Drought-induced oxidative stress in Canarian laurel forest tree species growing under controlled conditions

We studied photoprotection and antioxidative protection in the three major species of the Canarian laurel forest (Laurus azorica (Seub.) Franco, Persea indica (L.) K. Spreng and Myrica faya Aiton). Trees were exposed to drought under controlled conditions by withholding water until leaf relative water content (RWC) reached 50-55%. Drought reduced photosynthetic rate (P(N)) and was associated with decreased quantum yield of photosystem II (PSII) electron transport and increased non-photochemical quenching (NPQ) in L. azorica and M. faya, but did not increase NPQ in P. indica. Drought-treated trees of L. azorica had the highest de-epoxidation state (DPS) of the xanthophyll cycle and the highest zeaxanthin (Z) concentration, suggesting that this species had more effective photoprotective mechanisms than M. faya and P. indica. Moreover, beta-carotene remained unaltered in L. azorica trees during drought, suggesting that the chloroplasts of this species are better protected against oxidative stress than those of M. faya and P. indica. Increased antioxidation by ascorbate peroxidase, superoxide dismutase and glutathione reductase in L. azorica removed activated oxygen species (AOS) generated during drought treatment. Although M. faya was able to increase its energy dissipation rate by forming Z and thus increasing the DPS of the xanthophyll cycle, it did not respond to drought-induced oxidative stress with the result that beta-carotene degradation occurred. Persea indica did not activate an energy dissipation mechanism in response to drought treatment, hence formation of AOS was likely high in the drought-treated trees. In general, L. azorica was most resistant and P. indica most sensitive to photoinhibition and oxidative stress during drought.     

Water stress responses of seedlings of four Mediterranean oak species

Effects of water stress on phenology, growth, stomatal activity and water status were assessed from April to November 1996 in 2-year-old seedlings of Quercus frainetto Ten. (Quercus conferta Kit.), Quercus pubescens Willd., Quercus macrolepis Kotschy (Quercus aegilops auct.) and Quercus ilex L. growing in containers in northern Greece. All four species developed more than 50% of their total leaf area before the beginning of June--an adaptation to arid climates. Well-irrigated plants tended to develop greater individual leaf area, number of leaves per plant, total plant leaf area, height and root:shoot ratios than water-stressed plants, but the difference between treatments was not significant for any parameter in any species. Quercus macrolepis appeared to be the most drought-tolerant of the four species. It maintained the highest number of leaves of the smallest size and increased the proportion of fine roots during drought. In all species, drought caused significant decreases in stomatal conductance and predawn and midday water potentials from mid-July until the end of August, when the lowest soil water content and highest mean daily air temperatures and midday leaf temperatures occurred; however, the responses were species-specific. Among the four species, Quercus macrolepis sustained the highest stomatal conductance despite very low water potentials, thus overcoming drought by means of desiccation tolerance. Quercus ilex decreased stomatal conductance even before severe water stress occurred, thereby avoiding desication during drought. Quercus pubescens had the highest water potential despite a high stomatal conductance, indicating that its leaf water status was independent of stomatal activity. Quercus frainetto was the least drought-resistant of the four species. During drought it developed very low water potentials despite markedly reduced stomatal aperture.     

Quantifying stomatal and non-stomatal limitations to carbon assimilation resulting from leaf aging and drought in mature deciduous tree species

Gas exchange techniques were used to investigate light-saturated carbon assimilation and its stomatal and non-stomatal limitations over two seasons in mature trees of five species in a closed deciduous forest. Stomatal and non-stomatal contributions to decreases in assimilation resulting from leaf age and drought were quantified relative to the maximum rates obtained early in the season at optimal soil water contents. Although carbon assimilation, stomatal conductance and photosynthetic capacity (V(cmax)) decreased with leaf age, decreases in V(cmax) accounted for about 75% of the leaf-age related reduction in light-saturated assimilation rates, with a secondary role for stomatal conductance (around 25%). However, when considered independently from leaf age, the drought response was dominated by stomatal limitations, accounting for about 75% of the total limitation. Some of the analytical difficulties associated with computing limitation partitioning are discussed, including path dependence, patchy stomatal closure and diffusion in the mesophyll. Although these considerations may introduce errors in our estimates, our analysis establishes some reasonable boundaries on relative limitations and shows differences between drought and non-drought years. Estimating seasonal limitations under natural conditions, as shown in this study, provides a useful basis for comparing limitation processes between years and species.     

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.     

Changes in foliar carbon isotope composition and seasonal stomatal conductance reveal adaptive traits in Mediterranean coppices affected by drought

We estimated water-use efficiency and potential photosynthetic assimilation of Holm oak(Quercus ilex L.) on slopes of NW and SW aspects in a replicated field test examining the effects of intensifying drought in two Mediterranean coppice forests. We used standard techniques for quantifying gas exchange and carbon isotopes in leaves and analyzed total chlorophyll, carotenoids and nitrogen in leaves collected from Mediterranean forests managed under the coppice system. We postulated that responses to drought of coppiced trees would lead to differential responses in physiological traits and that these traits could be used by foresters to adapt to predicted warming and drying in the Mediterranean area. We observed physiological responses of the coppiced trees that suggested acclimation in photosynthetic potential and water-use efficiency:(1) a significant reduction in stomatal conductance(p0.01) wasrecorded as the drought increased at the SW site;(2) foliar δ13C increased as drought increased at the SW site(p0.01);(3) variations in levels of carotenoids and foliar nitrogen, and differences in foliar morphology were recorded, and were tentatively attributed to variation in photosynthetic assimilation between sites. These findings increase knowledge of the capacity for acclimation of managed forests in the Mediterranean region of Europe.     

Induction of photosynthesis and importance of limitations during the induction phase in sun and shade leaves of five ecologically contrasting tree species from the temperate zone

We examined the principal differences in photosynthetic characteristics between sun and shade foliage and determined the relative importance of biochemical and stomatal limitations during photosynthetic induction. Temperate-zone broadleaf and conifer tree species, ranging widely in shade tolerance, were investigated from one locality in the Czech Republic. The study species included strongly shade-tolerant Abies alba Mill. and Tilia cordata Mill., less shade-tolerant Fagus sylvatica L. and Acer pseudoplatanus L. and sun-demanding Picea abies (L.) Karst. In the fully activated photosynthetic state, sun foliage of all species had significantly higher maximum CO(2) assimilation rates, maximum stomatal conductance and maximum rates of carboxylation than shade foliage. Compared with shade leaves, sun leaves had significantly higher nocturnal stomatal conductances. In all species, shade foliage tended to have higher induction states 60 s after leaf illumination than sun foliage. Sun and shade foliage did not differ in the rate of disappearance of the transient biochemical limitation during the induction phase. Longer time periods were required to reach 90% photosynthetic induction and 90% stomatal induction in sun foliage than in shade foliage of the less shade-tolerant F. sylvatica and A. pseudoplatanus and in sun-demanding P. abies; however, in sun foliage of the strongly shade-tolerant species T. cordata and A. alba, the time needed for photosynthetic induction was similar to, or less than, that for shade foliage. Shade but not sun needles of P. abies and A. alba had significantly slower induction kinetics than the broadleaf tree species. Among species, the sun-demanding P. abies exhibited the shortest stomatal induction times in both sun and shade leaves. Independently of shade tolerance ranking, the transient stomatal and total limitations that characterize photosynthetic induction were relieved significantly earlier in shade foliage than in sun foliage. Sun foliage generally exhibited a hyperbolic photosynthetic induction response, whereas a sigmoidal induction response was more frequent in shade foliage. The different relative proportions of transient biochemical and stomatal limitations during photosynthetic induction in sun and shade foliage indicate an essential role of stomata in photosynthetic limitation during induction, mainly in shade foliage, with a consequent influence on the shape of the photosynthetic induction curve.     

Drought inhibits photosynthetic capacity more in females than in males of Populus cathayana

We investigated sex-related photosynthetic responses to drought in the dioecious species, Populus cathayana Rehd. Plants were subjected to two watering regimes (100% and 30% of field capacity) in a semi-controlled environment. Drought significantly decreased leaf area (LA), total number of leaves (TNL), specific leaf area (SLA), relative water content, net photosynthetic rate (P(n)), transpiration (E), stomatal conductance (g(s)), intercellular CO(2) concentration (C(i)), light saturation point (L(SP)), apparent quantum yield (Phi), carboxylation efficiency (CE), light-saturated photosynthetic rate (P(max)), maximum efficiency of PSII (F(v)/F(m)) and maximum effective quantum yield of PSII (Yield), and increased the total chlorophyll concentration (TC), CO(2) compensation point (Gamma), non-photochemical quenching coefficient, peroxidase (POD) activity and carbon isotope composition (delta(13)C). Moreover, differences between males and females were detected in many of these responses. In the drought treatment, males exhibited significantly higher LA, TNL, TC, concentration of carotenoids (Caro), P(n), E, g(s), C(i), L(SP), Phi, CE, P(max), F(v)/F(m), photochemical quenching coefficient, POD activity and delta(13)C, but a lower SLA, chlorophyll a/b ratio, carotenoids/total chlorophyll ratio and Gamma than females. However, Caro, L(SP), Gamma, Phi, CE and POD activity were apparently associated with sex-related resource demands, because significant differences in these traits were detected between the sexes under well-watered conditions. Our results indicate that drought stress limits photosynthetic capacity more in females than in males.     

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.     

Acclimation of leaves to contrasting irradiance in juvenile trees differing in shade tolerance

Leaves developing in different irradiances undergo structural and functional acclimation, although the extent of trait plasticity is species specific. We tested the hypothesis that irradiance-induced plasticity of photosynthetic and anatomical traits is lower in highly shade-tolerant species than in moderately shade-tolerant species. Seedlings of two evergreen conifers, shade-tolerant Abies alba Mill. and moderately shade-tolerant Picea abies Karst., and two deciduous angiosperm species, highly shade-tolerant Fagus sylvatica L. and moderately shade-tolerant Acer pseudoplatanus L., were grown in deep shade (LL, 5% of full irradiance) or in full solar irradiance (HL) during 2003 and 2004. Steady state responses of quantum yield of PSII (Phi(PSII)), apparent electron transport rate (ETR), nonphotochemical quenching (NPQ) and photochemical quenching (qP) were generally modified by the light environment, with slower declines in Phi(PSII) and qP and greater maximal ETR and NPQ values in HL plants in at least one season; however, no link between quantitative measures of plasticity of these traits and shade tolerance was found. Plasticity of nine anatomical traits (including palisade cell length, which was reduced in LL) showed no relationship with shade tolerance, but was less in conifers than in deciduous trees, suggesting that leaf life span may be a significant correlate of plasticity. When LL-acclimated plants were exposed to HL conditions, the degree and duration of photoinhibition (measured as a decline in maximum quantum yield) was greatest in F. sylvatica, much lower in P. abies and A. alba, and lowest in A. pseudoplatanus. Thus, as with the other traits studied, vulnerability to photoinhibition showed no relationship with shade tolerance.     

Drought effects on hydraulic conductivity and xylem vulnerability to embolism in diverse species and provenances of Mediterranean cedars

We studied hydraulic traits of young plants of the Mediterranean cedar species Cedrus atlantica (Endl.) G. Manetti ex Carrière (Luberon, France), C. brevifolia (Hook. f.) Henry (Cyprus), C. libani A. Rich (Hadeth El Jebbe, Lebanon) and C. libani (Armut Alani, Turkey). With an optimum water supply, no major differences were observed among species or provenances in either stem hydraulic conductivity (Ks) or leaf specific conductivity (Kl) measured on the main shoot. A moderate soil drought applied for 10 weeks induced marked acclimation through a reduction in Ks, particularly in the Lebanese provenance of C. libani, and a decrease in tracheid lumen size in all species. Cedrus atlantica, which had the smallest tracheids, was the species most vulnerable to embolism: a 50% loss in hydraulic conductivity (PsiPLC50) occurred at a water potential of -4.4 MPa in the well-watered treatment, and at -6.0 MPa in the moderate drought treatment. In the other species, PsiPLC50 was unaffected by moderate soil drought, and only declined sharply at water potentials between -6.4 and -7.5 MPa in both irrigation treatments. During severe drought, Ks of twigs and stomatal conductance (g(s)) were measured simultaneously as leaf water potential declined. For all species, lower vulnerability to embolism based on loss of Ks was recorded on current-year twigs. The threshold for stomatal closure (10% of maximum g(s)) was reached at a predawn water potential (Psi(pd)) of -2.5 MPa in C. atlantica (Luberon) and at -3.1 MPa in C. libani (Lebanon), whereas the other provenance and species had intermediate Psi(pd) values. Cedrus brevifolia, with a Psi(pd) (-3.0 MPa) close to that of C. libani (Lebanon), had the highest stomatal conductance of the study species. The importance of a margin of safety between water potential causing stomatal closure and that causing xylem embolism induction is discussed.     

Decline in stomatal response to leaf water deficit in Corylus maxima cuttings

Many woody species can be propagated from leafy cuttings. However, following rooting, cuttings of Corylus maxima Mill. cv. Purpurea do not always survive the transition from a highly supportive rooting environment (e.g., fog) to a more natural environment where evaporative demand is higher. We found that it is not the supply of water to leaves, but stomatal dysfunction that leads to severe water deficits in the rooted cuttings. Two hours after well-rooted cuttings were transferred from the rooting environment, we were able to relate visible signs of leaf water deficit to high stomatal conductance (g(s)) and low relative water content (R). Small expanding leaves (L3) had unusually high g(s) and lower R than fully expanded leaves (L1). Although high cuticular conductances (g(c)) were occasionally observed in L3, SEM confirmed that increased total leaf conductance (g) was mainly a result of abnormally wide stomatal opening. We measured changes in the ability of stomata to control water loss during rooting by determining stomatal responsiveness to leaf water deficit in detached L1 and L3 harvested from cuttings during the first 75 days after severance from stock plants. Reduced stomatal responsiveness was observed within 7 days of severance, prior to adventitious root formation, and was more pronounced in L3 than in L1. A period of acclimatization after rooting (no leaf wetting, but a vapor pressure deficit of 0.20 kPa) reduced g(s) by 50% in L3 but not in L1, and partially restored stomatal responsiveness in L1 but not in L3. After rooting, the original leaves on the cutting retained substantial capacity for photosynthesis (e.g., in L1, 8 micromol m(-2) s(-1) at a photosynthetic photon flux density of 400 micromol m(-2) s(-1)). The implications of the results for post-rooting acclimatization procedures are discussed.     

Energy dissipation in drought-avoiding and drought-tolerant tree species at midday during the Mediterranean summer

Photosynthetic performance was monitored during two consecutive summers in four co-occurring evergreen Mediterranean tree species growing on a south-facing rocky slope. In response to midday water stress, the drought-avoiding species Pinus halepensis Mill. exhibited marked stomatal closure (g(s)) but no changes in stem water potential (Psi(s)), whereas the drought-tolerant species Quercus coccifera L., Q. ilex ssp. ballota (Desf.) Samp. and Juniperus phoenicea L. displayed declines in midday g(s) and Psi(s). The higher resistance to CO(2) influx in needles of P. halepensis compared with the other species did not result in either a proportional increase in non-radiative dissipation of excess energy or photo-inactivation of photosystem II (PSII). No significant differences were found among species either in the de-epoxidation state of the xanthophyll cycle (DPS) or in the pool of its components on a total chlorophyll basis (VAZ). Despite contrasting midday assimilation rates, the three drought-tolerant species all exhibited a pronounced drop in photochemical efficiency at midday that was characterized by a decrease in the excitation capture efficiency of the open PSII centers. Although photoinhibition was not fully reversed before dawn, it apparently did not result in cumulative photo-damage. Thus, the drought-avoiding and drought-tolerant species employed different mechanisms for coping with excess light during the midday depression in photosynthesis that involved contrasting midday photochemical efficiencies of PSII and different degrees of dynamic photoinhibition as a photo-protective mechanism. These behaviors may be related to the different mechanisms employed by drought-avoiding and drought-tolerant species to withstand water deficit.     

Water relations of seedlings of three Quercus species: variations across and within species grown in contrasting light and water regimes

We compared seedling water relations of three Mediterranean Quercus species (the evergreen shrub Q. coccifera L., the evergreen tree Q. ilex L. subsp. ballota (Desf.) Samp. and the deciduous or marcescent tree Q. faginea L.). We also explored seedling potential for acclimation to contrasting growing conditions. In March, 1-year-old seedlings of the three species were planted in pots and grown outdoors in a factorial combination of two irrigation regimes (daily (HW) and alternate day watering (LW)) and two irradiances (43 and 100% of full sunlight). At the end of July, predawn and midday water potentials (Psi(pd), Psi(md)) were measured, and pressure-volume (P-V) curves were obtained for mature current-year shoots. Species exhibited similar Psi(pd) and Psi(md) values, but differed in leaf morphology and water relations. The evergreens possessed larger leaf mass per area (LMA) and were able to maintain positive turgor pressure at lower water potentials than the deciduous species because of their lower osmotic potential at full turgor. However, the three species had similar relative water contents at the turgor loss point because Q. faginea compensated for its higher osmotic potential with greater cell wall elasticity. Values of Psi(pd) had a mean of -1.12 MPa in LW and -0.63 MPa in HW, and Psi(md) had a mean of -1.13 MPa in full sunlight and -1.64 MPa in shade, where seedlings exhibited lower LMA. However, the P-V curve traits were unaffected by the treatments. Our results suggest that Q. faginea seedlings combine the water-use characteristics of mesic deciduous oak and the drought-tolerance of xeric evergreen oak. The ability of Q. coccifera to colonize drier sites than Q. ilex was not a result of higher drought tolerance, but rather may be associated with other dehydration postponement mechanisms including drought-induced leaf shedding. The lack of treatment effects may reflect a relatively low contrast between treatment regimes, or a low inherent responsiveness of these traits in the study species, or both.     

盐胁迫对国槐和核桃幼苗光合作用的影响

研究了国槐与核桃半年生蛭石盆栽苗在盐 (NaCl溶液处理浓度为 5 0 ,10 0 ,2 0 0mmol·L- 1)胁迫条件下光合作用的变化。结果表明 ,核桃受盐胁迫后净光合速率明显下降 ,并且随盐浓度增大、胁迫时间延长而下降更多。国槐受盐胁迫后净光合速率增加 ,在短时间胁迫下增加多 ,随胁迫时间延长增加幅度变小。国槐与核桃在 2 0 0mmol·L- 1NaCl胁迫 2 4d内叶绿素a和b的含量变化不大 ,但核桃的叶绿素a/b之值下降。并且国槐的类胡萝卜素含量在盐胁迫后先增加后与对照相平 ,而核桃则明显下降。两个树种CO2 响应曲线的主要差别为国槐光呼吸速率下降为对照的4 3 7% ,核桃则比对照增加了 71.6 0 % ;国槐的CO2 补偿点比对照下降了 13.5 8% ,而核桃则比对照上升了 194 .15 %。两个树种在光响应曲线上的差别为国槐的暗呼吸速率比对照增加了 3倍多 ,而核桃则下降为对照的 6 5 .2 8%。核桃发生明显的光抑制     

The ecological and functional correlates of nocturnal transpiration

Contrary to the conventional theory of optimal stomatal control, there is substantial transpiration at night in many tree species, but the functional significance of this phenomenon remains uncertain. To investigate the possible roles of nocturnal transpiration, we compared and contrasted the correlations of both nocturnal and diurnal sap flow with a range of traits in 21 temperate deciduous tree species. These traits included soil water affinity, shade tolerance, cold hardiness, nitrogen concentration of tissues, minimum transpiration rate of excised leaves, growth rate, photosynthetic capacity, stomatal length and density, and the water potential and relative water content of leaves at the wilting point. Nocturnal sap flow was higher in species with higher leaf nitrogen concentrations, higher rates of extension growth and lower shade tolerances. Diurnal sap flow was higher in species with higher leaf nitrogen concentrations and photosynthetic capacities on a leaf area basis. Because leaf metabolism and dark respiration, in particular, are strongly related to leaf nitrogen concentration, our findings suggest that nocturnal transpiration functions to sustain carbohydrate export and other processes driven by dark respiration, and that this function is most important in fast- growing shade-intolerant tree species.     

黄土丘陵区沙棘林几个水分生理生态特征研究

为探讨沙棘对半干旱黄土丘陵区水分条件的适应性,1998年对沙棘的水分生理生态及抗旱性进行了分析研究.试验结果表明(1)沙棘适应半干旱黄土丘陵区生境形成了耐旱、御旱的双重抗旱方式.在旱季,沙棘体内束缚水含量随干旱加剧而上升,束缚水与自由水比值随干旱加剧而增大,叶片维持较高的含水量,蒸腾减弱,叶水势降低,临界饱和亏增大,表现出较强的抗旱性.(2)沙棘光合速率具明显的日变化,1000 1200出现第1次峰值,1400 1500出现第2次峰值.沙棘光合速率与环境因子(气温、相对湿度和光合有效辐射)间有显著的相关关系,相关系数为0.8466 0.9351,其中光合有效辐射对光合速率影响最显著,气温次之,相对湿度最小;沙棘光合速率与气孔导度及细胞间CO2浓度间有十分显著的相关关系,相关系数为0.9852 0.9856.(3)沙棘蒸腾速率与气孔导度具明显的日变化,两者的变化趋势相似,5、7月日变化曲线呈单峰型,6、8、9月日变化曲线呈双峰型.在生长季(5-     

Leaf photosynthetic traits of 14 tropical rain forest species in relation to leaf nitrogen concentration and shade tolerance

Variability of leaf traits related to photosynthesis was assessed in seedlings from 14 tree species growing in the tropical rain forest of French Guiana. Leaf photosynthetic capacity (maximum rate of carboxylation and maximum rate of electron transport) was estimated by fitting a biochemical model of photosynthesis to response curves of net CO2 assimilation rate versus intercellular CO2 mole fraction. Leaf morphology described by leaf mass per unit leaf area (LMA), density and thickness, as well as area- and mass-based nitrogen (N) and carbon (C) concentrations, were recorded on the same leaves. Large interspecific variability was detected in photosynthetic capacity as well as in leaf structure and leaf N and C concentrations. No correlation was found between leaf thickness and density. The correlations between area- and mass-based leaf N concentration and photosynthetic capacity were poor. Conversely, the species differed greatly in relative N allocation to carboxylation and bioenergetics. Principal component analysis (PCA) revealed that, of the recorded traits, only the computed fraction of total leaf N invested in photosynthesis was tightly correlated to photosynthetic capacity. We also used PCA to test to what extent species with similar shade tolerances displayed converging leaf traits related to photosynthesis. No clear-cut ranking could be detected among the shade-tolerant groups, as confirmed by a one-way ANOVA. We conclude that the large interspecific diversity in photosynthetic capacity was mostly explained by differences in the relative allocation of N to photosynthesis and not by leaf N concentration, and that leaf traits related to photosynthetic capacity did not discriminate shade-tolerance ranking of these tropical tree species.     

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.     

Shrub species affect distinctively the functioning of scattered Quercus ilex trees in Mediterranean open woodlands

A low tree stand density has been showed as necessary to thrive with summer drought in semiarid Mediterranean open woodlands. Shrub encroachment of these open woodlands is currently recommended to guarantee the persistence of the system, due to the nursery effect of shrubs on tree seedling. However, the increase in abundance and cover of a shrub understory in these water limited woodlands could bring consequences to tree overstory functioning. The present study analyzes the physiological status of scattered Quercus ilex L. trees in paired adjacent plots with and without the presence of a shrubby understory in CW Spain. Two contrasting shrub strategies were addressed in order to take into account possible species-specific effects: a dense-shallow rooting shrub (Cistus landanifer L.) and a sparse-deep rooting shrub (Retama sphaerocarpa (L.) Boiss). Leaf water potential (at predawn and midday), leaf gas exchange parameters (net photosynthetic rate and stomatal conductance), leaf nitrogen content and chlorophyll fluorescence transients (maximum photochemical efficiency and performance index, sensuStrasser et al., 2004) were measured during three consecutive summers. Trees growing with Cistus as understory showed significant lower leaf water potential, leaf gas exchange parameters, leaf nitrogen content and chlorophyll photochemical efficiency than trees growing without shrub competence. However, the presence of the legume Retama did not affect significantly the physiological state of Q. ilex. Thus, we conclude that the presence of a shrubby understory has the potential to modify the functioning of scattered trees, but these effects are species-specific.     

Seasonal variability of photosynthetic characteristics influences growth of eight tropical tree species at two sites with contrasting precipitation in Panama

Relative to closed-canopy tropical forests, tree seedlings planted in open grown areas are exposed to higher light intensity, air temperatures, vapor pressure deficit, and greater seasonal fluxes of plant available water than mature tropical forests. The species-specific adaptive capacity to respond to variable precipitation and seasonality in open grown conditions, therefore, is likely to affect species performance in large-scale reforestation efforts. In the present study, we compared the photosynthetic characteristics of eight tropical tree species within and between seasons at two study sites with contrasting dry season intensities. All species except Pseudosamanea guachapele reduced leaf physiological function between the wet and dry seasons. The contrasting severity of seasonal drought stress at the study sites constrained growth rates and photosynthetic characteristics differently. Variation of photosynthetic characteristics at the species level was high, particularly in the dry season. Faster growing species at the less seasonal site, Terminalia amazonia, Inga punctata, Colubrina glandulosa, and Acacia mangium, exhibited a greater adaptive capacity than the other species to down-regulate leaf photosynthesis between seasons. As the dry season was more severe at the more seasonal site, most species strongly reduced physiological function regardless of relative growth rates, except two species (Tectona grandis and P. guachapele) with widespread distributions and relatively high drought tolerance. Our results underscore the need to consider seasonal drought tolerance when selecting tree species for specific reforestation sites.