The response of stomatal conductance to seasonal drought in tropical forests

Stomata regulate CO₂ uptake for photosynthesis and water loss through transpiration. The approaches used to represent stomatal conductance (g_s) in models vary. In particular, current understanding of drivers of the variation in a key parameter in those models, the slope parameter (i.e. a measure of intrinsic plant water‐use‐efficiency), is still limited, particularly in the tropics. Here we collected diurnal measurements of leaf gas exchange and leaf water potential (Ψ_leaf), and a suite of plant traits from the upper canopy of 15 tropical trees in two contrasting Panamanian forests throughout the dry season of the 2016 El Niño. The plant traits included wood density, leaf‐mass‐per‐area (LMA), leaf carboxylation capacity (V_c,max), leaf water content, the degree of isohydry, and predawn Ψ_leaf. We first investigated how the choice of four commonly used leaf‐level g_s models with and without the inclusion of Ψ_leaf as an additional predictor variable influence the ability to predict g_s, and then explored the abiotic (i.e. month, site‐month interaction) and biotic (i.e. tree‐species‐specific characteristics) drivers of slope parameter variation. Our results show that the inclusion of Ψ_leaf did not improve model performance and that the models that represent the response of g_s to vapor pressure deficit performed better than corresponding models that respond to relative humidity. Within each g_s model, we found large variation in the slope parameter, and this variation was attributable to the biotic driver, rather than abiotic drivers. We further investigated potential relationships between the slope parameter and the six available plant traits mentioned above, and found that only one trait, LMA, had a significant correlation with the slope parameter (R² = 0.66, n = 15), highlighting a potential path towards improved model parameterization. This study advances understanding of g_s dynamics over seasonal drought, and identifies a practical, trait‐based approach to improve modeling of carbon and water exchange in tropical forests.     

Optimal stomatal conductance in relation to photosynthesis in climatically contrasting Eucalyptus species under drought

Models of stomatal conductance (g_s) are based on coupling between g_s and CO₂ assimilation (A_net), and it is often assumed that the slope of this relationship (‘g₁’) is constant across species. However, if different plant species have adapted to different access costs of water, then there will be differences in g₁ among species. We hypothesized that g₁ should vary among species adapted to different climates, and tested the theory and its linkage to plant hydraulics using four Eucalyptus species from different climatic origins in a common garden. Optimal stomatal theory predicts that species from sub‐humid zones have a lower marginal water cost of C gain, hence lower g₁ than humid‐zone species. In agreement with the theory that g₁ is related to tissue carbon costs for water supply, we found a relationship between wood density and g₁ across Eucalyptus species of contrasting climatic origins. There were significant reductions in the parameter g₁ during drought in humid but not sub‐humid species, with the latter group maintaining g₁ in drought. There are strong differences in stomatal behaviour among related tree species in agreement with optimal stomatal theory, and these differences are consistent with the economics involved in water uptake and transport for carbon gain.     

13种栎属植物叶片的气孔特征及其相关性分析和分类学意义

以13种栎属(Quercus Linn.)植物〔包括产自辽宁省的槲栎组(Sect. Quercus)8种和麻栎组(Sect. Aegilops)2种以及引自北美洲的沼生栎组(Sect. Erythobalanus)2种和引自波兰的白栎组(Sect. Lepidabalanus)1种〕为试材,观察了这些种类叶片的气孔形态,并分析了气孔器和气孔的形态参数,还对各形态参数间的相关性进行了分析;在此基础上,对13种栎属植物进行了聚类分析。结果表明：栎属植物叶片的气孔仅分布于下表皮;气孔器排列属无规则型;气孔保卫细胞呈肾形,内壁加厚;气孔下陷,呈椭圆形或狭长椭圆形。供试种类的气孔和气孔器的形态参数差异明显,变异系数差异较大;气孔器的长轴长度、短轴长度和面积分别为18.24~27.46μm、14.63~23.18μm和221.56~501.70μm2,变异系数分别为7.73%~15.90%、7.10%~17.44%和14.13%~32.73%,气孔器指数为0.73~0.85;气孔的长轴长度、短轴长度、面积和密度分别为8.69~15.41μm、1.94~8.49μm、15.15~104.93μm2和462.32~984.44 mm-2,变异系数分别为12.03%~22.17%、13.65%~34.10%、27.95%~54.13%和8.10%~16.99%,气孔指数为0.22~0.57;总体上,按照多数气孔器和气孔参数从大至小的变化趋势依次排序为沼生栎组、白栎组、麻栎组、槲栎组。相关性分析结果表明：供试种类的气孔密度与气孔器和气孔的长轴及短轴长度均呈极显著(P<0.01)负相关;气孔器面积与气孔器指数和气孔指数分别呈极显著和显著(P<0.05)正相关;气孔面积与气孔器和气孔的长轴和短轴长度、气孔器指数和气孔指数呈极显著正相关,但与气孔密度呈显著负相关。聚类分析结果表明：在欧氏距离10处可将13种栎属植物分为3类,第Ⅰ类包含槲栎组的8种,第Ⅱ类包含麻栎组和白栎组的3种,第Ⅲ类包含沼生栎组的2种。研究结果显示：栎属植物叶片的气孔特征具有一定的稳定性,可作为栎属植物组间分类及亲缘关系分析的依据之一。     

Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring

Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO₂ concentrations in the proximity of a natural CO₂ spring. Trees at the spring site have been growing for generations in elevated atmospheric CO₂ concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid ‘runaway embolism’. Trees at the CO₂ spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO₂ was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO₂ spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO₂ spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO₂ spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO₂ on plant water use at the organ and whole-tree scale.     

Correlations of stomatal conductance with hydraulic and chemical factors in several deciduous tree species in a natural habitat

Recent research in whole-plant stomatal physiology, conducted largely with potted plants in controlled environments, suggests that stomatal conductance ( g _s) might be more closely linked to plant chemical variables than to hydraulic variables. To test this in a field situation, seasonal g _s was examined in relation to a number of plant and environmental variables in 11 temperate, deciduous forest tree species. Stomatal conductance was generally better correlated with environmental variables (air temperature, vapor pressure deficit, PPFD) than with plant variables, and slightly better correlated with plant hydraulic variables (shoot water and osmotic potentials) than with plant chemical variables (xylem sap ABA concentration, xylem sap pH). We examined a model, developed previously for maize, which describes regulation of g _s by xylem sap ABA concentration with leaf water status acting to modify stomatal sensitivity to the ABA signal. This model explained slightly more variation in seasonal g _s in the forest trees than did single plant variables but not more variation than most single environmental variables. Response surface models, especially those incorporating environmental variables, were more consistently successful at explaining g _s across species.     

Day length and evening temperature predict circannual variation in activity duration of the colony of the Indian cliff swallow,Hirundo fluvicola

In avian species, circannual rhythms, in a number of biological variables, including locomotor activity, have been studied under both laboratory and natural environmental conditions. However, information on circannual rhythm in daily activity duration of Indian cliff swallow (Hirundo fluvicola) is not available. It is a communal mud nesting, non-migratory species and inhabits low under arch bridges. Although it figures in the IUCN Red List under the least concerned category, it is important to study its behavioral ecology that may be of utmost importance from conservation stand point. In the present investigation, we examined the circannual rhythm in daily activity duration in this species at a communal roosting site under Kharoon river bridge on the Raipur–Bhilai highway (21°15′07.54′′N; 81°32′30.65′′E), Raipur, India, for eight consecutive days, every month from August 2009 to October 2011. On each study day, observations were made in two sessions, morning and evening. We monitored and recorded emergence time of the first bird from the nest in the morning and entry time of the last bird to the nest in the evening. We also recorded the ambient temperature and humidity of the study site simultaneously. Average daily activity duration (the time difference between the first bird’s emergence time and the last bird’s entry time from/to the nest, respectively) was computed for each month. A significant circannual rhythm in activity duration was validated. The peak of activity rhythm occurred on 22nd June with a spread between 19th June and 25th June. The activity duration was the shortest in winter and the longest in summer. The daily activity duration was positively correlated to the day length, sunset time, and morning and evening temperatures; whereas negatively correlated with the sunrise time, and morning and evening humidity. The multiple linear regression models suggest that day length alone explains 96% of the total variance in activity duration, whereas only 0.5% variance was attributable to evening temperature. We conclude that the day length is the strongest predictor of circannual variation in daily activity duration of the colony of Indian cliff swallow; whereas the evening temperature has very little effect. Further, we recommend that comparable studies under natural conditions might be very helpful to explore the effect of environmental cues on other intriguing behavioral decisions made by this and other avian species.     

Herbarium specimens show patterns of fruiting phenology in native and invasive plant species across New England

Premise of the Study

Patterns of fruiting phenology in temperate ecosystems are poorly understood, despite the ecological importance of fruiting for animal nutrition and seed dispersal. Herbarium specimens represent an under‐utilized resource for investigating geographical and climatic factors affecting fruiting times within species, patterns in fruiting times among species, and differences between native and non‐native invasive species.

Methods

We examined over 15,000 herbarium specimens, collected and housed across New England, and found 3159 specimens with ripe fruits, collected from 1849–2013. We examined patterns in fruiting phenology among 37 native and 18 invasive woody plant species common to New England. We compared fruiting dates between native and invasive species, and analyzed how fruiting phenology varies with temperature, space, and time.

Key Results

Spring temperature and year explained a small but significant amount of the variation in fruiting dates. Accounting for the moderate phylogenetic signal in fruiting phenology, invasive species fruited 26 days later on average than native species, with significantly greater standard deviations.

Conclusions

Herbarium specimens can be used to detect patterns in fruiting times among species. However, the amount of intraspecific variation in fruiting times explained by temporal, geographic, and climatic predictors is small, due to a combination of low temporal resolution of fruiting specimens and the protracted nature of fruiting. Later fruiting times in invasive species, combined with delays in autumn bird migrations in New England, may increase the likelihood that migratory birds will consume and disperse invasive seeds in New England later into the year.     

Distribution profile of stomatal conductance and its interrelations to transpiration rate and water dynamics in young maize laminas under sulfate deprivation

Seven-day-old maize (Zea mays) plants were grown hydroponically for 10 days in S-deprived nutrient solution. The distribution profiles according to the position on the stem of the S-deprived laminas’ stomatal conductance, transpiration rate, photosynthetic rate, dry mass, water content, and specific surface area were monitored relative to control among others. Photochemical efficiency of photosystem II remained unaffected by the deprivation, as well as the specific surface area of all but the embryonic laminas after d2. In S-deficient plants, the embryonic (L0) and the uppermost lamina or the one below it presented mostly significant changes. The response ratios (Rr) of the L0 stomatal conductance oscillated; the oscillation started with an increase at d2. The corresponding Rr values of L0 transpiration and photosynthetic rates started oscillating at d4 in the same fashion. At d8, an increasing gradient appeared in water-content Rr values from L1 to the uppermost lamina. At d10, all but the embryonic laminas presented significantly reduced Rr values in water content. Changes in dry mass and surface area of laminas were synchronized. In control, the transpiration rate expressed per DM unit remained constant during the examined period, while under the deprivation it followed a power function of surface area.     

Genetic variation in circadian regulation of nocturnal stomatal conductance enhances carbon assimilation and growth

Circadian resonance, whereby a plant's endogenous rhythms are tuned to match environmental cues, has been repeatedly shown to be adaptive, although the underlying mechanisms remain elusive. Concomitantly, the adaptive value of nocturnal transpiration in C₃ plants remains unknown because it occurs without carbon assimilation. These seemingly unrelated processes are interconnected because circadian regulation drives temporal patterns in nocturnal stomatal conductance, with maximum values occurring immediately before dawn for many species. We grew individuals of six Eucalyptus camaldulensis genotypes in naturally lit glasshouses and measured sunset, predawn and midday leaf gas exchange and whole‐plant biomass production. We tested whether sunrise anticipation by the circadian clock and subsequent increases in genotype predawn stomatal conductance led to rapid stomatal opening upon illumination, ultimately affecting genotype differences in carbon assimilation and growth. We observed faster stomatal responses to light inputs at sunrise in genotypes with higher predawn stomatal conductance. Moreover, early morning and midday stomatal conductance and carbon assimilation, leaf area and total plant biomass were all positively correlated with predawn stomatal conductance across genotypes. Our results lead to the novel hypothesis that genotypic variation in the circadian‐regulated capacity to anticipate sunrise could be an important factor underlying intraspecific variation in tree growth.     

Variability of photosynthetic gas exchange parameters, dark respiration, and stomatal numbers in species of Polygonum

Within the genus Polygonum a large variation was found between species with regard to stomatal number, gas phase resistance, intracellular resistance and dark respiration. Interspecific variation in CO₂ compensation concentration and intercellular CO₂ concentration at constant external concentration were comparatively small. Correlations were found between stomatal number and gas phase resistance, stomatal number and Γ, and Γ and the product of dark respiration rate and intracellular resistance. The influence of dark respiration and stomatal number on photosynthetic gas exchange is discussed. It was concluded that dark respiration in light was enhanced by 22% as a mean value in 9 Polygonum species and by 62% in Polygonum lapathifolium .     

陕北黄土高原辽东栎林分布区北部边界形成的光合作用机制

为阐明辽东栎林分布区边界形成的原因,沿秦岭到陕北黄土高原的水热梯度,选取秦岭北坡(鸡窝子)和陕北黄土高原森林区南部(陈家山)、中部(槐树庄)及北部(下寺湾)4个辽东栎林样地,测定了辽东栎叶片的气体交换与叶绿素荧光参数、气孔密度和气孔大小.结果表明: 随着水热梯度由南到北呈现出由凉湿到偏温干的变化,辽东栎叶片最大净光合速率(P_{n max})和光饱和点(L_sp)分别由13.34 μmol CO₂·m^-2·s^-1、1489.9 μmol·m^-2·s^-1显著减少到6.99 μmol CO₂·m^-2·s^-1、1055.6 μmol·m^-2·s^-1,暗呼吸速率(R_d)、光补偿点(L_cp)分别由0.313 μmol CO₂·m^-2·s^-1、7.49 μmol·m^-2·s^-1显著增加到1.080 μmol CO₂·m^-2·s^-1、31.96 μmol·m^-2·s^-1;初始荧光(F_o)与非光化学猝灭系数(NPQ)呈增加趋势,而PSⅡ实际光化学量子产量(Φ_PSⅡ)、表观电子传递速率(ETR)、光化学猝灭系数(q_P)等均显著降低.辽东栎的光合能力沿此梯度逐渐减弱,除黄土高原的陈家山外,其他3个样地辽东栎叶片的气孔密度和气孔大小无显著差异.陕北森林区北部辽东栎叶片的光化学活性和电子传递速率等降低,导致光合速率下降可能是其分布区边界形成的重要影响因素之一.     

Stomatal control by fed or endogenous xylem ABA in sunflower: interpretation of correlations between leaf water potential and stomatal conductance in anisohydric species

The stomatal conductance of several anisohydric plant species, including field-grown sunflower, frequently correlates with leaf water potential (φ₁), suggesting that chemical messages travelling from roots to shoots may not play an important role in stomatal control. We have performed a series of experiments in which evaporative demand, soil water status and ABA origin (endogenous or artificial) were varied in order to analyse stomatal control. Sunflower plants were subjected to a range of soil water potentials under contrasting air vapour pressure deficits (VPD, from 0.5 to 2.5 kPa) in the field, in the glasshouse or in a humid chamber. Sunflower plants were also fed through the xylem with varying concentrations of artificial ABA, in the glasshouse and in the field. Finally, detached leaves were fed directly with varying concentrations of ABA under three contrasting VPDs. A unique relationship between stomatal conductance (g_s) and the concentration of ABA in the xylem sap (xylem [ABA]) was observed in all cases. In contrast, the relationship between φ₁ and g_s varied substantially among experiments. Its slope was positive for droughted plants and negative for ABA-fed whole plants or detached leaves, and also varied appreciably with air VPD. All observed relationships could be modelled on the basis of the assumption that φ₁ had no controlling effect on g_s. We conclude that stomatal control depended only on the concentration of ABA in the xylem sap, and that φ₁ was controlled by water flux through the plant (itself controlled by stomatal conductance). The possibility is also raised that differences in stomatal ‘strategy’ between isohydric plants (such as maize, where daytime φ₁ does not vary appreciably with soil water status) and anisohydric plants (such as sunflower) may be accounted for by the degree of influence of φ₁ on stomatal control, for a given level of xylem [ABA]. We propose that statistical relationships between φ₁ and g_s are only observed when φ₁ has no controlling action on stomatal behaviour.     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

The nitrogen-sink is involved in the regulation of nitrogenase activity in white clover after defoliation

The objective of the present study was to develop an empirical cold hardiness model applicable to several taxa of deciduous trees. Cold hardiness expressed as lowest survival temperature of Acer rubrum, Betula nigra, Liquidambar styracifiua, Fraxinus pennsylvanica, Prunus serotina and Quercus alba was evaluated at approximately weekly intervals during the winters of three consecutive years. Plant samples and meteorological data were collected from Georgia Experiment Station, Griffin, Georgia. Maximum, minimum and average temperatures, hourly chill and heat accumulation. day length and time of year were used as input variables for model development. The statistical method of stepwise procedure of regression analysis was employed to select variables for the model. Based on the assumption that model components should be the same for all taxa included in this study and all three winters, the following independent model variables were selected as valid inputs: day length, number of accumulated hours with temperature above 20°C and number of accumulated hours with temperature below 10°C. Equation coefficients of species-specific models were determined for each species. Cold hardiness predictions were compared to actual observations for each species. The model components were interpreted as representing two processes: (1) internally regulated and independent of actual temperature, and (2) externally regulated and dependent on the amount of accumulated chill or heat. The model allowed for comparisons of cold hardening and dehardening between the studied taxa and between years.     

The boundary layer and the apparent responses of stomatal conductance to wind speed and to the mole fractions of CO2 and water vapour in the air

The experiments and simulations reported in this paper show that, for stomata sensitive to both CO₂ and water vapour concentrations, responses of stomatal conductance (g^w_s) to boundary layer thickness have two components, one resulting from changes in intercellular CO₂ concentration (χ^c_i) and another from changes in leaf surface water vapour saturation deficit (D^w_s). The experiments and simulations also show that the boundary layer conductance (g^w_b) can significantly alter the apparent response of g^w_s to ambient air CO₂ mole fraction (χ^c_a) and water vapour mole fraction (χ^w_a). Because of the feedback loop involved the responses of g^w_s for χ^c_a and χ^w_a each include responses to both χ^c_i and D^w_s. The boundary layer alters the state of the variables sensed by the guard cells—i.e. χ^c_i and D^w_s—and so it is a source of feedback. Thus, when scaling up from responses of stomata to the response of g^w_s for a whole leaf, the effect of the boundary layer must be considered. The results indicate that, for given responses of g^w_s to χ^c_i and D^w_s, the apparent responses of g^w_s to D^w_a and χ^c_a depend on the size of the leaf and wind speed, showing that this effect of the boundary layer should be considered when comparing data measured under different conditions, or with different methods.     

Regulation of Photosynthesis of C3 Plants in Response to Progressive Drought: Stomatal Conductance as a Reference Parameter

We review the photosynthetic responses to drought in field-growngrapevines and other species. As in other plant species, therelationship between photosynthesis and leaf water potentialand/or relative water content in field-grown grapevines dependson conditions during plant growth and measurements. However,when light-saturated stomatal conductance was used as the referenceparameter to reflect drought intensity, a common response patternwas observed that was much less dependent on the species andconditions. Many photosynthetic parameters (e.g. electron transportrate, carboxylation efficiency, intrinsic water-use efficiency,respiration rate in the light, etc.) were also more stronglycorrelated with stomatal conductance than with water statusitself. Moreover, steady-state chlorophyll fluorescence alsoshowed a high dependency on stomatal conductance. This is discussedin terms of an integrated down-regulation of the whole photosyntheticprocess by CO₂ availability in the mesophyll. A study with sixMediterranean shrubs revealed that, in spite of some markedinterspecific differences, all followed the same pattern ofdependence of photosynthetic processes on stomatal conductance,and this pattern was quite similar to that of grapevines. Furtheranalysis of the available literature suggests that the above-mentionedpattern is general for C₃ plants. Even though the patterns describeddo not necessarily imply a cause and effect relationship, theycan help our understanding of the apparent contradictions concerningstomatal vs. non-stomatal limitations to photosynthesis underdrought. The significance of these findings for the improvementof water-use efficiency of crops is discussed.