Low temperature during winter elicits differential responses among populations of the Mediterranean evergreen cork oak (Quercus suber)

Populations of cork oak (Quercus suber L.) were assessed for seasonal and inter-population variability in, and temperature responses of, the ratio between light-induced variable and maximum fluorescence of chlorophyll, Fv/Fm, considered a surrogate for the maximum photochemical efficiency of photosystem II (PSII). Seedlings from 10 populations throughout the distribution range of Q. suber in the Mediterranean basin were grown in a common garden in central Spain. The Fv/Fm ratio of dark-adapted leaves was measured at dawn every month for 2 years. Air temperature was recorded at a nearby climatic station. During the summer, when maximum air temperatures reached 40 degrees C, there were no significant differences in Fv/Fm among populations, but significant differences were seen during the winter. In colder months, Fv/Fm ranged in all populations between 0.5-0.6 and 0.2-0.3 in 2001 and 2002, respectively. The variance explained by the population effect was greatest during winter months, especially in 2002, reaching a peak value of 10% when minimum air temperature was below -10 degrees C. Populations originating from warmer sites showed the largest decline in Fv/Fm between the end of 2001 and the beginning of 2002. Thus, a negative linear relationship was established between mean annual temperature at the population source and population mean Fv/Fm recorded in the coldest month in 2002 and normalized by the Fv/Fm spring measurement.     

Susceptibility to low-temperature photoinhibition in three conifers differing in successional status

Susceptibility to photoinhibition of the evergreen conifers Abies alba Mill., Picea abies (L.) Karst. and Pinus mugo Turra was investigated in an unheated greenhouse during winter and spring 2003. Photosynthetic performance of the seedlings was assessed by chlorophyll a fluorescence and analyses of chlorophyll and total carotenoid concentrations in needles. During winter months, maximum quantum yield of PSII photochemistry (ratio of variable to maximum fluorescence, Fv/Fm) was significantly greater in A. alba than in P. abies and P. mugo. Abies alba also sustained higher maximum apparent electron transport rate (ETRmax) than P. abies and P. mugo. Total concentrations of chlorophyll and carotenoids in needles decreased during the winter in P. mugo and P. abies, but remained stable in A. alba. For all species, Fv/Fm decreased from December until February and then increased to a maximum in April. Photoinhibition was greatest (Fv/Fm < 0.80) in all seedlings in February, the month with the lowest mean temperature. Saturating photosynthetic photon flux (PPFsat) and ETRmax were positively related to air temperature. All species had lower values of ETRmax and PPFsat in winter than in spring. Non-photochemical quenching of chlorophyll fluorescence (NPQ) was highest at low air temperatures. Differences among species in susceptibility to winter photoinhibition resulted from their specific light preferences and led to different mechanisms to cope with photoinhibitory stress. The more shade-tolerant A. alba sustained a higher photosynthetic capacity in winter than P. abies and P. mugo. Winter photoinhibition in P. abies, P. mugo and, to a lesser extent, in A. alba may reflect adaptive photoprotection of the photosynthetic apparatus in winter.     

Genetic and environmental control of seasonal carbohydrate dynamics in trees of diverse Pinus sylvestris populations

We explored environmental and genetic factors affecting seasonal dynamics of starch and soluble nonstructural carbohydrates in needle and twig cohorts and roots of Scots pine (Pinus sylvestris L.) trees of six populations originating between 49 degrees and 60 degrees N, and grown under common garden conditions in western Poland. Trees of each population were sampled once or twice per month over a 3-year period from age 15 to 17 years. Based on similarity in starch concentration patterns in needles, two distinct groups of populations were identified; one comprised northern populations from Sweden and Russia (59-60 degrees N), and another comprised central European populations from Latvia, Poland, Germany and France (49-56 degrees N). Needle starch concentrations of northern populations started to decline in late spring and reached minimum values earlier than those of central populations. For all populations, starch accumulation in spring started when minimum air temperature permanently exceeded 0 degrees C. Starch accumulation peaked before bud break and was highest in 1-year-old needles, averaging 9-13% of dry mass. Soluble carbohydrate concentrations were lowest in spring and summer and highest in autumn and winter. There were no differences among populations in seasonal pattern of soluble carbohydrate concentrations. Averaged across all populations, needle soluble carbohydrate concentrations increased from about 4% of needle dry mass in developing current-year needles, to about 9% in 1- and 2-year-old needles. Root carbohydrate concentration exhibited a bimodal pattern with peaks in spring and autumn. Northern populations had higher concentrations of fine-root starch in spring and autumn than central populations. Late-summer carbohydrate accumulation in roots started only after depletion of starch in needles and woody shoots. We conclude that Scots pine carbohydrate dynamics depend partially on inherited properties that are probably related to phenology of root and shoot growth.     

Light-energy processing and freezing-tolerance traits in red spruce and black spruce: species and seed-source variation

Red spruce (Picea rubens Sarg.) and black spruce (Picea mariana (Mill.) B.S.P.) are genetically and morphologically similar but ecologically distinct species. We determined intraspecific seed-source and interspecific variation of red spruce and black spruce, from across the near-northern margins of their ranges, for several light-energy processing and freezing-tolerance adaptive traits. Before exposure to low temperature, red spruce had variable fluorescence (Fv) similar to black spruce, but higher photochemical efficiency (Fv/Fm), lower quantum yield, lower chlorophyll fluorescence (%), and higher thermal dissipation efficiency (qN), although the seed-source effect and the seed-source x species interaction were significant only for Fv/Fm. After low-temperature exposure (-40 degrees C), red spruce had significantly lower Fv/Fm, quantum yield and qN than black spruce, but higher chlorophyll fluorescence and relative fluorescence. Species, seed-source effect, and seed-source x species interaction were consistent with predictions based on genetic (e.g., geographic) origins. Multi-temperature exposures (5, -20 and -40 degrees C) often produced significant species and temperature effects, and species x temperature interactions as a result of species-specific responses to temperature exposures. The inherent physiological species-specific adaptations of red spruce and black spruce were largely consistent with a shade-tolerant, late-successional species and an early successional species, respectively. Species differences in physiological adaptations conform to a biological trade-off, probably as a result of natural selection pressure in response to light availability and prevailing temperature gradients.     

不同种源香椿芽菜品质分析及光合特性比较

通过田间试验,以国内不同产区的9个种源的香椿1年生实生苗为试验材料,对芽菜品质进行了分析比较,测定了芽菜采摘后不同种源香椿苗木的光合特性指标。试验结果表明:(1)不同种源香椿芽菜的感官品质、可溶性糖、可溶性蛋白质、Vc含量存在极显著差异,游离氨基酸含量存在显著差异,济南、成都和泰安种源的芽菜品质较佳。(2)不同种源香椿的净光合速率、气孔导度、蒸腾速率、胞间CO2浓度和Fv/Fm存在极显著差异,叶绿素相对浓度存在显著差异,泰安、济南、栾川、旬阳4个种源的香椿生长速度最快;(3)芽菜采摘时的一些品质指标与芽菜采摘后植株的光合特性指标存在一定的相关关系,不同种源香椿芽菜的感官品质与Vc含量、采摘后植株叶片的Fv/Fm存在显著的负相关关系,可溶性蛋白质含量与采摘后植株叶片的叶绿素浓度存在显著的正相关关系,游离氨基酸含量与采摘后植株叶片的Ci存在显著的负相关关系。     

Excitation energy partitioning and quenching during cold acclimation in Scots pine

We studied the influence of two irradiances on cold acclimation and recovery of photosynthesis in Scots pine (Pinus sylvestris L.) seedlings to assess mechanisms for quenching the excess energy captured by the photosynthetic apparatus. A shift in temperature from 20 to 5 degrees C caused a greater decrease in photosynthetic activity, measured by chlorophyll fluorescence and oxygen evolution, in plants exposed to moderate light (350 micromol m(-2) s(-1)) than in shaded plants (50 micromol m(-2) s(-1)). In response to the temperature shift, maximal photochemical efficiency of photosystem II (PSII), measured as the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) of dark-adapted samples, decreased to 70% in exposed seedlings, whereas shaded seedlings maintained Fv/Fm close to initial values. After a further temperature decrease to -5 degrees C, only 8% of initial Fv/Fm remained in exposed plants, whereas shaded plants retained 40% of initial Fv/Fm. Seven days after transfer from -5 to 20 degrees C, recovery of photochemical efficiency was more complete in the shaded plants than in the exposed plants (87 and 65% of the initial Fv/Fm value, respectively). In response to cold stress, the estimated functional absorption cross section per remaining PSII reaction center increased at both irradiances, but the increase was more pronounced in exposed seedlings. Estimates of energy partitioning in the needles showed a much higher dissipative component in the exposed seedlings at low temperatures, pointing to stronger development of non-photochemical quenching at moderate irradiances. The de-epoxidation state of the xanthophyll cycle pigments increased in exposed seedlings at 5 degrees C, contributing to the quenching capacity, whereas significant de-epoxidation in the shaded plants was observed only when temperatures decreased to -5 degrees C. Thermoluminescence (TL) measurements of PSII revealed that charge recombinations between the second oxidation state of Mn-cluster S2 and the semireduced secondary electron acceptor quinone Q(B)- (S2Q(B)-) were shifted to lower temperatures in cold-acclimated seedlings compared with control seedlings and this effect depended on irradiance. Concomitant with this, cold-acclimated seedlings demonstrated a significant shift in the S2 recombination with primary acceptor Q(A)- (S2Q(A)-) characteristic TL emission peak to higher temperatures, thus narrowing the redox potential gap between S2Q(B)- and S2Q(A)-, which might result in increased probability for non-radiative radical pair recombination between the PSII reaction center chlorophyll a (P680+) and Q(A)- (P680+)Q(A)-) (reaction center quenching) in cold-acclimated seedlings. In Scots pine seedlings, mechanisms of quenching excess light energy in winter therefore involve light-dependent regulation of reaction center content and both reaction center-based and antenna-based quenching of excess light energy, enabling them to withstand high excitation pressure under northern winter conditions.     

水杨酸诱导桉树对焦枯病的抗性与叶绿素荧光参数的关系

以巨尾桉(Eucalyptus grandis×Eucalyptus urophylla)为试验材料,研究不同浓度水杨酸(SA)诱导桉树对焦枯病抗性与叶绿素荧光参数的关系。结果表明,SA能降低Fo值,提高Fm、Fv值,表现为先上升后下降的变化趋势。SA处理的桉树叶片Fv/Fo、Fv/Fm值,第3天达到最大值,说明SA 1～3 mmol.L-1处理的抗焦枯病的持久期为3 d。SA 1 mmol.L-1处理的桉树叶片Fo值最小,Fm、Fv、Fv/Fo和Fv/Fm值最高;说明SA 1mmol.L-1处理的效果最好,能有效地减轻光合机构的损伤,一定程度上抑制焦枯病菌的侵害。     

Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open

Photosynthetic light and temperature response curves were measured seasonally in seedlings of white spruce (Picea glauca (Moench.) Voss) grown for two years in the understory of aspen (Populus tremuloides Michx.) or in the open in central Alberta. Light-saturated rate of net photosynthesis, the optimum temperature for net photosynthesis, transpiration rate, photochemical efficiency, and stomatal and mesophyll conductances increased from spring to summer and declined thereafter, whereas dark respiration rate and compensation and saturation points were highest in spring. Depression of photosynthetic parameters was greater in open-grown seedlings than in understory seedlings during the periods in spring and autumn when night frosts were common. Net photosynthetic rates were similar in understory and open-grown seedlings in summer, but they were significantly lower in open-grown seedlings in spring and autumn. Significantly lower transpiration rates and stomatal conductances in open-grown seedlings than in understory seedlings were also observed at 15 and 25 degrees C in the autumn. Shoot and needle growth were less in open-grown seedlings than in understory seedlings. In summer, when irradiances were low in the aspen understory, understory white spruce seedlings maintained a positive carbon balance by decreasing their compensation and saturation points and increasing their photochemical efficiency compared to spring and autumn.     

Leaf angle responds to nitrogen supply in eucalypt seedlings. Is it a photoprotective mechanism?

We examined the adjustment of leaf angle (L theta) and foliar chlorophyll and xanthophyll chemistry in Eucalyptus nitens (Deane and Maiden) Maiden seedlings maintained in various nitrogen (N)-supply treatments over a 6-month period. Adjustment of L theta toward the vertical was greatest under conditions of foliar N deficiency and became incrementally more horizontal with increasing foliar N concentration. Photochemical efficiency (Fv/Fm) and quantum yield were lower in seedlings with low foliar N (low-N seedlings) in winter, but not in autumn. Low-N seedlings generally had low area-based chlorophyll concentrations and high xanthophyll-cycle conversion ratios, particularly during months of low temperature. Under mild temperature conditions, high concentrations of zeaxanthin and antheraxanthin were associated with lower electron transport rates (ETR). Incident light, Fv/Fm, ETR and total chlorophyll concentration were negatively correlated with L theta, with horizontal leaf orientation measured as 0 degrees and vertical leaf orientation as 90 degrees . Xanthophyll conversion ratio was positively correlated with L theta. Adjustments in L theta may play a role in photoprotection of E. nitens seedlings by assisting the leaf to balance its utilization and dissipation of energy.     

Photosynthetic response of white spruce families to drought stress

In the context of climate change, an increased frequency of drought stresses might occur at a regional scale in boreal forests. To assess photosynthetic responses to drought treatment, seedlings of 12 open-pollinated families of white spruce (Picea glauca (Moench) Voss) differing in their growth performance were grown in a controlled environment. Gas exchange and chlorophyll fluorescence parameters as well as shoot xylem water potential (WP) were measured for 21 successive days after watering was stopped. Net photosynthesis decreased as stomatal conductance decreased. Net photosynthesis was not affected by drought until WP reached –2.0 MPa when stomata were closed. Initial fluorescence (F and basic fluorescence after induction (F00) were not affected by drought. A progressive decrease in maximal (Fm) and variable fluorescences (Fv), maximum photosystem II (PS II) efficiency (Fv = Fm), effective quantum yield of PS II (FII), photochemical efficiency of open PS II (Fp), and photochemical quenching (qP) was observed at WP < - 1.0 MPa, whereas non-photochemical quenching (qN) remained high throughout the drought treatment. White spruce families with inferior growth performance showed higher values of Fm, Fv, Fv = Fm, Fp, and qN at WP< - 2.0MPa. The results indicated that chlorophyll fluorescence variables can be used as drought markers in relation to present or predicted climate conditions. These could be used for selecting planting stock adapted to drought periods or dry environments. These markers showed that slow-growing genotypes are better adapted to drought conditions than intermediate or fast-growing genotypes in present and predicted drought conditions.     

Effects of timing of soil frost thawing on Scots pine

Effects of the timing of soil thawing in the spring on Scots pine (Pinus sylvestris L.) were studied under controlled laboratory conditions. Sixteen 6-year-old saplings were lifted from the field, replanted in containers and placed in four treatments in controlled environment (CE) chambers with four replicate saplings per chamber. The saplings were held in the CE chambers during one simulated winter and one simulated growing season. The soil was frozen to -2 degrees C during a second simulated winter in the CE chambers, and the soil thawing treatments began at the end of the second simulated winter. Soil thawing began at various times before (no delay in thawing) and after (delay in thawing) chamber air conditions were changed from simulated winter to simulated summer. Delayed soil thawing subjected saplings to stress, with the severity of stress depending on the length of the delay in thawing. If there was no delay or only a short delay in soil thawing, stress was minor and reversible. A 2-week delay in soil thawing led to death of the saplings. Stress was apparent as decreases in the variable to maximal chlorophyll fluorescence ratio (Fv/Fm), chlorophyll a/b ratio and needle water potential. In needles of stressed saplings, apoplastic electrical resistance first decreased and then increased and there were anomalies in the electrical impedance spectra of the stems. Stress from the soil thawing treatments affected both root and shoot growth.     

Carbon dioxide gas exchange of cembran pine (Pinus cembra) at the alpine timberline during winter

Winter CO(2) gas exchange of the last three flushes of cembran pine (Pinus cembra L.) was studied under ambient conditions at the alpine timberline, an ecotone with strong seasonal changes in climate. During the coldest months of the year, December to March, gas exchange was almost completely suppressed and even the highest irradiances and temperatures did not cause a significant increase in net photosynthesis compared to spring and fall. In general, daily CO(2) balance was negative between December and March except during extended warm periods in late winter. However, because twig respiration was also reduced to a minimum during the December-March period, daily carbon losses were minimal. Total measured carbon loss during the winter months was small, equalling the photosynthetic production of one to two warm days in spring or summer when average air temperature was above 6 degrees C.     

Rapid freezing induces winter injury symptomatology in red spruce foliage

Red spruce (Picea rubens Sarg.) suffers frequent and extensive injury to current-year foliage during the winter. Experimental freezing of red spruce foliage at cooling rates > 10 degrees C min(-1) induced visible symptomatology similar to natural winter injury at the branch, needle and cellular levels. Such damage was associated with a low-temperature exotherm near -10 to -12 degrees C, a loss in needle fluorescence, massive cellular disruption, foliar discoloration, and low needle survival. Susceptibility of individual trees to rapid freezing injury was associated with historical winter injury patterns and alterations in foliar nutrition. We conclude that anthropogenic deposition may alter the sensitivity of trees to winter injury caused by rapid temperature changes.     

干旱胁迫对杨树气体交换与荧光参数的影响

以白杨派和黑杨派杨树的盆栽苗为试材,分析了不同干旱胁迫程度对5个杨树的气体交换参数与荧光参数(Fv/Fm、Fmv/Fms)的影响,并对不同树种的耐旱性进行了综合评价。结果表明:①5种杨树净光合速率与蒸腾速率均随苗木叶水势的下降而呈明显下降趋势;在水分胁迫初期,欧洲黑杨和银白杨的净光合速率呈现明显的上升趋势。②在中度干旱胁迫前,5个供试杨树的水分利用效率大部分随叶水势的下降有增大现象。③不同树种的Fv/Fm随叶水势降低总体上呈逐渐下降趋势。④与Fv/Fm相比,Fmv/Fms能更直接地反映光合机构的实际光能捕获能力。⑤当苗木叶水势为-1.5 MPa时,5个树种耐旱性的综合评价结果为:转Bt基因欧洲黑杨>欧洲黑杨>银白杨>银灰杨为欧洲山杨,综合指数评价结果与采用单一指标中的水分利用效率完全一致,与净光合速率相近;水分利用效率证明是一种较为可靠的树种耐旱性评价指标。     

Age-related trends in red spruce foliar plasticity in relation to declining productivity

Phenotypic plasticity in needle morphology with increasing tree size and age was investigated by comparing four age classes of red spruce (Picea rubens Sarg.) ranging from juvenile (3-12 years old) to mature (over 100 years old). With increase in tree age there were significant increases in leaf mass per unit area (LMA), mesophyll and vascular bundle area as a percentage of total needle cross-sectional area, and stomatal density. Within the vascular bundle, both xylem cross-sectional area and tracheid lumen area increased significantly, whereas air space as a percentage of total cross-sectional area decreased. These morphological changes were associated with a significant decrease in photosynthetic capacity and stomatal conductance, and an increase in (13)C enrichment. Although both photosynthetic capacity and whole-tree conductance decreased significantly between age classes 3 and 12 years, they did not differ between age classes 53 and 127 years, even though needle (13)C enrichment was significantly greater in the 127-year age class. Thus there appear to be compensatory mechanisms that maintain photosynthetic capacity as trees increase in size and vascular complexity, which in red spruce and other species, may affect leaf hydraulic conductance. Although increased LMA may contribute to reduced photosynthetic capacity in red spruce, similar relationships are not seen in other conifers.     

Rapid freezing injury in red spruce: seasonal changes in sensitivity and effects of temperature range

On calm, cold days in winter, sun-exposed needles of red spruce (Picea rubens Sarg.) may warm 10 to 20 degrees C above ambient air temperature, and undergo rapid (>/= 1 degrees C min(-1)) fluctuations in temperature as light breezes or passing clouds alter the energy balance of the foliage. It has been proposed that the resulting rapid freeze-thaw cycles (freezing stress) cause a type of winter injury in montane red spruce that is characterized by necrosis of sun-exposed foliage. In autumn and winter, we monitored rapid freezing stress response of needle sections from 10 montane red spruce trees by subjecting needles to rapid freezing over the temperature span typically recorded in the field. In autumn, experimental rapid freezing stress produced severe injury only at temperatures considerably lower than expected for that time of year. In winter, rapid freezing caused occasional, moderate injury in fully hardened foliage of trees susceptible to both slow and rapid freezing. Seasonal changes in sensitivity to rapid and slow freezing were correlated, suggesting that environmental factors that are known to affect sensitivity to slow freezing may also affect sensitivity to rapid freezing. Experimental manipulation of the start and end temperatures of rapid freezing stress events showed that moderate to severe needle injury can occur in susceptible trees at temperature spans slightly more extreme than those typically recorded in the field. The extent of injury was similar at different starting temperatures if rapid freezing occurred over the same temperature span. Year-old foliage was consistently less sensitive to rapid freezing stress than current-year foliage, but some year-old foliage was damaged when the rapid freezing stress regime caused severe injury in current-year foliage. We conclude that rapid freeze-thaw cycles can explain light to moderate injury of current-year foliage, but they do not explain the more severe and widespread pattern of foliar damage that has occurred intermittently over at least the last 18 years.     

Springtime resumption of photosynthesis in balsam fir (Abies balsamea)

Photosynthesis in balsam fir (Abies balsamea (L.) Mill.) was measured in the field at two locations in New Brunswick, Canada from late winter to late spring in 2004 and 2005. No photosynthesis was detectable while the soil remained below 0 degrees C throughout the rooting zone. In both years, photosynthesis began once soil temperature rose to 0 degrees C. In potted seedlings in growth chambers, there was no photosynthesis at an air temperature of 10 degrees C if the pots were frozen. These findings suggest that, once air temperatures permit photosynthesis, it is the availability of unfrozen soil water that triggers the onset of photosynthesis. In the field, full recovery of photosynthetic capacity following the onset of soil thaw was dependent on air temperature and took 5 weeks in 2005, but 10 weeks in 2004. There were two substantial frost events during the recovery period in 2004 that may explain the extended recovery period. In 2005, recovery was complete after the accumulation of 200 growing degree days above 0 degrees C after the start of soil thaw.     

Winter photoinhibition in the field involves different processes in four co-occurring Mediterranean tree species

Photoinhibition was examined in four co-occurring Mediterranean evergreen tree species during two consecutive winters. In response to low temperatures and saturating light, Juniperus phoenicea L., Pinus halepensis Mill., Quercus coccifera L. and Q. ilex ssp. ballota (Desf.) Samp. exhibited marked chronic photoinhibition, indicated by low predawn maximal photochemical efficiency of photosystem II (PSII) (Fv/Fm). Low Fv/Fm values were correlated with high concentrations of xanthophyll cycle components (VAZ) and with the maintenance of high concentrations of zeaxanthin overnight (DPSpd). In all species, however, chronic photoinhibition was enhanced as the winter progressed in the absence of changes in DPSpd, suggesting cumulative damage toward the end of winter. Photoinhibition differed among species: P. halepensis always displayed significantly higher Fv/Fm values; and Q. coccifera had the lowest Fv/Fm values, showing a high sensitivity to the combination of high light and low temperatures. Differences among species were not fully explained by differences in the xanthophyll pool or its de-epoxidation state. Chronic photoinhibition overlapped with a dynamic photoinhibition as shown by the low values of photochemical efficiency of the open reaction centers of PSII at midday. Winter photoprotective strategies differed among species and may involve photoprotective mechanisms in addition to those associated with xanthophylls. The observed species-specific differences matched results obtained for the same species in summer; however, comparison of the two seasons suggests that the higher VAZ concentration observed in winter has an additional structural photoprotective role.     

Effects of radiational heating at low air temperature on water balance, cold tolerance, and visible injury of red spruce foliage

Recent studies have shown that winter needle mortality in red spruce (Picea rubens Sarg.) is increased by exposure to direct solar radiation, possibly as a result of photo-oxidative damage, accelerated winter desiccation, or reduced cold tolerance due to heating of sun-exposed needles. In an experiment at controlled subfreezing air temperatures of -10 to -20 degrees C, visible radiation was less effective than infrared radiation in producing needle desiccation and visible injury during freeze-thaw cycles. However, visible radiation produced a red-brown color in injured needles, similar to natural winter injury, whereas injured needles exposed to infrared radiation were yellow and injured needles kept in darkness were dark brown. Thus, visible radiation was necessary to produce the red-brown color of damaged needles, but not the injury itself. Needle desiccation was not strongly correlated with visible injury, but the pattern of variation in visible injury among trees and the positive correlation between electrolyte leakage and visible injury suggested that freezing damage following freeze-thaw cycles might cause the visible injury. This was confirmed by a second experiment that showed loss of cold hardiness in needles thawed by radiational heating for six consecutive days. Even with a constant nighttime temperature of -10 degrees C, six days of radiational heating of needles to above freezing caused a small (2.8 degrees C) mean decrease in needle cold tolerance, as measured by electrolyte leakage. Continuous darkness at -10 degrees C for six days resulted in an estimated 5.6 degrees C mean increase in needle cold tolerance. Freezing injury stimulated desiccation: cooling at 4 degrees C h(-1) to -43 or -48 degrees C increased the dehydration rate of isolated shoots by a factor of two to three during the first day after thawing. Within three days at 15 to 22 degrees C and 50% relative humidity, the mean water content of these shoots fell to 60% or lower, compared to 90% or greater for unfrozen controls or shoots subject to less severe freezing stress. In some but not all severely freeze-stressed shoots, accelerated needle desiccation and abscission were accompanied by a red-brown color typical of red spruce winter needle injury. We conclude that severe winter desiccation in red spruce may often be due to prior freezing injury, increased as a result of exposure to direct solar radiation. Furthermore, freezing injury in red spruce may sometimes cause desiccation and abscission of green needles.     

12种金花茶组植物光合生理特性比较

为了探讨不同金花茶品种的光合生理特性,从而为金花茶的引种栽培提供参考,以3年生的12个金花茶品种为试验材料,对其叶片光合参数、叶绿素含量和叶绿素荧光参数进行测定。结果表明:广西本地品种普通金花茶和越南引进品种多毛金花茶具有较低的光饱和点、光补偿点及较高的叶绿素含量,表明其利用弱光的能力最强;与其他品种相比,普通金花茶具有较强的光合作用能力和较高的光化学转化效率,其在相同的环境条件下可积累更多的光合产物并且生长更旺盛;越南引进品种罗斯曼金花茶对光环境的适应能力较强,对光照要求较低,具有较强的光合能力。越南引种金花茶的F_v/F_m值表现正常,表明6种越南引种金花茶适于在广西种植。普通金花茶和多毛金花茶的F_v/F_m和F_v/F_o值最高,表明其捕获激发光能的能力最强,并且其光系统Ⅱ的潜在活性最大。12种金花茶品种的光合生理特性具有显著差异,越南引种金花茶表现出了对广西生态环境的适应性。