Leaf-air Vapour Pressure Deficit Research Articles

CORRELATION BETWEEN CO2 ASSIMILATION RATES AND OTHER GAS EXCHANGE PARAMETERS OF Hevea WITH REFERENCE TO COMMENCEMENT OF TAPPING

The present study was supplement to a longer experiment to determine the feasibility of early commencement of tapping of two Hevea genotypes i.e. RRIC121 and RRISL 211. The main objective of this study was to develop a correlations between CO2 assimilation rates and other leaf gas exchange parameters of Hevea with reference to the commencement of tapping. Tapped and untapped trees of two Hevea genotypes, i.e. RRIC121 and RRISL 211 at the same age were selected for the study. All cultural practices were performed according to the Rubber Research Institute of Sri Lanka. To evaluate the significance of differences between all possible pairs of treatment means, mean separation of treatments was performed with Duncan’s Multiple Range Test (DMRT). The strength of the relationships between yield and photosynthetic gas exchange parameters was estimated by linear correlation analysis. The leaf gas exchange parameters were measured under optimal environmental conditions clearly showed the intercellular CO2 concentration (Ci) gradually decrease with increasing light intensity in both clones. The same trend was observed in the different canopy layers under both tapping treatments. The highest leaf-air vapour pressure deficit (lvpd) was shown in the leaves of the upper canopy and lowest in the bottom canopy layer. Furthermore, the water use efficiency (WUE) of clone RRISL 211 was higher than that of clone RRIC 121 under tapping. This was primarily because of the greater photosynthetic rates of RRISL 211, rather than lower transpiration rates.When considering the correlation analysis of gas exchange parameters of Hevea, CO2 assimilation rate (A) had highly significant correlations with gs, Tr and intensity of PAR. However, the absence of a correlation between PAR and WUE showed that, when PAR increases both A and Tr increase in similar magnitudes. Therefore, WUE remains approximately constant. The presence or absence of correlations in gas exchange parameters of tapped and untapped treatments were broadly similar to the overall correlation analysis. In the tapped treatments, because of tapping, the photosynthetic rate is stimulated relative to the transpiration rate. The positive latex yield response to WUE and lvpd of top leaves indicates that, in top leaves when lvpd increases transpiration also increases accordingly. Hence WUE and ivpd increase latex flow within the tree and finally increase the latex yield with increasing latex volume. On the other hand, latex yield has a negative correlation with gs. The study findings clearly revealed that, in top leaves reduced Ci increases the CO2 gradient and uptake of CO2 for more photosynthesis, which increases the latex yield. Faster utilization of absorbed Ci may enhance the yield potential.

Aerosol pollution alters the diurnal dynamics of sun and shade leaf photosynthesis through different mechanisms

Anthropogenic aerosols have been shown to perturb CO2 exchange between the vegetation and the atmosphere. However, the climate effects of aerosols through carbon cycle feedback still have significant uncertainties. Taking advantage of the periodic fluctuations of aerosol loading in Beijing, we intensively measured the diurnal course of leaf microclimates and photosynthesis under different aerosol conditions during the growing season in 2014 and 2015. We found that increasing aerosol loadings altered the diurnal course of microclimates and thus sun and shade leaf photosynthesis. Our mechanistic photosynthesis model experiments further showed that aerosol-induced increase in sun leaf photosynthesis occurred around noon and afternoon, mainly by alleviating the depression of photosynthesis caused by high leaf temperature and leaf-air vapour pressure deficit. Meanwhile, aerosols enhanced shade leaf photosynthesis throughout the day by mitigating the light limitation within the canopy, with the highest increase occurring around noon. Overall, our study suggested that aerosol's diffuse fertilization effect, cooling effect and the accompanying low leaf-air vapour pressure deficit collectively drove the changes in the diurnal courses of sun and shade leaf photosynthesis. Our results provided an important benchmark for assessing how anthropogenic aerosols regulate ecosystem C balance under different meteorological conditions.

Effect of highly processed calcined kaolin residues on apple water use efficiency

Processed calcined kaolin particle films (PKPF) have been shown to repel some insects and reduce various environmental stresses often resulting in improved yield and quality of horticultural crops. PKPF studies in the literature have various water use efficiency (WUE) responses. The purpose of this study was to evaluate 2 application rates of PKPF (3% and 12%) on the whole tree water use efficiency and compare gas exchange values with carbon isotopic discrimination and seasonal water use efficiency in order to determine if there are consistent effects on WUE in apple and infer the mechanisms of action. Fruit yield and total biomass influence WUE whether WUE is measured directly in whole plant chambers, estimated by biomass/unit evapotranspiration, or Δ13C. Δ13C analysis of WUE generally found that the untreated control had lower values and therefore higher WUE than PKPF treatments. Whole plant gas exchange analysis of WUE generally lacked the sensitivity to identify the treatment differences observed using Δ13C analysis and those treatment differences identified were contrary to Δ13C analysis. A lysimeter study supported the reduced WUE of PKPF treated plants by demonstrating increased E with a range of PKPF residue amounts. The lysimeter results suggest that for well-watered and presumably non-stressed conditions, gas exchange is limited by leaf temperature and the concomitant leaf-air vapor pressure deficit that influence stomatal conductance. When leaf temperature is reduced by PKPF, the response results in increased stomatal conductance and gas exchange, as measured by increased E. The reduction in WUE by PKPF treatment is balanced by an increase in overall gas exchange and increased yield and quality.

Field-acclimated Gossypium hirsutum cultivars exhibit genotypic and seasonal differences in photosystem II thermostability

Previous investigations have demonstrated that photosystem II (PSII) thermostability acclimates to prior exposure to heat and drought, but contrasting results have been reported for cotton (Gossypium hirsutum). We hypothesized that PSII thermotolerance in G. hirsutum would acclimate to environmental conditions during the growing season and that there would be differences in PSII thermotolerance between commercially-available U.S. cultivars. To this end, three cotton cultivars were grown under dryland conditions in Tifton Georgia, and two under irrigated conditions in Marianna Arkansas. At Tifton, measurements included PSII thermotolerance (T15, the temperature causing a 15% decline in maximum quantum yield), leaf temperatures, air temperatures, midday (1200 to 1400h) leaf water potentials (ΨMD), leaf-air vapor pressure deficit (VPD), actual quantum yield (ΦPSII) and electron transport rate through PSII (ETR) on three sample dates. At Marianna, T15 was measured on two sample dates. Optimal air and leaf temperatures were observed on all sample dates in Tifton, but PSII thermotolerance increased with water deficit conditions (ΨMD=−3.1MPa), and ETR was either unaffected or increased under water-stress. Additionally, T15 for PHY 499 was ∼5°C higher than for the other cultivars examined (DP 0912 and DP 1050). The Marianna site experienced more extreme high temperature conditions (20–30 days Tmax≥35°C), and showed an increase in T15 with higher average Tmax. When average T15 values for each location and sample date were plotted versus average daily Tmax, strong, positive relationships (r2 from .954 to .714) were observed between Tmax and T15. For all locations T15 was substantially higher than actual field temperature conditions. We conclude that PSII thermostability in G. hirsutum acclimates to pre-existing environmental conditions; PSII is extremely tolerant to high temperature and water-deficit stress; and differences in PSII thermotolerance exist between commercially-available cultivars.

Generalizing plant-water relations to landscapes

Changing climate and land use patterns make it increasingly important that the hydrology of catchments and ecosystems can be reliably characterized. The aim of this paper is to identify the biophysical factors that determine the rates of water vapor loss from different types of vegetation, and to seek, from an array of currently available satellite-borne sensors, those that might be used to initialize and drive landscape-level hydrologic models. Spatial variation in the mean heights, crowd widths, and leaf area indices (LAI) of plant communities are important structural variables that affect the hydrology of landscapes. Canopy stomatal conductance (G) imposes physiological limitation on transpiration by vegetation. The maximum value of G (Gmax) is closely linked to canopy photosynthetic capacity, which can be estimated via remote sensing of foliar chlorophyll or nitrogen contents. G can be modeled as a nonlinear multipliable function of: (i) leaf–air vapor pressure deficit, (ii) water potential gradient between soil and leaves, (iii) photosynthetically active radiation absorbed by the canopy, (iv) plant nutrition, (v) temperature and (vi) the CO2 concentration of the air. Periodic surveys with Light Detection and Ranging (LiDAR) and interferometric RADAR, along with high-resolution spectral coverage in the visible, near-infrared, and thermal infrared bands, provide, along with meteorological data gathered from weather satellites, the kind of information required to model seasonal and interannual variation in transpiration and evaporation from landscapes with diverse and dynamic vegetation.

Morphological analysis and photosynthetic performance of improved papaya genotypes

Brazil is the largest world producer of papaya and the third largest exporter despite of only 1.5 to 2.0% of its production is exported. Such an underexplored exportation potential highlight the necessity for physiological studies on new cultivars and hybrids to verify their agronomic and commercial viability. Two Brazilian states, Bahia and Espirito Santo, are responsible for 80% of national production. Papaya can also be an agricultural alternative to north / northeast of Rio de Janeiro, because the region is close to consumer centers and have similar environmental conditions of the most productive regions. Nevertheless, it is worth to develop cultivars that can express the highest yield potential in this region. The aim of this work was to characterize physiologically two hybrids developed for the north/northeast of Rio de Janeiro (UENF/Caliman 01 and JS12) in comparison with three top commercial genotypes (Golden, Sunrise Solo 7212 and Tainung) of Caricapapaya L. The cv. Golden presented the lowest shoot and root growth, the lowest height, shrunk diameter, specific leaf weight, less efficiency in electrons transport per sample area and show the lowest ability to synthesized total chlorophylls in comparison with the others genotypes. At noon, this genotype showed higher stomatal conductance, related to the leaf-air vapor pressure deficit, which lead to higher transpiration rate and intrinsic water use efficiency. No differences were detected in the photosynthetic rates among the five genotypes suggesting that the UENF`s hybrids are endowed with similar photosynthetic capacity and morphological characteristics to the top commercial genotypes. The relevance of this characterization to drive future successful genetic improvement programs will be discussed.

Response of Photosynthesis and Water Relations of Rice (Oryza sativa) to Elevated Atmospheric Carbon Dioxide in the Subhumid Zone of Sri Lanka

AbstractThe objective of the present paper is to determine the response of the physiological parameters related to biomass production and plant water relations in a standard Sri Lankan rice (Oryza sativa) variety (BG‐300) to elevated CO2 (i.e. 570 µmol/mol). During two seasons, rice crops were grown under three different experimental treatments; namely, at 570 µmol/mol (i.e. ‘elevated’) and 370 µmol/mol (‘ambient’) CO2 within open top chambers, and at ambient CO2 under open field conditions. Leaf net photosynthetic rate in the elevated treatment increased by 22–75 % in comparison to the ambient. However, the ratio between intercellular and ambient CO2 concentrations remained constant across different CO2 treatments and seasons. CO2 enrichment decreased individual leaf stomatal conductance and transpiration rate per unit leaf area, and increased both leaf and canopy temperatures. However, the overall canopy stomatal conductance and daily total canopy transpiration rate of the elevated treatment were approximately the same as those achieved under ambient conditions. This was because of the significantly greater leaf area index and greater leaf–air vapour pressure deficit under CO2 enrichment. The leaf chlorophyll content increased significantly under elevated CO2; however, the efficiency (i.e. photochemical yield) of light energy capture by Photosystem II (i.e. Fv/Fm) in chlorophyll a did not show a significant and consistent variation with CO2 enrichment.

Survival on the Exposed Limestone Pavement in the Burren: Photosynthesis and Water Relations of Three Co–Occurring Plant Species

Examination of the abundance of plant species occurring in limestone terrain at Mullagh More, in the Burren, Co. Clare, indicates that 'shade' plants are not restricted to microenvironmental conditions with reduced light levels (grikes), but are also common in exposed situations. Investigations of three species found in exposed locations (Teucrium scorodonia L., Mycelis muralis L. Dumort. and Corrylus avellana L.) indicate that they all have a limited capacity to acclimate their maximum photosynthetic rate (Pm) to the prevailing light climate and that as much as 70% of the incident irradiance is not utilised in photosynthesis. Diurnal reductions in stomatal conductance (Gs) and maximum photosynthetic rate (P,,), were observed with all three species. These changes in Pm and Gs were associated with concomitant decreases in shoot water potential (TP) and increases in leaf-air vapour pressure deficit (VPD), suggesting that water availability could be an important factor even under the prevailing mild climatic conditions. This was supported by parallel species-specific declines in the ratio of variable (Fv) to maximum (Fm) fluorescence (F,/F,Qj for a larger number of species found in these limestone habitats. Shoot-water status was also closely coupled to short-term wetting and drying episodes caused by periodic rainfall events. On the basis of this evidence, water limitation is likely to be a common and overriding ecological determinant in this limestone environment, despite the mild climatic conditions. Also, under these environmental conditions a limited acclimatory capacity does not preclude an ability to persist in exposed situations. Bruce Osborne (corresponding author; email: Bruce.Osborne@ucd.ie.),

Temperature-Dependent Gas Exchange and Stomatal/Non-Stomatal Limitation to CO<sub>2</sub> Assimilation of Quercus Liaotungensis under Midday High Irradiance

The effects of varying leaf temperature (T 1) on some ecophysiological characteristics of photosynthesis for Quercus liaotungensis Koiz. under ambient radiation stress around midday on clear summer days were investigated using an IRGA equipped with a temperature-controlled cuvette. Net photosynthetic rate (P N) decreased as T 1 increased from 30 to 35 °C as a result of stomatal closure, whereas non-stomatal limitation led to decreased P N in the T 1 range of 35–45 °C. Decreased transpiration rate (E) and stomatal conductance (g s) at leaf temperatures above 30 °C were interpreted as a combined ‘feedward’ effect as a result of enhanced leaf-air vapour pressure deficit (VPD) and stomatal closure. Changes in E from T 1 30 to 20 °C depended on VPD when g s was maintained constant. Water use efficiency (WUE) varied inversely with T 1 by following a hyperbola. A decrease in intercellular CO2 concentration (C i) occurred as a result of stomatal closure and a relatively high carboxylation capacity, whereas inactivation of mesophyll carboxylation in combination with photorespiration might be associated with the observed increase in C i in the T 1 range of 40 to 45 °C.

Vegetative propagation of Cordia alliodora (Ruiz & Pavon) Oken: the effects of IBA concentration, propagation medium and cutting origin

The effects of different concentrations of IBA, rooting media and cutting origins on the rooting of leafy stem cuttings of Cordia alliodora (Ruiz & Pavon) Oken were investigated in three experiments using non-mist propagators. During the rooting period, changes in dry mass, photosynthetic rate, stomatal conductance and relative water content (RWC) of the cuttings were assessed. In Experiment 1, cuttings treated with 1.6% IBA achieved 70% rooting after 9 weeks. Bud growth in cuttings was inhibited by increasing concentrations of IBA, which were associated with higher final rooting percentages. No significant treatment differences in RWC or dry mass were found between IBA concentrations, although the dry mass of cuttings in all treatments tended to increase during the rooting period. In Experiment 2, higher rooting percentages were recorded in gravel (89%) and sand (88%) than in sawdust (76%). Although the water content of sawdust was relatively high, cuttings in this treatment displayed significantly lower stomatal conductances ( g s), which may be attributed to death of stem tissue resulting from anoxia. In Experiment 3, highly significant differences ( P < 0.01) were recorded between node positions with respect to rooting percentage, values ranging from 21.3% for Node 2 to 46.3% for Node 5 (basal). Cuttings actively photosynthesized during the rooting period, with photosynthetic rates ( P n) of 0.68–6.70 μmol CO 2 m −2 s −1 recorded. However, no clear relationship between P n and rooting ability was apparent. Contrary to previous studies with this species, it was concuded that C. alliodora cuttings may be rooted relatively easily using the propagation system described. However, the close relationships recorded between P n, irradiance and leaf-air vapour pressure deficit (VPD) suggest that propagator microclimate should be carefully managed if optimal rooting percentages are to be achieved.

Discrimination Processes and Shifts in Carboxylation during the Phases of Crassulacean Acid Metabolism.

The magnitude and extent of Crassulacean acid metabolism (CAM) activity in two Clusia species was manipulated to investigate the regulation of the distinct CAM phases. First, in response to leaf-air vapor pressure deficit at night, changes in leaf conductance altered on-line carbon-isotope discrimination throughout the theoretical range for dark CO2 uptake during CAM. These ranged from the limit set by phosphoenolpyruvate carboxylase (PEPc) (-6[per mille (thousand) sign], [delta]13C equivalent of -2[per mille (thousand) sign]) to that imposed by diffusion limitation (+4[per mille (thousand) sign], [delta]13C equivalent of -12[per mille (thousand) sign]), but the lowest carbon-isotope discrimination occurred when P[square root]pa was only 0.7. Second, when the availability of external or internal sources of CO2 was reduced for both field- and greenhouse-grown plants, CO2 uptake by day via PEPc during phase II largely compensated. Third, by reducing the dark period, plants accumulated low levels of acidity, and CO2 uptake occurred throughout the subsequent light period. Discrimination switched from being dominated by PEPc (phase II) to ribulose 1,5-bisphosphate carboxylase/oxygenase (phase III), with both enzymes active during phase IV. Under natural conditions, photochemical stability is maintained by extended PEPc activity in phase II, which enhances acid accumulation and delays decarboxylation until temperature and light stress are maximal at midday.

Gas Exchange of Field-Grown Pinus radiata - Relationships With Foliar Nutrition and Water Potential, and With Climatic Variables

Gas exchange measurements were conducted on Pinus radiata to investigate relationships between these and leaf-air vapour pressure deficit, photosynthetic photon flux density, and foliar temperature, water potential and nutrition in the field. Multiple non-linear regressions indicated strong relationships between gas exchange and foliar [P] (but of no other nutrient), leaf-air vapour pressure deficit, photosynthetic photon flux density, foliar water potential and temperature. The final regression produced for relationships between gas exchange and these variables explained 81% of the variance in the data. Micro-climate and foliar data from another site were used to predict gas exchange using the regressions and calculated parameters. Good agreement was obtained between the predicted values and carbon assimilation measured at that site. The relationship was poorer for leaf conductance.

Gas exchange characteristics of mangosteen (Garcinia mangostana L.) leaves.

Gas exchange responses of mangosteen (Garcinia mangostana L.) leaves to photosynthetic photon flux density (PPFD), internal CO(2) concentration (C(i)), leaf-air vapor pressure deficit (VPD), leaf temperature (T(l)) and time of day were investigated in plants grown in three shade treatments. Maximal photosynthetic rate (P(n(max))) per unit leaf area at light saturation did not differ significantly among plants grown in the different shade treatments despite significant morphological differences. Light compensation point (9-15 micro mol m(-2) s(-1)) and quantum yield (0.022-0.023) did not differ significantly among treatments, whereas light saturation point was significantly higher for leaves grown in 20% shade than for leaves grown in 50 or 80% shade (951, 645 and 555 micro mol m(-2) s(-1), respectively). Shade treatments significantly affected assimilation responses to varying CO(2) concentrations. At CO(2) concentrations between 600 and 1000 micro mol mol(-1), leaves from the 20% shade treatment recorded higher P(n) (6.44 micro mol m(-2) s(-1)) than leaves from the 80% shade treatment (4.57 micro mol m(-2) s(-1)). Stomatal conductance (g(s)) decreased with increasing CO(2) concentrations. Vapor pressure deficits higher than 2.5 kPa significantly decreased P(n) and g(s), whereas P(n) remained steady over a 24-33 degrees C temperature range in leaves in 80% shade and over a 27-36 degrees C range in leaves in 20 and 50% shade. Highest carbon gain during a 12-hour photoperiod was observed for leaves grown in 50% shade. The results imply that mangosteen is a shade-tolerant, lower canopy tree adapted to humid tropical lowlands. Providing low VPD in the nursery may result in maximal growth. However leaves of seedlings grown in 50 or 20% shade tolerated high VPD and temperatures better than leaves of seedlings grown in 80% shade, indicating some adaptability to the seasonally wet and dry tropics of northern Australia.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : I. VPD effects on the transient stomatal response to lightflecks.

The effects of leaf-air vapor pressure deficit (VPD) on the transient and steady-state stomatal responses to photon flux density (PFD) were evaluated in Piper auritum, a pioneer tree, and Piper aequale, a shade tolerant shrub, that are both native to tropical forests at Los Tuxtlas, Veracruz, México. Under constant high-PFD conditions, the stomata of shade-acclimated plants of both species were sensitive to VPD, exhibiting a nearly uniform decrease in gs as VPD increased. Acclimation of P. auritum to high light increased the stomatal sensitivity to VPD that was sufflcient to cause a reduction in transpiration at high VPD's. At low PFD, where gs was already reduced, there was little additional absolute change with VPD for any species or growth condition. The stomatal response to 8-min duration lightflecks was strongly modulated by VPD and varied between the species and growth light conditions. In P. aequale shade plants, increased VPD had no effect on the extent of stomatal opening but caused the rate of closure after the lightfleck to be faster. Thus, the overall response to a lightfleck changed from hysteretic (faster opening than closure) to symmetric (similar opening and closing rates). Either high or low VPD caused gs not to return to the steady-state value present before the lightfleck. At high VPD the value after was considerably less than the value before whereas at low VPD the opposite occurred. Shade-acclimated plants of P. auritum showed only a small gs response to lightflecks, which was not affected by VPD. Under sunfleck regimes in the understory, the stomatal response of P. aequale at low VPD may function to enhance carbon gain by increasing the induction state. At high VPD, the shift in the response enhances water use efficiency but at the cost of reduced assimilation.

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species : II. Stomatal versus biochemical limitations during photosynthetic induction.

The relative importance of biochemical and stomatal limitations on assimilation (A) during photosynthetic induction were compared in sun and shade plants of Piper auritum, a pioneer tree, and shade plants of Piper aequale, a shade tolerant shrub native to a Mexican tropical rainforest. For non-induced leaves, increases in A during induction depended on the dynamics of stomatal conductance (gs) and ribulose-1,5-bisphosphate carboxylase (RuBisCO) activation. At high leaf-air vapor pressure deficit (VPD), more of the limitation during induction was stomatal. Calculations of mesophyll conductance revealed longer time constants for shade than for sun plants. However, no differences in the time course of RuBisCO activity between sun- and shade-plants were found. We conclude on the basis of the similar RuBisCO responses that differences in induction can be accounted for by the differences in stomatal behavior. Differences in the time course of mesophyll conductance may be due to an artifact caused by stomatal patchiness. Experiments on induction loss of previously induced leaves revealed that under these circumstances biochemical limitations can be important. A more rapid induction loss was evident in sun as compared to shade plants. The rapid loss of induction in sum plants was not due to the decreases in gs and RuBisCO activity, which both occurred slowly. Instead, a limitation, probably in RuBP regeneration capacity, appeared to develop during the low light periods. This limitation was much smaller or absent in shade plants.

Midday depression of net CO2 exchange of leaves of an emergent rain forest tree in French Guiana

Photosynthesis of tropical rain forest plants has primarily been studied in relation to variation in the light environment. Two aspects have been emphasized: acclimation to the contrasting light intensity of understorey, gap or canopy environment (Bjorkman el al. 1972, Doley el al. 1988, Kwesiga el al. 1986, Langenheimn el al. 1984, Pearcy 1987, Thompson el al. 1988, Walters & Field 1987) and the response of understorey plants to lightflecks (Chazdon & Pearcy 1986, Pearcy 1988, Pearcy et al. 1985). Photosynthetic response to other environmental factors has been studied less, perhaps because it is thought that other factors impose only a minor limitation on photosynthesis. Langenheim et al. (1984) and Pearcy (1987) found a decrease in photosynthetic rate with increasing leaf-air vapour pressure deficit, (VPD); Chiariello et al. (1987), and Pearcy (1987) reported a weak response to temperature; and Thompson et al. (1988) have investigated the reduction of photosynthetic rate at low nutrient levels. Few studies have determined if these factors limit photosynthesis in natural conditions. Pearcy (1987) did not find evidence that temperature or VPD significantly limited CO2 uptake. Doley el al. (1988) found that during the first two days following a rainfall event, gas exchange rates in the upper crown of an emergent tree closely followed the daily course of irradiance, while during the subsequent days, they declined markedly following the morning peak. Whitehead et al. (1981) in two planted forest tree species, and Mooney el al. (1983) in an understorey shrub, have both shown a midday decrease of stomatal conductance which was attributed to an increase in VPD, although the consequences for

Water and nutrient relations of woody perennials from tropical dry forests

Abstract. Dry forests occupy a larger area in the tropics than rain forests. They grow under a wide range of rainfall conditions; the determining characteristics are the occurrence of a dry season of 2–6 months duration, and the dominance of deciduous woody perennials. The knowledge of the ecophysiological properties of woody perennials from these forests, essential for the development of forest restoration strategies, is still scanty. This paper describes the ecophysiological behaviour of 10 species of woody perennials growing in a secondary dry forest, which has been recovering since 1944 on the hills of the Botanical Garden of Caracas. Four species, Astronium graveolens, Bauhinia megalandra, Sapindus saponaria and Tabebuia chrysantha, were initially planted while the others, Bursera simaruba, Capparis flexuosa, Erythroxylon cumanense, E. densum, E. orinocense and Eugenia casearioides, reproduced from seed sources existing in forest remnants growing nearby. Specific leaf areas measured are relatively high, covering a range from 11 to 34 m2/kg. Nitrogen and phosphorus levels are also high compared to data reported from dry forests elsewhere, therefore no nutrient limitation for photosynthetic productivity is apparent. Osmotic pressure (π) of leaf sap, extracted from frozen samples taken during the growing season, is correlated with the corresponding content of soluble sugars and the ions Mg and K; Ca-ions do not play a significant role in explaining the variance of π. Most species maintained a diluted leaf sap during the rainy season, characterized by π values between 5–15 bar. The osmotic pressure increased strongly in older leaves and during the dry season. Four species showed more stable π values throughout the growing season, with C. flexuosa and B. simaruba characterized by higher and lower π values, respectively. Proline was found to be a reliable indicator of water stress in these woody species, the amount of proline measured in leaf sap being logarithmically correlated with the corresponding π value. Osmotic pressure of leaf sap and leaf xylem tension was higher during the dry season for all species, while the contrary was true for leaf conductance. Leaf conductance was better correlated with leaf-air vapour pressure deficit than with leaf xylem tension. The most drought-tolerant species were C. flexuosa, E. casearioides and the three Erythroxylon species. Drought resistance of B. simaruba, B. megalandra and A. graveolens was associated with their high sensitivity to leaf-air VPD and lower leaf conductances. The other two species occupied intermediate positions.

The carbon isotope ratio of plant organic material reflects temporal and spatial variations in CO2 within tropical forest formations in Trinidad.

A method of monitoring and collecting CO2 samples in the field has been developed which has been used to study both temporal and spatial variations in canopy CO2 isotopic signatures in two contrasting tropical forest formations in Trinidad. These have been related to vertical gradients in the carbon isotope ratio (δ13C) of organic material in conjunction with measurements of other environmental parameters. The δ13C of leaf material from two canopies showed a gradient with respect to height, more negative values being found low in the understorey. The deciduous secondary forest, (Simla) showed a difference of 4.6‰ and the semi-evergreen seasonal canopy (Aripo), 2.8‰. The range of δ13C values at Simla was 4‰ less negative than those at Aripo. In order to relate these measurements to the interaction between diffusion or carboxylation limitation, and source CO2 effects, variations in environmental parameters through the canopy have been compared with changes in CO2 partial pressure (P a) and isotopic composition δ13C throughout the day during the dry season. Values of P a20 m above the ground at Aripo varied from 380 vpm at dawn to 340 vpm at midday, at which time the partial pressure 15 cm above the ground was 375 vpm. The CO2 partial pressure did not stabilise during the course of the day, and there was good correlation (r 2=0.82) between δa and P a, with more negative values of δa occuring in the understorey. Diuraal changes of 2‰ were evident at all canopy positions. In the more open canopy at Simla, these gradients were similar, but less marked. Leaf-air vapour pressure deficit (VPD) showed no relationship with height, possibly as a result of minimal water flux from both the soil and the canopy due to low soil water content; VPD was 1.5 kPa higher at midday than dawn. A 3° C temperature gradient between the understorey and upper canopy was observed at Aripo but not in the more open Simla canopy. CO2 partial pressure stabilised for only 4 h in the middle of the day, while other parameters showed no stable period. The proportion of floor respired CO2 reassimilated at Aripo has been calculated as 26%, 19%, and 8% for the periods 0600-1000, 1000-1400, and 1400-1800 hours. In order to quantify source CO2 effects, measurements of the environmental parameters and assimilation rate must be made at all canopy positions and throughout the day.

Seasonal Variation in Stomatal Responses of Two Cultivars of Lychee (Litchi chinenis Sonn)

The effect of cultivar and environment on the stomatal conductance and plant water relations of lychee (Litchi chinensis Sonn.) was investigated. Diurnal changes in stomatal conductance (gs) and leaf water potentials (Ψ1) were determined for leaves of irrigated trees of cv. 'Bengal' and cv. 'Kwai May Pink' between July and December 1990. Leaves of Bengal always had much higher gs than Kwai May Pink in winter, but as summer approached, this difference became less. Ψ1 values at midday for Bengal were always much lower than for Kwai May Pink. The linear model, E = G(Ψ1-Ψsoil), where E is transpiration rate and G is whole plant conductance, was found to be valid for nearly all the data sets collected. The values of G for Kwai May Pink were higher than for Bengal, especially in summer, and the average values of G (Bengal 4.1 and Kwai May Pink 6.3 mmol H2O m-2 s-1 Mpa-1) indicate that the lychee has a relatively efficient water transport system compared with other fruit tree species. Laboratory measurements of the responses of these cultivars confirmed observed differences in gs in the field. The responses of each cultivar to irradiance (I), leaf temperature (Tl) and leaf-air water vapour pressure deficit (D) were obtained and used to model the orchard data. The equation (see pdf) where Topt is the temperature for maximum gs, gdark is the basal gs in the dark at given T1 and D, and kI, kT and kD are constants fitted by non-linear least squares, provided an acceptable fit for both cultivars (R2 = 0.68 for Bengal and 0.55 for Kwai May Pink). The fit was not improved by including Ψ1 in the model. There was a significant difference between cultivars in kT, the temperature sensitivity coefficient. Possible implications of inter-cultivar differences in temperature sensitivity are discussed.

The Effect of Air Humidity on Photosynthesis of Ryegrass and White Clover at Three Temperatures

Increasing leaf-air vapour pressure deficit (VPD) decreased the stomatal conductance and the photosynthetic rate of leaves of ryegrass (Lolium perenne L.) and white clover (Trifolhim repens L.) at light saturation and at lower irradiance. In ryegrass both conductance and photosynthesis, and in clover photosynthesis, decreased less with increasing VPD in low irradiance than they did at an irradiance which saturated photosynthesis. In ryegrass, relative to their values at 10 mb, photosynthesis and conductance decreased less with increasing VPD at 25 °C than at 20 or 16·5 °C. In white clover, relative conductance (but not photosynthesis) was less reduced at 25 than at 16·5 °C. Measurements of VPD of air in the leaf canopy of a field-grown crop are combined with the observed responses of photosynthesis to VPD and temperature in a model. This shows that high VPD is likely to depress photosynthesis significantly and that, during a typical day, the rate of light saturated photosynthesis may remain fairly steady, because the depression of photosynthesis due to rising VPD is offset by the stimulation due to rising temperature.