes revealed a clear picture, indicating consistent associations of antioxidant enzyme activities with the type of root turnover strategy and with the preferred habitat. We conclude that antioxidant enzyme activities in plant roots are associated with the species' ecological strategies and can be used as traits for the characterization of the species' position along plant economics spectrum.     

Susceptibility to low-temperature photoinhibition and the acquisition of freezing tolerance in winter and spring wheat: The role of growth temperature and irradiance

Five winter and five spring wheat ( Triticum aestivum L.) cultivars were grown under either control conditions (20°C/250 photosynthetic photon flux density (PPFD) [μmol m⁻² s⁻¹]), high irradiance (20°C/800 PPFD) or at low temperature (either 5°C/250 PPFD or 5°C/50 PPFD). To eliminate any potential bias, the wheat cultivars were arbitrarily chosen without any previous knowledge of their freezing tolerance or photosynthetic competence. We show that the differential susceptibilities to photoinhibition exhibited between spring and winter wheat cultivars, as assessed by chlorophyll fluorescence cannot be explained on the basis of either growth irradiance or low growth temperature per se. The role of excitation pressure is discussed. We assessed the correlation between susceptibility to low-temperature photoinhibition, maximum ribulose 1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) and NADP-dependent malate dehydrogenase (EC 1.1.1.82) activities, chlorophyll and protein concentrations and freezing tolerance determined by electrolyte leakage. Susceptibility to photoinhibition is the only parameter examined that is strongly and negatively correlated with freezing tolerance. We suggest that the assessment of susceptibility to photoinhibition may be a useful predictor of freezing tolerance and field survival of cereals.     

Mowing Reduces Exotic Annual Grasses but Increases Exotic Forbs in a Semiarid Grassland

Cheatgrass (Bromus tectorum) and other exotic winter‐active plants can be persistent invaders in native grasslands, growing earlier in the spring than native plants and pre‐empting soil resources. Effective management strategies are needed to reduce their abundance while encouraging the reestablishment of desirable native plants. In this 4‐year study, we investigated whether mowing and seeding with native perennial grasses could limit growth of exotic winter‐actives, and benefit growth of native plants in an invaded grassland in Colorado, United States. We established a split‐plot experiment in October 2008 with 3 mowing treatments: control, spring‐mowed, and spring/summer‐mowed (late spring, mid‐summer, and late summer), and 3 within‐plot seeding treatments: control, added B. tectorum seeds, and added native grass seeds. Cover of plant species and aboveground biomass were measured for 3 years. In March and June of 2010, 2011, and March of 2012, B. tectorum and other winter‐annual grasses were half as abundant in both mowing treatments as in control plots; however, cover of non‐native winter‐active forbs increased 2‐fold in spring‐mowed plots and almost 3‐fold in spring/summer‐mowed plots relative to controls. These patterns remained consistent 1 year after termination of treatments. Native cool‐season grasses were most abundant in spring‐mowed plots, and least abundant in control plots. There was higher cover of native warm‐season grasses in spring/summer‐mowed plots than in control plots in July 2011 and 2012. The timing of management can have strong effects on plant community dynamics in grasslands, and this experiment indicates that adaptive management can target the temporal niche of undesirable invasive species.     

Winter cold tolerance and the geographic range separation of Bromus tectorum and Bromus rubens,two severe invasive species in North America

The invasive grasses Bromus rubens and Bromus tectorum are responsible for widespread damage to semiarid biomes of western North America. Bromus. tectorum dominates higher and more northern landscapes than its sister species B. rubens, which is a severe invader in the Mojave desert region of the American Southwest. To assess climate thresholds controlling their distinct geographic ranges, we evaluated the winter cold tolerance of B. tectorum and B. rubens. Freezing tolerance thresholds were determined using electrolyte leakage and whole‐plant mortality. The responses of the two species to winter cold and artificial freezing treatments were similar in 2007–2008 and 2009–2010. When grown at minimum temperatures of 10 °C, plants of both species had cold tolerance thresholds near ?10 °C, while plants acclimated to a daily minimum of ?10 to ?30 °C survived temperatures down to ?31 °C. In the winter of 2010–2011, a sudden severe cold event on December 9, 2010 killed all B. rubens populations, while B. tectorum was not harmed; all tested plants were 7–8 weeks old. Controlled acclimation experiments demonstrated that 8‐week‐old plants of B. rubens had a slower acclimation rate to subzero temperatures than B. tectorum and could not survive a rapid temperature drop from 1 to ?14 °C. Four‐month‐old B. rubens populations were as cold tolerant as B. tectorum. Our results show that severe and sudden freeze events in late autumn can kill young plants of B. rubens but not B. tectorum. Such events could exclude B. rubens from the relatively cold, Intermountain steppe biome of western North America where B. tectorum predominates.     

Temperature and functional traits influence differences in nitrogen uptake capacity between native and invasive grasses

Performance differences between native and exotic invasive plants are often considered static, but invasive grasses may achieve growth advantages in western North America shrublands and steppe under only optimal growing conditions. We examine differences in N uptake and several morphological variables that influence uptake at temperatures between 5 and 25 °C. We contrast two native perennial grasses in western North America: Elymus elymoides and Pseudoroegneria spicata; two invasive annual grasses: Bromus tectorum and Taeniatherum caput-medusae; and one highly selected non-native perennial grass: Agropyron cristatum. The influence of temperature on N uptake is poorly characterized, yet these invasive annual grasses are known to germinate in warm soils in the autumn, and both experience cool soils during the short growing season following snowmelt in the spring. To further explore the influence of temperature on the correlation between morphological variables and N uptake, our data are applied to a previously published path model and one proposed here. Differences in N uptake between native and invasive grasses were small at the lowest temperature, but were large at the highest temperature. At lower temperatures, uptake of N by annuals and perennials was correlated with leaf N and mass. At higher temperatures, uptake by annuals was correlated only with these leaf traits, but uptake by perennials was correlated with these leaf traits as well as root N and mass. Consequently, our results imply that annual grasses face fewer morphological constraints on N uptake than perennial grasses, and annual grasses may gain further advantage in warmer temperature conditions or during more frequent warm periods.     

Cold tolerance,water relations and accumulation of osmolytes in <Emphasis Type="Italic">Bituminaria bituminosa</Emphasis>

We determined the cold (freezing) tolerance of five Spanish populations of the perennial shrub Bituminaria bituminosa (L.) C.H. Stirton (Fabaceae), as the temperature at which 50 % of leaf electrolytes are released (LT₅₀) using leaves of field-grown plants, obtained in two winters and one spring. The freezing tolerance was greater in winter and reflected the minimum temperatures at the original sites from which the populations were obtained. Tolerance in vitro was related to osmotic adjustment in the leaves; more negative osmotic potential values and more positive pressure potential values (MPa) were associated with greater tolerance. Tolerance and osmotic potential were not related to leaf cation contents but to leaf amino acids, soluble sugar and proline contents.     

Antioxidant gene-enzyme responses in Medicago truncatula genotypes with different degree of sensitivity to salinity

Antioxidant responses and nodule function of Medicago truncatula genotypes differing in salt tolerance were studied. Salinity effects on nodules were analysed on key nitrogen fixation proteins such as nitrogenase and leghaemoglobin as well as estimating lipid peroxidation levels, and were found more dramatic in the salt-sensitive genotype. Antioxidant enzyme assays for catalase (CAT, EC 1.11.1.6), superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and guaiacol peroxidase (EC 1.11.1.7) were analysed in nodules, roots and leaves treated with increasing concentrations of NaCl for 24 and 48 h. Symbiosis tolerance level, depending essentially on plant genotype, was closely correlated with differences of enzyme activities, which increased in response to salt stress in nodules (except CAT) and roots, whereas a complex pattern was observed in leaves. Gene expression responses were generally correlated with enzymatic activities in 24-h treated roots in all genotypes. This correlation was lost after 48 h of treatment for the sensitive and the reference genotypes, but it remained positively significant for the tolerant one that manifested a high induction for all tested genes after 48 h of treatment. Indeed, tolerance behaviour could be related to the induction of antioxidant genes in plant roots, leading to more efficient enzyme stimulation and protection. High induction of CAT gene was also distinct in roots of the tolerant genotype and merits further consideration. Thus, part of the salinity tolerance in M. truncatula is related to induction and sustained expression of highly regulated antioxidant mechanisms.     

Dormancy of alpine and subalpine perennial forbs

Depth of dormancy of alpine and subalpine perennial forbs in autumn was investigated, which was judged by the number of days required for growth initiation at 24 °C. The depth of dormancy differed depending on Raunkiaer’s life-form and shoot habits. Chamaephytes with perennial shoot-axes showed shallower dormancy than hemicryptophytes with annual shoot-axes, and geophytes with annual shoot-axes showed the deepest dormancy. The results strongly suggest that the dormancy is more endogenously controlled in forbs less hardy to freezing stress. Potential growth ability of alpine herbaceous chamaephytes in autumn is an adaptive advantage, since they utilize the short vegetative period as long as possible. All of the species with annual shoot-axes had winter buds covered with scales. In plants with perennial shoot-axes, percentage of winter buds covered with scales increased with increasing depth of dormancy. The results indicate that the shoot apices are well protected by bud scales in forbs with a long endogeneous-dormant period.     

Comparative physiological and proteomic response to abrupt low temperature stress between two winter wheat cultivars differing in low temperature tolerance

Abrupt temperature reduction in winter wheat at either autumn seedling stage prior to vernalisation or early spring crown stage can cause severe crop damage and reduce production. Many studies have reported the physiological and molecular mechanisms underlying cold acclimation in winter wheat by comparing it with spring wheat. However, processes associated with abrupt temperature reduction in autumn seedling stage prior to vernalisation in winter wheat are less understood. In this study, physiological and molecular responses of winter wheat seedlings to abrupt low temperature (LT) stress were characterised in the relatively LT‐tolerant winter wheat cultivar Shixin 828 by comparing it with the relatively LT‐sensitive cultivar Shiluan 02‐1 using a combination of physiological, proteomics and biochemical approaches. Shixin 828 was tolerant to abrupt LT stress, while Shiluan 02‐1 exhibited high levels of reactive oxygen species (ROS) and leaf cell death. Significant increases in relative abundance of antioxidant‐related proteins were found in Shixin 828 leaves, which correlate with observed higher antioxidant enzyme activity in Shixin 828 compared to Shiluan 02‐1. Proteomics analysis also indicated that carbohydrate metabolism‐related proteins were more abundant in Shiluan 02‐1, correlating with observed accumulation of soluble sugars in Shiluan 02‐1 leaves. Amino acid analysis revealed a strong response to LT stress in wheat leaves. A negative effect of exogenous sucrose on LT tolerance was also found. This study indicates that high ROS scavenging capacity and high abundance of photosynthesis‐related proteins might play a role in winter wheat response to abrupt LT stress. In contrast, excess accumulation of soluble sugars might be disadvantageous for LT tolerance in the wheat cultivar Shiluan 02‐1.     

Influence of simulated snow cover on the cold tolerance and freezing injury of yellow-cedar seedlings

It has been hypothesized that yellow‐cedar [Chamaecyparis nootkatensis (D. Don) Spach] decline may result from root freezing injury following climate change‐induced reductions in protective snow cover. To test this hypothesis, we measured the freezing tolerance and injury expression of yellow‐cedar seedlings in three treatments that differed in the insulative protection they provided to soils during winter and spring: (1) full exposure to ambient temperatures (exposed treatment), (2) continuous protection from ambient temperatures via addition of perlite over pots (full protection), and (3) perlite protection only during winter and exposure to ambient temperatures during spring (partial protection). Foliage from all treatments was cold tolerant enough to prevent foliar freezing injury throughout the study period. However, on all sample dates, roots of seedlings from all treatments were only tolerant to about ?5 °C – a level considerably warmer than the reported maximum cold tolerance for the species and well above the soil temperature recorded in the exposed treatment. As a result of this limited root cold tolerance, visibly uninjured roots of seedlings from the exposed treatment had significantly higher relative electrolyte leakage (REL) throughout the winter and early spring than seedlings in soil protection treatments. Seedlings from the exposed treatment also had significantly higher foliar REL values and greater visual foliar injury than seedlings from the other treatments starting in early spring. For both roots and foliage, REL measurements consistently detected tissue damage before visual injury was evident. Patterns of injury from both REL and visual injury assessments showed the same pattern: damage began with freezing injury to roots and subsequently became evident as foliar browning after spring temperatures increased. All seedlings in the exposed treatment eventually had 100% fine root damage and died. This progression of initial root damage followed by foliar browning and mortality after the onset of warming conditions is consistent with reports of yellow‐cedar decline symptom development in the field.     

Effect of drought and low light on growth and enzymatic antioxidant system of Picea asperata seedlings

The combined effects of drought and low light on biomass partition, foliar nitrogen concentration, membrane stability and active oxygen species (AOS) and antioxidant system were investigated in dragon spruce (Picea asperata Mast.) seedlings grown at two watering regimes (well-watered, 100% of field capacity and drought, 30% of field capacity) and light availabilities (HL, 100% of full sunlight and low light, 15% of full sunlight). Under high light condition drought not only reduced foliar nitrogen concentration (Nmass) and membrane stability index (MSI) but also significantly increased biomass partitioning to roots, AOS, ascorbic acid (AsA) content and antioxidant enzyme activities including superoxide dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase(GR, EC 1.6.4.2). However, no prominently drought-induced differences in biomass partitioning to root, SOD, GR activities, hydrogen peroxide (H₂O₂) and MSI were observed in low light seedlings. On the other hand, significant interaction of drought and low light was found on MSI, the antioxidant enzymes activities (SOD, POD, CAT, APX, GR), H₂O₂ and superoxide radical (O₂ ⁻). These results suggested that seedlings grown at the understory were more sensitive to drought than low light.     

Responses of antioxidant gene, protein and enzymes to salinity stress in two genotypes of perennial ryegrass (Lolium perenne) differing in salt tolerance

Salinity could damage cellular membranes through overproduction of reactive oxygen species (ROS), while antioxidant capacities play a vital role in protecting plants from salinity caused oxidative damages. The objective of this study was to investigate the toxic effect of salt on the antioxidant enzyme activities, isoforms and gene expressions in perennial ryegrass (Lolium perenne L.). Salt-tolerant ‘Quickstart II’ and salt-sensitive ‘DP1′ were subjected to 0 and 250 mM NaCl for 12 d. Salt stress increased the content of lipid peroxidation (MDA), electrolyte leakage (EL) and hydrogen peroxide (H₂O₂), to a greater extent in salt-sensitive genotype. Salt-stressed plant leaves exhibited a greater activity of superoxide dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7), ascorbate peroxidase (APX, EC 1.11.1.11) at 4 d after treatment (DAT), but a lower level of enzyme activity at 8 and 12 d, when compared to the control. Catalase (CAT, EC 1.11.1.6) activity was greater at 4 DAT and thereafter decreased in salt tolerant genotype relative to the control, whereas lower than the control during whole experiment period for salt-sensitive genotype. There were different patterns of five isoforms of SOD, POD and two isoforms of APX between two genotypes. Antioxidant gene expression was positively related to isoenzymatic and total enzymatic activities during 12-d salt-treated leaves of two genotypes, with a relatively higher level in salt-tolerant genotype. Thus, salt tolerance could be related to the constitutive/induced antioxidant gene, leading to more efficient enzyme stimulation and protection in perennial ryegrass.     

Cold tolerance of micro-arthropods from Alaskan taiga

Abstract. Mean supercooling points for a variety of soil and litter arthropods including mites, springtails, a heteropteran and immature spiders from a central Alaskan taiga site ranged from -6.3 to -28.5°C during autumn. Variation in supercooling ability of five species of cryptostigmatid mites occurred throughout the year with increased cold tolerance in autumn and early winter concomitant with the temperature pattern of the habitat. No correlation between the level of supercooling and water content of the mites was evident. Changes in the frequency distribution of individual supercooling points occurred in autumn, winter, spring and summer samples which were species specific. All arthropods tested were susceptible to freezing, and the mites utilize supercooling to avoid freezing.     

Patterns of nitrogen conservation in shrubs and grasses in the Patagonian Monte,Argentina

We analysed the main plant strategies to conserve nitrogen in the Patagonian Monte. We hypothesized that the two main plant functional groups (xerophytic evergreen shrubs and mesophytic perennial grasses) display different mechanisms of nitrogen conservation related to their structural and functional characteristics. Evergreen shrubs are deep-rooted species, which develop vegetative and reproductive growth from spring to late summer coupled with high temperatures, independently from water inputs. In contrast, perennial grasses are shallow-rooted species with high leaf turnover, which display vegetative growth from autumn to spring and reproductive activity from mid-spring to early-summer, coupled with precipitation inputs. We selected three evergreen shrubs (Larrea divaricata Cav., Atriplex lampa Gill. ex Moq. and Junellia seriphioides (Gilles and Hook.) Moldenke) and three perennial grasses (Stipa tenuis Phil., S. speciosa Trin. and Rupr. and Poa ligularis Nees ex Steud.), characteristic of undisturbed and disturbed areas of the Patagonian Monte. N concentration in expanded green and senesced leaves was estimated in December 1997 (late spring) and June 1998 (late autumn). Deep-rooted evergreen shrubs displayed small differences in N concentration between green and senesced leaves (low N-resorption efficiency), having high N concentration in senesced leaves (low N-resorption proficiency). Shallow-rooted perennial grasses, conversely, showed high N-resorption efficiency and high N-resorption proficiency (large differences in N concentration between green and senesced leaves and very low N concentration in senesced leaves, respectively). The lack of a strong mechanism of N resorption in evergreen shrubs apparently does not agree with their ability to colonize N-poor soils. These results, however, may be explained by lower N requirements in evergreen shrubs resulting from lower growth rates, lower N concentrations in green leaves, and lower leaf turnover as compared with perennial grasses. Long-lasting N-poor green tissues may, therefore, be considered an efficient mechanism to conserve N in evergreen shrubs in contrast with the mechanism of strong N resorption from transient N-rich tissues displayed by perennial grasses. Evergreen shrubs with low N-resorption efficiency provide a more N-rich substrate, with probably higher capability of N mineralization than that of perennial grasses, which may eventually enhance N fertility and N availability in N-poor soils.     

Ecotype-dependent control of growth,dormancy and freezing tolerance under seasonal changes in <Emphasis Type="Italic">Betula pendula</Emphasis> Roth

Woody plants in the temperate and boreal zone undergo annual cycle of growth and dormancy under seasonal changes. Growth cessation and dormancy induction in autumn are prerequisites for the development of substantial cold hardiness in winter. During evolution, woody plants have developed different ecotypes that are closely adapted to the local climatic conditions. In this study, we employed distinct photoperiodic ecotypes of silver birch (Betula pendula Roth) to elucidate differences in these adaptive responses under seasonal changes. In all ecotypes, short day photoperiod (SD) initiated growth cessation and dormancy development, and induced cold acclimation. Subsequent low temperature (LT) exposure significantly enhanced freezing tolerance but removed bud dormancy. Our results suggested that dormancy and freezing tolerance might partially overlap under SD, but these two processes were regulated by different mechanisms and pathways under LT. Endogenous abscisic acid (ABA) levels were also altered under seasonal changes; the ABA level was low during the growing season, then increased in autumn, and decreased in winter. Compared with the southern ecotype, the northern ecotype was more responsive to seasonal changes, resulting in earlier growth cessation, cold acclimation and dormancy development in autumn, higher freezing tolerance and faster dormancy release in winter, and earlier bud flush and growth initiation in spring. In addition, although there was no significant ecotypic difference in ABA level during growing season, the rates and degrees of ABA alterations were different between the ecotypes in autumn and winter, and could be related to ecotypic differences in dormancy and freezing tolerance.