Soil temperature,gas exchange and nitrogen status of 5-year-old Norway spruce seedlings

Five-year-old Norway spruce (Picea abies (L.) Karst.) seedlings were subjected to three simulated growing seasons in controlled environment chambers. Plants were acclimated to a soil temperature of 16 degrees C during the first and third growing seasons, but were allocated at random to soil temperature treatments of 9, 13, 18 and 21 degrees C during the second growing season. Low soil temperature during the second growing season depressed stomatal conductance and photosynthetic rate (A) per unit of projected leaf area, although intercellular CO2 concentrations did not differ significantly between treatments. At all soil temperatures, total chlorophyll concentration first decreased and then increased, although the rate of increase and the final concentration increased with soil temperature, which may explain the effect of soil temperature on A. Neither chlorophyll a/b ratio nor leaf nitrogen concentration was significantly affected by soil temperature. Treatment differences disappeared during the third simulated growing season when plants were again acclimated to a soil temperature of 16 degrees C.     

Effects of duration of a simulated winter thaw on dieback and xylem conductivity of Betula papyrifera

Stems or roots + stems of potted, 2-year-old paper birch (Betula papyrifera L.) were subjected to simulated winter thaws of various durations in climate-controlled chambers. The simulated thaws induced dieback of shoots of the treated plants. Although the stem thaw treatment did not significantly increase dieback, there were significant (P < 0.05) correlations between growing degree days above 4 degrees C and both shoot dieback and percent reduction in conductive xylem. All trees that received > 60 growing degree days (GDD) > 4 degrees C died back to some extent. Plants in the root + stem thaw treatment that received more than 60 GDD > 4 degrees C showed a significant (P < 0.05) increase in dieback and a significant (P < 0.05) loss of conducting xylem after a period of growth and recovery in the greenhouse, especially in the xylem of 1-year-old stems. Furthermore, higher correlations between GDD > 4 degrees C during a thaw and both the extent of dieback and the loss in conductive xylem were found in trees subjected to the root + stem thaw treatments than in trees exposed only to the stem thaw treatments (P < 0.05). The root + stem thaw treatments also resulted in highly significant relationships (P < 0.05-0.001) between loss in conductive xylem and dieback. The occurrence of dieback in response to winter thaws, and its close correlation with irreversible losses of xylem conductivity due to embolisms, coupled with an inability to refill the xylem because of root damage, support the view that these processes may be key factors in initiating birch decline.     

Stomatal responses to drought at a Mediterranean site: a comparative study of co-occurring woody species differing in leaf longevity

We studied stomatal responses to decreasing predawn water potential (Psipd) and increasing leaf-to-air water vapor pressure difference (VPD) of co-occurring woody Mediterranean species with contrasting leaf habits and growth form. The species included two evergreen oaks (Quercus ilex subsp. ballota (Desf.) Samp. and Q. suber L.), two deciduous oaks (Q. faginea Lam. and Q. pyrenaica Willd.) and two deciduous shrubs (Pyrus bourgaeana Decne. and Crataegus monogyna Jacq.). Our main objective was to determine if stomatal sensitivity is related to differences in leaf life span and leaf habit. The deciduous shrubs had the least conservative water-use characteristics, with relatively high stomatal conductance and low stomatal sensitivity to soil and atmospheric drought. As a result, Psipd decreased greatly in both species during the growing season, resulting in early leaf abscission in the summer. The deciduous oaks showed intermediate water-use characteristics, having maximum stomatal conductances and CO2 assimilation rates similar to or even higher than those of the deciduous shrubs. However, they had greater stomatal sensitivity to soil drying and showed less negative Psipd values than the deciduous shrubs. The evergreen oaks, and especially the species with the greatest leaf longevity, Q. ilex, exhibited the most conservative water-use behavior, having lower maximum stomatal conductances and greater sensitivity to VPD than the deciduous species. As a result, Psipd decreased less during the growing season in the evergreens than in the deciduous species, which may contribute to greater leaf longevity by avoiding irreversible damage during the summer drought. However, the combination of low maximum CO2 assimilation rates and high stomatal sensitivity to drought must have a negative impact on the final carbon budget of leaves with a long life span.     

Chlorophyll fluorescence and CO(2) assimilation of black spruce seedlings following frost in different temperature and light conditions

Effects of artificial frosts on light-saturated photosynthesis (A(max)) and ground, maximal and variable fluorescence variables (F(o), F(m), and F(v) and F(v)/F(m)) were monitored on 1-year-old foliage of black spruce seedlings (Picea mariana (Mill.) BSP) grown at high (25 degrees C), moderate (15 degrees C) and low (5 degrees C) temperatures and moderate (240 &mgr;mol m(-2) s(-1)) and low (80 &mgr;mol m(-2) s(-1)) irradiances. Photoinhibition of 1-year-old foliage was greater in seedlings grown in moderate light than in seedlings grown in low light. Photoinhibition increased with decreasing growth chamber temperature at both irradiances. Most changes in F(v)/F(m) were caused by changes in F(v). Exposure to -4 degrees C decreased both F(v)/F(m) and A(max) compared with control values. The effect of the -4 degrees C frost treatment was greater in seedlings grown in low light than in seedlings grown in moderate light, probably because seedlings grown in moderate light were already partially photoinhibited before the frost treatment. Following -4 degrees C treatment, neither F(v)/F(m) nor A(max) recovered in seedlings grown in low light. Light-saturated photosynthesis decreased with decreasing growth chamber temperature. Light-saturated photosynthesis was more sensitive to the -3 and -4 degrees C frost treatments in seedlings grown at 25 degrees C than in seedlings grown at 15 and 5 degrees C. The A(max) of seedlings grown at 15 degrees C was sensitive only to the -4 degrees C frost treatment, whereas A(max) of seedlings grown at 5 degrees C was not sensitive to any of the frost treatments. Recovery of A(max) following frost took longer in seedlings grown at high temperatures than in seedlings grown at low temperatures. For seedlings grown at the same temperature but under different irradiances, both A(max) and F(v)/F(m) reflected damage to the photosynthetic system following a moderate frost. However, for seedlings grown at the same irradiance but different temperatures, A(max) provided a more sensitive indicator of frost damage to the photosynthetic system than F(v)/F(m) ratio.     

Stomatal and leaf growth responses to partial drying of root tips in willow

Root tips of intact willow (Salix dasyclados Wimm., Clone 81-090) plants were partially dried by exposure to ambient greenhouse air and then kept in water-vapor-saturated air for up to 3 days. The drying treatment increased abscisic acid (ABA) concentrations in both the root tips subjected to drying and in the xylem sap, while it reduced leaf stomatal conductance and leaf extension rate. Despite the decrease in stomatal conductance, leaf water potentials were unaffected by the root drying treatment, indicating that the treatment reduced hydraulic conductivity between roots and foliage. After roots subjected to drying were returned to a nutrient solution or excised, ABA concentrations in the remaining roots and in the xylem sap, stomatal conductance of mature leaves and extension rate of unfolding leaves all returned to values observed in control plants. The 4-fold increase in xylem sap ABA concentration following the root drying treatment was not solely the result of reduced sap flow, and thus may be considered a potential cause, not merely a consequence, of the observed reduction in stomatal conductance.     

Patterns of hydraulic architecture and water relations of two tropical canopy trees with contrasting leaf phenologies: Ochroma pyramidale and Pseudobombax septenatum

Many authors have attempted to explain the adaptive response of tropical plants to drought based on studies of water relations at the leaf level. Little attention has been given to the role of the xylem system in the control of plant water requirements. To evaluate this role, we studied the hydraulic architecture and water relations parameters of two tropical canopy trees with contrasting leaf phenologies: deciduous Pseudobombax septenatum (Jacq.) Dug and evergreen Ochroma pyramidale (Cav. ex lamb) Urban, both in the family Bombacaceae. The hydraulic architecture parameters studied include hydraulic conductivity, specific conductivity, leaf specific conductivity, and Huber value. Water relations parameters include leaf water potential, stem and leaf water storage capacitance, transpiration, stomatal conductance, and vulnerability of stems to cavitation and loss of hydraulic conductivity by embolisms. Compared to temperate trees, both species showed a pattern of highly vulnerable stems (50% loss of conductivity due to embolism at water potentials less than 1 MPa) with high leaf specific conductivities. The vulnerability of xylem to water-stress-induced embolism was remarkably similar for the two species but the leaf specific conductivity of petioles and leaf-bearing stems of the evergreen species, Ochroma (e.g., 9.08 and 11.4 x 10(-4) kg s(-1) m(-1) MPa(-1), respectively), were 3.4 and 2.3 times higher, respectively, than those of the deciduous species, Pseudobombax (e.g., 2.64 and 5.15 x 10(-4) kg s(-1) m(-1) MPa(-1), respectively). A runaway embolism model was used to test the ability of Ochroma and Pseudobombax stems to maintain elevated transpiration rates during the higher evaporative demand of the dry season. The percent loss of leaf area predicted by the runaway embolism model for stems of Pseudobombax ranged from 5 to 30%, not enough to explain the deciduous phenology of this tree species without analysis of root resistance or leaf and petiole vulnerability to embolism.     

Contrasting seasonal leaf habits of canopy trees between tropical dry-deciduous and evergreen forests in Thailand

We compared differences in leaf properties, leaf gas exchange and photochemical properties between drought-deciduous and evergreen trees in tropical dry forests, where soil nutrients differed but rainfall was similar. Three canopy trees (Shorea siamensis Miq., Xylia xylocarpa (Roxb.) W. Theob. and Vitex peduncularis Wall. ex Schauer) in a drought-deciduous forest and a canopy tree (Hopea ferrea Lanessan) in an evergreen forest were selected. Soil nutrient availability is lower in the evergreen forest than in the deciduous forest. Compared with the evergreen tree, the deciduous trees had shorter leaf life spans, lower leaf masses per area, higher leaf mass-based nitrogen (N) contents, higher leaf mass-based photosynthetic rates (mass-based P(n)), higher leaf N-based P(n), higher daily maximum stomatal conductance (g(s)) and wider conduits in wood xylem. Mass-based P(n) decreased from the wet to the dry season for all species. Following onset of the dry season, daily maximum g(s) and sensitivity of g(s) to leaf-to-air vapor pressure deficit remained relatively unchanged in the deciduous trees, whereas both properties decreased in the evergreen tree during the dry season. Photochemical capacity and non-photochemical quenching (NPQ) of photosystem II (PSII) also remained relatively unchanged in the deciduous trees even after the onset of the dry season. In contrast, photochemical capacity decreased and NPQ increased in the evergreen tree during the dry season, indicating that the leaves coped with prolonged drought by down-regulating PSII. Thus, the drought-avoidant deciduous species were characterized by high N allocation for leaf carbon assimilation, high water use and photoinhibition avoidance, whereas the drought-tolerant evergreen was characterized by low N allocation for leaf carbon assimilation, conservative water use and photoinhibition tolerance.     

Impacts of seasonal air and soil temperatures on photosynthesis in Scots pine trees

Seasonal courses of light-saturated rate of net photosynthesis (A360) and stomatal conductance (gs) were examined in detached 1-year-old needles of Scots pine (Pinus sylvestris L.) from early April to mid-November. To evaluate the effects of soil frost and low soil temperatures on gas exchange, the extent and duration of soil frost, as well as the onset of soil warming, were manipulated in the field. During spring, early summer and autumn, the patterns of A360 and gs in needles from the control and warm-soil plots were generally strongly related to daily mean air temperatures and the frequency of severe frost. The warm-soil treatment had little effect on gas exchange, although mean soil temperature in the warm-soil plot was 3.8 degrees C higher than in the control plot during spring and summer, indicating that A360 and gs in needles from control trees were not limited by low soil temperature alone. In contrast, prolonged exposure to soil temperatures slightly above 0 degrees C severely restricted recovery of A360 and especially gs in needles from the cold-soil treatment during spring and early summer; however, full recovery of both A360 and gs occurred in late summer. We conclude that inhibition of A360 by low soil temperatures is related to both stomatal closure and effects on the biochemistry of photosynthesis, the relative importance of which appeared to vary during spring and early summer. During the autumn, soil temperatures as low as 8 degrees C did not affect either A360 or gs.     

Effects of simulated thaw on xylem cavitation,residual embolism,spring dieback and shoot growth in yellow birch

Yellow birch seedlings (Betula alleghaniensis Britt.) that had lost more than 90% of their stem hydraulic conductivity during ambient winter temperatures were exposed to 0 and 20 days of a simulated winter thaw followed by a 48-h freezing treatment at 0, -5, -10, -20 and -30 degrees C. After measuring freezing injury to shoots and roots, the seedlings were placed in a greenhouse where recovery of xylem conductivity and new growth were measured. Shoot xylem cavitation was measured as percent loss of hydraulic conductivity. Shoot freezing injury was assessed by electrolyte leakage (EL) and root freezing injury was assessed by EL and triphenyl tetrazolium chloride reduction. Seedlings pretreated with thaw had higher stem water contents and suffered more freezing damage to roots and shoots (at -20 and -30 degrees C, respectively) than unthawed seedlings. After 3 weeks in a greenhouse, seedlings from the 0, -5 and -10 degrees C freezing treatments showed complete recovery of xylem conductivity, with substantially increased stem water contents. Poor recovery of hydraulic conductivity was observed only in seedlings that were subjected to freezing treatments at -20 and -30 degrees C, regardless of thaw treatment. Of these embolized seedlings, however, only those not previously thawed showed recovery of hydraulic conductivity or regained stem water content after 9 weeks in the greenhouse. Shoot dieback, bud burst and length of new shoots were significantly related to the extent of stem xylem cavitation and freezing injury. We conclude that (1) the simulated winter thaw predisposed yellow birch seedlings to freezing damage in shoots and roots by dehardening tissues and increasing their water content; (2) root freezing damage in turn affected the seedlings' ability to refill embolized stem xylem, resulting in considerable residual xylem embolism after spring refilling; (3) further recovery of stem xylem conductivity was attributable to growth of new vessels; (4) and the permanent residual embolism, together with root and shoot freezing injury, caused increased dieback, bud mortality and reduced growth of new shoots.     

Assimilation and stomatal conductance responses of red spruce to midwinter frosts and the constituent ions of acid mist

Red spruce (Picea rubens Sarg.) seedlings growing outside in open-top chambers were sprayed twice weekly with artificial mists at either pH 2.5 or 5.6, for five months during the 1988 growing season. The mists contained one of the following: water, pH 5.6 (control); (NH(4))(2)SO(4), pH 5.6; NH(4)NO(3), pH 5.6; HNO(3), pH 2.5; H(2)SO(4), pH 2.5; or (NH(4))(2)SO(4) + NH(4)NO(3), pH 2.5. During January 1989, the light responses of assimilation and stomatal conductance were assessed in the laboratory following a 4-day equilibration at 12 degrees C. The aerial portions of the intact trees were then subjected to a mild (-10 degrees C) frost for three hours during the night and the rate of recovery of light-saturated assimilation (A(max)) was determined the following day using the same branches as were used for the assimilation studies before the frost treatment. The same trees were then subjected to a second frost of -18 degrees C for three hours during the following night and the recovery of A(max) of the same branches was measured the next day. All of the acid mist treatments increased A(max) and apparent quantum yield relative to the control treatment when measured before the frost treatments. Frosts of -10 and -18 degrees C resulted in a significant decline in A(max) of seedlings in all treatments except the control. Stomatal conductance increased with increasing irradiance in seedlings in the acid mist treatments that did not contain SO(4) (2-) ion. Stomatal conductance of seedlings in acid mist treatments containing SO(4) (2-) ion was insensitive to changes in irradiance over the range 50-1500 micro mol m(-2) s(-1). It is concluded that acid precipitation increased the sensitivity of the assimilation response to midwinter frosts that follow a brief warm period. The SO(4) (2-) ion appears to be significant in causing increased sensitivity to frost and in causing stomatal insensitivity to light flux density.     

荒漠植物白刺叶片气孔性状对模拟增雨的响应

为研究荒漠植物气孔性状对气候变化的响应以及对植物固碳能力的影响,根据内蒙古磴口多年平均降水量和植物生长规律,设计2个增雨时段(生长季前期与后期),每个时段设计2个增雨梯度(50%与100%),对天然白刺灌丛进行增雨实验,利用数码图像显微镜处理系统,研究叶片气孔大小、气孔密度及气孔导度等气孔性状对增雨的响应。结果表明:与对照相比,各增雨处理可以增加气孔长度、宽度与气孔导度,但孔密度减小,且叶片下表皮密度大于上表皮。相同增雨季节,100%处理对气孔大小、气孔密度及气孔导度的影响均高于50%处理,2个梯度间差异不显著;气孔大小与气孔导度对生长季前期增雨响应更敏感,气孔密度对不同生长季节增雨响应不明显;气孔性状之间存在一定的协调性。在未来降雨增加的条件下,荒漠植物可以通过改变气孔性状(扩大气孔)通路来提高固碳能力。     

Leaf photosynthetic characteristics of beech (Fagus sylvatica) saplings during three years of exposure to elevated CO(2) concentration

Beech (Fagus sylvatica L.) seedlings were cultivated from seeds sown in pots or directly in the ground in outdoor chambers that were transparent to solar radiation, and provided either ambient air or CO(2)-enriched air (ambient + 350 &mgr;mol mol(-1)). The rooting volume was high in all experiments. In the short-term experiment, potted plants were assigned to a factorial CO(2) x nutrient treatment (optimal nutrient supply and severe nutrient shortage) for 1 year. In the long-term experiment, plants were grown directly in the ground and received an optimal supply of water and nutrients in both CO(2) treatments for 3 years. Nutrient stress caused carboxylation capacity (V(m)) to decrease in the potted seedlings exposed to CO(2)-enriched air during their first growing season. In the long-term experiment with optimal nutrient supply, CO(2)-enriched air did not affect V(m), but caused an upward acclimation of maximum electron transport rate (J(m)). Consequently, there was a 14% increase in the J(m)/V(m) ratio, indicating nitrogen reallocation to maintain an equilibrium between RuBP consumption and RuBP regeneration. Both V(m) and J(m) decreased during the growing season in both CO(2) treatments. Although upward acclimation of J(m) was no longer apparent at the end of the third growing season, plants in CO(2)-enriched air maintained a higher J(m)/V(m) ratio than plants in ambient air, indicating that photosynthetic acclimation always occurred. Second flush leaves appeared during each growing season. When expressed on the basis of foliar nitrogen concentration, their photosynthetic characteristics (V(m) and J(m)) were enhanced compared with other leaves. Because the number of second flush leaves was also increased in the elevated CO(2) treatment, this response should be taken into account when modeling the effects of elevated CO(2) concentration on canopy photosynthesis. Stomatal conductance decreased in response to atmospheric CO(2) enrichment; however, the stomatal response to irradiance followed a single relationship based on two stomatal conductance models.     

Frost hardiness of Picea rubens growing in spruce decline regions of the Appalachians

It has been proposed that pollutants predispose Picea rubens Sarg. growing in the high Appalachians to frost damage. The pattern of autumn hardening of P. rubens growing at Whiteface Mountain, NY, and Newfound Gap, NC, was monitored by detaching shoots at 1-3 weekly intervals, air freighting them to Scotland, and freeze-testing them. The temperatures that produced freezing injury from August 1986 to January 1987 were compared with minimum air temperatures recorded in those months at nearby meteorological stations over 22 previous years. There was only weak evidence that the onset or degree of frost hardening was inadequate to protect the trees from direct freezing injury (as opposed to winter desiccation). Historically, minimum air temperatures occasionally fell below the lethal temperature for a 10% kill (LT(10)), but they rarely fell below the LT(50). The trees hardened rapidly in the autumn (max. 2.2 degrees C day(-1)) to between -30 degrees C and -40 degrees C by January (LT(50)), including trees showing visible decline on Clingman's Dome, TN. Individual trees differed in hardiness by up to 10 degrees C. It is concluded that any pollutant-induced susceptibility to freezing injury is insufficient, on its own, to account for forest decline in the Appalachians.     

Planting stress in newly planted jack pine and white spruce. 2. Changes in tissue water potential components

Water relations of bare-root jack pine (Pinus banksiana Lamb.) and white spruce (Picea glauca (Moench) Voss) planted in a greenhouse and on a boreal cut-over site were examined during the first growing season. In field-planted trees, maximum stomatal conductances (g(wv)) were initially low (< 0.10 cm s(-1)). Base and minimum xylem pressure potentials (Psi(x(base)) and Psi(x(min))) were less than -1.5 and -1.7 MPa for jack pine and -2.0 and -2.6 MPa for white spruce, respectively. During the growing season, maximum g(wv) increased in both species to around 0.2 cm s(-1). Base and minimum xylem pressure potentials also increased in both species to around -0.5 and -1.0 MPa in jack pine and -1.0 and -1.5 MPa in white spruce, respectively. Minimum xylem pressure potentials in white spruce fell below the turgor loss point during the first half of the growing season. Osmotic potential at the turgor loss point Psi(pi(TLP)) decreased after field planting to around -2.7 and -2.3 MPa in jack pine and white spruce, respectively. In the greenhouse, minimum values of Psi(pi(TLP)) were -2.2 and -2.3 MPa in jack pine and white spruce, respectively. Maximum bulk modulus of elasticity was greater in white spruce and underwent greater seasonal change than in jack pine. Relative water content (RWC) at turgor loss ranged between 71 and 74% in jack pine and 80 and 87% in white spruce. Available turgor (T(avail)), defined as the integral of turgor over the range of RWC between Psi(x(base)) and xylem pressure potential at the turgor loss point, was similar in jack pine and white spruce just after field planting. For the rest of the growing season, however, T(avail) in jack pine was two to three times that in white spruce. Diurnal turgor (T(diurnal)), defined as the integral of turgor over the range of RWC between Psi(x(base)) and Psi(x(min)), as a percent of T(avail) was higher in field-planted white spruce than jack pine until the end of the season. Dynamics of tissue water potential components are discussed in relation to plantation establishment.     

Seasonal patterns of leaf gas exchange and water relations in dry rain forest trees of contrasting leaf phenology

Diurnal and seasonal patterns of leaf gas exchange and water relations were examined in tree species of contrasting leaf phenology growing in a seasonally dry tropical rain forest in north-eastern Australia. Two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret. were studied. The deciduous species had higher specific leaf areas and maximum photosynthetic rates per leaf dry mass in the wet season than the evergreens. During the transition from wet season to dry season, total canopy area was reduced by 70-90% in the deciduous species and stomatal conductance (g(s)) and assimilation rate (A) were markedly lower in the remaining leaves. Deciduous species maintained daytime leaf water potentials (Psi(L)) at close to or above wet season values by a combination of stomatal regulation and reduction in leaf area. Thus, the timing of leaf drop in deciduous species was not associated with large negative values of daytime Psi(L) (greater than -1.6 MPa) or predawn Psi(L) (greater than -1.0 MPa). The deciduous species appeared sensitive to small perturbations in soil and leaf water status that signalled the onset of drought. The evergreen species were less sensitive to the onset of drought and g(s) values were not significantly lower during the transitional period. In the dry season, the evergreen species maintained their canopies despite increasing water-stress; however, unlike Eucalyptus species from northern Australian savannas, A and g(s) were significantly lower than wet season values.     

Bole girdling affects metabolic properties and root, trunk and branch hydraulics of young ponderosa pine trees

Effects of trunk girdling on seasonal patterns of xylem water status, water transport and woody tissue metabolic properties were investigated in ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) trees. At the onset of summer, there was a sharp decrease in stomatal conductance (g(s)) in girdled trees followed by a full recovery after the first major rainfall in September. Eliminating the root as a carbohydrate sink by girdling induced a rapid reversible reduction in g(s). Respiratory potential (a laboratory measure of tissue-level respiration) increased above the girdle (branches and upper trunk) and decreased below the girdle (lower trunk and roots) relative to control trees during the growing season, but the effect was reversed after the first major rainfall. The increase in branch respiratory potential induced by girdling suggests that the decrease in g(s) was caused by the accumulation of carbohydrates above the girdle, which is consistent with an observed increase in leaf mass per area in the girdled trees. Trunk girdling did not affect native xylem embolism or xylem conductivity. Both treated and control trunks experienced loss of xylem conductivity ranging from 10% in spring to 30% in summer. Girdling reduced xylem growth and sapwood to leaf area ratio, which in turn reduced branch leaf specific conductivity (LSC). The girdling-induced reductions in g(s) and transpiration were associated with a decrease in leaf hydraulic conductance. Two years after girdling, when root-to-shoot phloem continuity had been restored, girdled trees had a reduced density of new wood, which increased xylem conductivity and whole-tree LSC, but also vulnerability to embolism.     

Field measurements of isoprene emission from trees in response to temperature and light

The atmospheric hydrocarbon budget is important for predicting ozone episodes and the effects of pollution mitigation strategies. Isoprene emission from plants is an important part of the atmospheric hydrocarbon budget. We measured isoprene emission capacity at the bottom, middle, and top of the canopies of a white oak (Quercus alba L.) tree and a red oak (Quercus rubra L.) tree growing adjacent to a tower in the Duke University Forest. Leaves at the top of the white oak tree canopy had a three- to fivefold greater capacity for emitting isoprene than leaves at the bottom of the tree canopy. Isoprene emission rate increased with increasing temperature up to about 42 degrees C. We conclude that leaves at the top of the white oak tree canopy had higher isoprene emission rates because they were exposed to more sunlight, reduced water availability, and higher temperature than leaves at the bottom of the canopy. Between 35 and 40 degrees C, white oak photosynthesis and stomatal conductance declined, whereas red oak (Quercus rubra) photosynthesis and stomatal conductance increased over this range. Red oak had lower rates of isoprene emission than white oak, perhaps reflecting the higher stomatal conductance that would keep leaves cool. The concentration of isoprene inside the leaf was estimated with a simplified form of the equation used to estimate CO(2) inside leaves.     

Net CO(2) assimilation of cacao seedlings following dark chilling

Overnight exposure of cacao (Theobroma cacao L.) seedlings to chilling temperatures between 4.7 and 15.8 degrees C reduced net CO(2) assimilation rate (A) and stomatal conductance to water vapor (g(s)), with temperatures below 10 degrees C causing severe inhibition. Net CO(2) assimilation rates of chilled seedlings recovered to those of nonchilled plants within 7 days. No differences in daytime intercellular CO(2) concentration (c(i)) with overnight temperature were observed on the first day after the chilling treatment, which indicates that the reduction in photosynthesis was not caused by the reduction in stomatal conductance. However, c(i) of chilled plants was much less than that of nonchilled plants on the second day after treatment, which suggests that chilling caused a change in stomatal response to CO(2) concentration. Even 7 days after treatment, when A had recovered to control values, g(s) of chilled leaves was only approximately 70% that of controls. Chilling did not inhibit A through an effect on leaf water potential, which was higher in chilled plants than in unchilled plants.     

Planting stress in newly planted jack pine and white spruce. 1. Factors influencing water uptake

Bareroot jack pine (Pinus banksiana Lamb.) seedlings (2 + 0) and bareroot white spruce (Picea glauca (Moench) Voss) transplants (1 1/2 + 1 1/2) were taken from cold storage and planted on a clearcut forest site in northeastern Ontario on several dates between May 6 and June 5 during which period soil temperature at 15 cm depth increased from 0 to 18 degrees C. Additional cold-stored trees were transferred to a greenhouse where they were grown in pots for 0, 7 or 28 days and then placed with their roots in aerated water maintained at one of a range of constant temperatures between 0 and 22 degrees C. In both species, daytime xylem pressure potentials (Psi(x)) and needle conductances (g(wv)) decreased with decreasing soil or water temperature. At all root temperatures, g(wv) was lower, and Psi(x) higher, in jack pine than in white spruce. After 28 days in the greenhouse, g(wv) of jack pine seedlings, and Psi(x) of white spruce, was higher than in plants just removed from cold storage. In both species, water-flow resistance through the soil-plant-atmosphere continuum (RSPAC) increased as root temperature decreased. At all root temperatures, RSPAC was higher in plants just removed from cold storage than in plants grown in the greenhouse for 28 days, during which time many new unsuberized roots were formed. At root temperatures above 10 degrees C, RSPAC of both species was higher in trees newly planted in mineral soil than in trees with roots in aerated water; presumably because the roots of planted trees had limited hydraulic contact with the soil. On the day following removal from cold storage, relative plant water flow resistance increased, in both species, more rapidly with declining root temperature than could be accounted for by the change with temperature in the viscosity of water, thus indicating an effect of temperature on root permeability. The same effect was evident in jack pine seedlings, but not white spruce transplants, that had been grown for 28 days in the greenhouse after removal from cold storage.     

Stomatal response of an anisohydric grapevine cultivar to evaporative demand, available soil moisture and abscisic acid

Stomatal responsiveness to evaporative demand (air vapour pressure deficit (VPD)) ranges widely between species and cultivars, and mechanisms for stomatal control in response to VPD remain obscure. The interaction of irrigation and soil moisture with VPD on stomatal conductance is particularly difficult to predict, but nevertheless is critical to instantaneous transpiration and vulnerability to desiccation. Stomatal sensitivity to VPD and soil moisture was investigated in Semillon, an anisohydric Vitis vinifera L. variety whose leaf water potential (Ψ(l)) is frequently lower than that of other grapevine varieties grown under similar conditions in the warm grape-growing regions of Australia. A survey of Semillon vines across seven vineyards revealed that, regardless of irrigation treatment, midday Ψ(l) was dependent on not only soil moisture but VPD at the time of measurement. Predawn Ψ(l) was more closely correlated to not only soil moisture in dry vineyards but to night-time VPD in drip-irrigated vineyards, with incomplete rehydration during high night-time VPD. Daytime stomatal conductance was low only under severe plant water deficits, induced by extremes in dry soil. Stomatal response to VPD was inconsistent across irrigation regime; however, in an unirrigated vineyard, stomatal sensitivity to VPD-the magnitude of stomatal response to VPD-was heightened under dry soils. It was also found that stomatal sensitivity was proportional to the magnitude of stomatal conductance at a reference VPD of 1kPa. Exogenous abscisic acid (ABA) applied to roots of Semillon vines growing in a hydroponic system induced stomatal closure and, in field vines, petiole xylem sap ABA concentrations rose throughout the morning and were higher in vines with low Ψ(l). These data indicate that despite high stomatal conductance of this anisohydric variety when grown in medium to high soil moisture, increased concentrations of ABA as a result of very limited soil moisture may augment stomatal responsiveness to low VPD.