The Response of Transpiration Resistance to Leaf Temperature as a Desiccation Resistance Mechanism in Tree Seedlings

Transpiration rate and leaf transfer resistance to water vapor loss were determined under a range of leaf temperatures for Quercus macrocarpa, Q. velutina, Q. alba, Q. rubra, and Acer saccharum. Transfer resistance increased with rising leaf temperatures between 20 and 40°C in all species, but the rate of increase in resistance was greatest in species which normally occupy xeric sites. Increased transfer resistance with rising leaf temperature may be significant in preventing rapid desiccation of leaves under the large evaporative stress imposed by high leaf temperature.     

Water Potential Components, Stomatal Function, and Liquid Phase Water Transport Resistances of Four Arctic and Alpine Species in Relation to Moisture Stress

Transpiration measurements of two alpine tundra species, Deschampsia caespitosa and Geum rossii, and two arctic tundra species, Dupontia fischeri and Carex aquatilis, were conducted under varying atmospheric and soil moisture stress regimes to determine if the stomatal response to water stress may play a role in the local distributions of these species. Under low soil moisture stress, stomata of the species restricted typically to wet meadow areas, Deschampsia and Dupontia, did not exhibit closure until leaf water potential declined. However, when soil moisture stress was low and atmospheric stress increased, Geum and particularly Carex exhibited partial stomatal closure before leaf water potential dropped, suggesting a direct response of the stomata to the vapor pressure gradient between the leaf and the atmosphere. Lower liquid phase water transport resistance from the soil to the leaves may also reduce the development of leaf moisture stress in Geum. Furthermore, Geum and possibly Carex appeared to undergo less of a loss of leaf turgor when leaf water potential decreased. This response may serve to maintain leaf cell turgor and to abate the reduction in leaf enlargement.     

The Effect of Leaf Water Potential, Leaf Temperature and Light Intensity on Leaf Diffusion Resistance and the Transpiration of Leaves of Malus sylvestris

The relationship between leaf resistance to water vapour diffusion and each of the factors leaf water potential, light intensity and leaf temperature was determined for leaves on seedling apple trees (Malus sylvestris Mill. cv. Granny Smith) in the laboratory. Leaf cuticular resistance was also determined and transpiration was measured on attached leaves for a range of conditions. Leaf resistance was shown to be independent of water potential until potential fell below — 19 bars after which leaf resistance increased rapidly. Exposure of leaves to CO₂-free air extended the range for which resistance was independent of water potential to — 30 bars. The light requirement for minimum leaf resistance was 10 to 20 W m^?2 and at light intensities exceeding these, leaf resistance was unaffected by light intensity. Optimum leaf temperature for minimum diffusion resistance was 23 ± 2°C. The rate of change measured in leaf resistance in leaves given a sudden change in leaf temperature increased as the magnitude of the temperature change increased. For a sudden change of 1°C in leaf temperature, diffusion resistance changed at a rate of 0.01 s cm^?1 min^?1 whilst for a 9°C leaf temperature change, diffusion resistance changed at a rate of 0.1 s cm^?1 min^?1. Cuticular resistance of these leaves was 125 s cm^?1 which is very high compared with resistances for open stomata of 1.5 to 4 s cm^?1 and 30 to 35 s cm^?1 for stomata closed in the dark. Transpiration was measured in attached apple leaves enclosed in a leaf chamber and exposed to a range of conditions of leaf temperature and ambient water vapour density. Peak transpiration of approximately 5 × 10^?6 g cm^?2 s^?1 occurred at a vapour density gradient from the leaf to the air of 12 to 14 g m^?3 after which transpiration declined due presumably to increased stomatal resistance. Leaves in CO₂-free air attained a peak transpiration of 11 × 10^?6 g cm^?2 s^?1 due to lower values of leaf resistance in CO₂ free air. Transpiration then declined in these leaves due to development of an internal leaf resistance (of up to 2 s cm^?1). The internal resistance was masked in leaves at normal CO₂ concentrations by the increase in stomatal resistance.     

Why are Sun Leaves Thicker than Shade Leaves? — Consideration based on Analyses of CO2 Diffusion in the Leaf

A ) depend not only on photosynthetic biochemistry but also on mesophyll structure. Because resistance to CO₂ diffusion from the substomatal cavity to the stroma is substantial, it is likely that mesophyll structure affects A through affecting diffusion of CO₂ in the leaf. To evaluate effects of various aspects of mesophyll structure on photosynthesis, we constructed a one-dimensional model of CO₂ diffusion in the leaf. When mesophyll thickness of the leaf is changed with the Rubisco content per unit leaf area kept constant, the maximum A occurs at an almost identical mesophyll thickness irrespective of the Rubisco contents per leaf area. On the other hand, with an increase in Rubisco content per leaf area, the mesophyll thickness that realizes a given photosynthetic gain per mesophyll thickness (or per leaf cost) increases. This probably explains the strong relationship between A and mesephyll thickness. In these simulations, an increase in mesophyll thickness simultaneously means an increase in the diffusional resistance in the intercellular spaces (R _ias), an increase in the total surface area of chloroplasts facing the intercellular spaces per unit leaf area (S _c ), and an increase in construction and maintenance cost of the leaf. Leaves can increase S _c and decrease R _ias also by decreasing cell size. Leaves with smaller cells are mechanically stronger. However, actual leaves do not have very small cells. This could be because actual leaves exhibiting considerable rates of leaf area expansion, adequate heat capacitance, high efficiency of N and/or P use, etc, are favoured. Relationships between leaf longevity and mesophyll structure are also discussed. Received 20 September 2000/ Accepted in revised form 4 January 2001     

Ozone concentration in leaf intercellular air spaces is close to zero

Transpiration and ozone uptake rates were measured simultaneously in sunflower leaves at different stomatal openings and various ozone concentrations. Ozone uptake rates were proportional to the ozone concentration up to 1500 nanoliters per liter. The leaf gas phase diffusion resistance (stomatal plus boundary layer) to water vapor was calculated and converted to the resistance to ozone multiplying it by the theoretical ratio of diffusion coefficients for water vapor and ozone in air (1.67). The ozone concentration in intercellular air spaces calculated from the ozone uptake rate and diffusion resistance to ozone scattered around zero. The ozone concentration in intercellular air spaces was measured directly by supplying ozone to the leaf from one side and measuring the equilibrium concentration above the other side, and it was found to be zero. The total leaf resistance to ozone was proportional to the gas phase resistance to water vapor with a coefficient of 1.68. It is concluded that ozone enters the leaf by diffusion through the stomata, and is rapidly decomposed in cell walls and plasmalemma.     

Changes in diffusion resistance of apple leaves undergoing rapid fluctuations in leaf temperature

Transpiration was measured in apple leaves (Malus sylvestris Mill.) which were enclosed in a leaf chamber and subjected to rapid changes in leaf temperature. Fluctuations in leaf temperature produced parallel fluctuations in transpiration. The change in transpiration rate with change in temperature was found to be less than the theoretical value calculated from the change in water vapour density gradient from leaf to air. The results suggest the presence of a small and rapidly varying resistance to water vapour loss from the leaf. The magnitude of this additional resistance increased to a maximum value of approximately 1.5 s cm^-1 as the magnitude of the temperature change increased to a maximum of approximately 12°C.     

Plant transpiration at high elevations: Theory,field measurements,and comparisons with desert plants

Summary The influence of elevational changes on plant transpiration was evaluated using leaf energy balance equations and well-known elevational changes in the physical parameters that influence water vapor diffusion. Simulated transpirational fluxes for large leaves with low and high stomatal resistances to water vapor diffusion were compared to small leaves with identical stomatal resistances at elevations ranging from sea level to 4 km. The specific influence of various air temperature lapse rates was also tested. Validation of the simulated results was accomplished by comparing actual field measurements taken at a low elevation (300 m) desert site with similar measurements for a high elevation (2,560 m) mountain research site. Close agreement was observed between predicted and measured values of transpiration for the environmental and leaf parameters tested.Substantial increases in solar irradiation and the diffusion coefficient for water vapor in air (D _wv) occurred with increasing elevation, while air and leaf temperatures, the water vapor concentration difference between the leaf and air, longwave irradiation, and the thermal conductivity coefficient for heat in air decreased with increasing elevation. These changes resulted in temperatures for sunlit leaves that were further above air temperature at higher elevations, especially for large leaves. For large leaves with low stomatal resistances, transpirational fluxes for low-elevation desert plants were close to those predicted for high-elevation plants even though the sunlit leaf temperatures of these mountain plants were over 10°C cooler. Simulating conditions with a low air temperature lapse rate (0.003° C m^-1 and 0.004° C m^-1) resulted in predicted transpirational fluxes that were greater than those calculated for the desert site. Transpiration for smaller leaves decreased with elevation for all lapse rates tested (0.003° C m^-1 to 0.010° C m^-1). However, transpirational fluxes at higher elevations were considerably greater than expected for all leaves, especially larger leaves, due to the strong influence of increased solar heating and a greater D _wv. These results are discussed in terms of similarities in leaf structure and plant habit observed among low-elevation desert plants and high-elevation alpine and subalpine plants.     

Selective oviposition by a leaf miner in response to temporal variation in abscission

Summary Responses to humidity of net photosynthesis and leaf conductance of single attached leaves were examined in populations of herbs from wet soil sites in Beltsville, Maryland and Davis, California, USA. Plants were grown in controlled environments under three conditions which differed in the magnitude of the day-night temperature difference and in daytime air saturation deficit. No population differences in response were found in Abutilon theophrasti. In Amaranthus hybridus stomatal conductance and net photosynthesis were more reduced by increasing leaf to air water vapor pressure difference (VPD) in the population from Beltsville, but only for the growth condition with a constant 25°C temperature. In Chenopodium album, stomatal conductance was more sensitive to VPD in the population from Davis, but only for the growth condition with 28/22°C day/night temperatures. Population differences in the sensitivity to VPD of leaf conductance were associated with differences in leaf area to root weight ratio. The relative reduction of net photosynthesis as VPD increased was greater than, equal to, or less than the relative decrease in substomatal carbon dioxide partial pressure. The pattern depended on species, and on growth condition. From these results one can not conclude that environmental humidity has been a strong selective force in determining sensitivity to humidity of stomatal conductance.     

SEASONAL PATTERNS OF CO2 AND WATER VAPOR EXCHANGE OF JUNCUS ROEMERIANUS SCHEELE IN A GEORGIA SALT MARSH

CO₂ and water vapor exchange studies of intact plants of black needle rush (Juncus roemerianus Scheele) were conducted in an undisturbed marsh community on Sapelo Island, Georgia. The seasonal patterns of the light and temperature responses of net photosynthesis, transpiration, leaf diffusive conductance, water-use efficiency and respiration were determined five times over the year. Internal resistances to CO₂ uptake were also evaluated. Net photosynthesis was highest in early spring, but declined only slightly through the year. A distinct and moderate temperature optimum of net photosynthesis was observed with decreasing rates above 30 C. Leaf conductances to water vapor were similar at all seasons and were high at cooler temperatures and decreased with increasing temperature. Transpiration was relatively high and constant during all seasons. The water-use efficiency of photosynthesis was high below 25 C, but decreased sharply above that temperature. Dark respiration was relatively low. Seasonal changes reflected changes in leaf density. Decreasing stomatal conductances and increasing respiration rates reduced net photosynthesis at higher temperatures. The stomatal resistance increased and internal resistances to CO₂ uptake decreased over the year, but the total resistance remained constant. The internal resistance to CO₂ uptake was consistently higher than the stomatal resistance. These seasonal response patterns show that J. roemerianus is well adapted to the seasonal changes in ambient temperature and irradiance and other microenvironmental factors in the high marsh. These physiological characteristics permit this C₃ species to maintain a high productivity in a seasonally hot and stressful environment.     

Effects of atmospheric humidity on net photosynthesis,transpiration, and stomatal resistance

Rhizomes ofHydrocotyle plants from three contrasting habitats were cloned and the ramets grown under controlled environmental conditions. Measurements of net photosynthesis, transpiration, and total leaf diffusion resistance were used to examine possible physiological adaptations to specific field environments. Increasing dryness of the growth chamber environment had large effects on gas exchange (CO₂ and water vapor) and on total diffusion resistance of plants from a pond, moderate effects on plants from a mesic forest, but plants from a coastal sand dune were unaffected by the experimentally imposed dryness. Thus the 3 Hydrocotyle types demonstrated adaptive physiological reponses to their specific field habitats. Periodic stomatal oscillations were induced in ramets from the pond by sharply increasing irradiance, but the adaptiveness of the oscillations cannot be determined with the evidence at hand.No stomatal closure could be induced by atmospheric dryness alone as long as soil and plant dessication were prevented. There were no observable differences in stomatal response to increasing atmospheric vapor pressure deficits.     

Leaf Water Potential Response to Transpiration by Citrus

This paper reports on further studies of a model for interpreting leaf water potential data for Citrus. Experimental data confirmed the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small. Data collected diurnally indicated that the relationship between leaf water potential and transpiration followed a sequence of steady states without hysteresis. No difference in water transport characteristics was found for Valencia orange on three rootstocks in well-watered soil, but the two rootstocks Cleopatra mandarin and Rangpur gave slightly greater leaf water stress in Valencia orange leaves than‘Troyer’ citrange rootstock at high transpiration rates under mild soil water deficits. In laboratory studies, previously unstressed seedlings had higher leaf water potentials than field trees at equivalent transpiration rates. After several drying cycles, however, leaf water potentials were similar to those observed in the field.     

The Nature of Systemic Invasion of Stem and Leaves of Sunflowers by Plasmopara helianthi Novot. var. helianthi Novot. with Mechanism of Sporulation and Zoospore Release

During the systemic development of Plasmopara helianthi Novot. var. helianthi Novot. the hyphae in the stem advance especially through the intercellular spaces of loose parenchyma forming haustoria in adjacent cells. The hyphae which reach leaf blades through petioles continue their growth along the main veins intercellularly in non-vascular tissues enclosing the vascular bundles. At the same time, the hyphae spread to intercellular spaces of spongy parenchyma where the growth is limited by the veinlets resulting in angular chlorotic lesions. Under humid conditions the sporangiophores arising from the aggregated hyphae in a substomatal cavity emerge through the stomatal pore on the lower side of the leaf and zoosporangia are borne terminally on sporangiophores. Soon after biflagellated zoospores are liberated into distilled water from the zoosporangia, they retract their flagellae and then lyse.     

New method for quantitative determination of water potential of mesophyll cells’ apoplast in substomatal cavity of the leaf

A new method for quantitative determination of water potential of mesophyll cells’ apoplast in substomatal cavity of the leaves of herbaceous (maize, millet, wheat, amaranth, and seepweed) and woody (larch, pine, and birch) plants by means of modern instruments designed to assess photosynthetic(СО₂/Н₂О) gas exchange is described. The method consists in determination in the light of such a level of humidity above the leaf surface, which would nullify transpiration without a noticeable suppression of photosynthetic assimilation of СО₂; this makes it possible to calculate the value of water potential at the interface between aqueous and gaseous phases of mesophyll cells’ apoplast in the substomatal cavity.     

Photosynthesis in the water-stressed C4 grass Setaria sphacelata is mainly limited by stomata with both rapidly and slowly imposed water deficits

A comparison of the effects of a rapid and a slowly imposed water deficit on photosynthesis was performed in Setaria sphacelata var. splendida (Stapf) Clayton, a C₄ NADP‐ME grass. Gas exchange was measured in rapidly and slowly dehydrated adult leaves either under atmospheric CO₂ partial pressure with an infrared gas analyser or under saturating CO₂ partial pressure with a leaf disc oxygen electrode. These measurements were used to calculate stomatal and non‐stomatal limitations to photosynthesis. These were further investigated using modulated chlorophyll a fluorescence measurements and photosynthetic pigment quantification. The decrease of net photosynthesis, leaf conductance and water use efficiency was more pronounced under rapid stress than in slow stress. However, photosynthesis is always mainly limited by stomata in both types of stress, albeit the contribution of non‐stomatal limitations increases at severe water deficits in slow stress experiments. The substomatal CO₂ partial pressure significantly increased in both types of stress, suggesting an increased resistance due to an internal barrier to CO₂ diffusion. Physical alterations in the structure of the intercellular spaces due to leaf shrinkage may account for these results. The maximal photochemical efficiency of photosystem II (PSII) was remarkably resistant to stress, as the F_v/F_m ratio decreased only at severe water deficit. On the contrary, the effective photochemical efficiency of PSII (ΔF/F′_m) measured under high actinic light decreased linearly in both types of stress, although in a more pronounced way under rapid stress. A similar variation in photochemical quenching suggests that the decrease of ΔF/F′_m is mainly due to the closure of PSII reaction centres. The non‐photochemical quenching did not change significantly except under severe dehydration indicating that the energization state of thylakoids remained stable under stress. The decrease observed in photosynthetic pigments may be an adaptation to stress rather than a limiting factor to photosynthesis. Results suggests that, although intrinsic mesophyll metabolic inhibitions occur, stomatal limitation to CO₂ diffusion is the main reason for the decrease in photosynthesis.     

Der Diffusionswiderstand der Spaltöffnungen; ein Vergleich des CO2-Gaswechsels von Blättern mit und ohne Epidermis

Summary The lower epidermis from leaves of Primula palinuri can be stripped off. Light-saturation curves of the CO₂-exchange were measured at 20°C and 300 ppm CO₂. Whereas the normal leaf reaches light-saturation at 0.3 cal cm^-2 min^-1, even 0.6 cal cm^-2 min^-1 is not sufficient to saturate the stripped leaf. Transpiration, apparent CO₂-uptake and leaf-temperature were measured simultaneously. The data were used to calculate the diffusion resistances for CO₂ with the usual methods, that is, from the diffusion resistances for water-vapour transport. The comparison of the CO₂-exchange of stripped and normal leaves makes it possible to determine the resistances—in particular those of the stomata—directly from the CO₂-exchange. Both methods agree well. When CO₂ exchanges only through the lower surface of the leaf the epidermis is—even with opened stomata—a considerable diffusion resistance. It lowers the CO₂-concentration in the intercellular system to 160 ppm and limits the CO₂-uptake.     

Morphological and anatomical determinants of mesophyll conductance in wild relatives of tomato (Solanum sect. Lycopersicon,sect. Lycopersicoides; Solanaceae)

Natural selection on photosynthetic performance is a primary factor determining leaf phenotypes. The complex CO₂ diffusion path from substomatal cavities to the chloroplasts – the mesophyll conductance (g_m) – limits photosynthetic rate in many species and hence shapes variation in leaf morphology and anatomy. Among sclerophyllous and succulent taxa, structural investment in leaves, measured as the leaf dry mass per area (LMA), has been implicated in decreased g_m. However, in herbaceous taxa with high g_m, it is less certain how LMA impacts CO₂ diffusion and whether it significantly affects photosynthetic performance. We addressed these questions in the context of understanding the ecophysiological significance of leaf trait variation in wild tomatoes, a closely related group of herbaceous perennials. Although g_m was high in wild tomatoes, variation in g_m significantly affected photosynthesis. Even in these tender‐leaved herbaceous species, greater LMA led to reduced g_m. This relationship between g_m and LMA is partially mediated by cell packing and leaf thickness, although amphistomy (equal distribution of stomata on both sides of the leaf) mitigates the effect of leaf thickness. Understanding the costs of increased LMA will inform future work on the adaptive significance of leaf trait variation across ecological gradients in wild tomatoes and other systems.     

Energy balance of an isolated scots pine

The transpiration of an isolated Scots pine (P. sylvestris), determined by weighing, was related to stomatal resistance and environmental factors. The tree had been sheared to 1.5 m diameter at the base and 1.8 m high. The transpiration (2.97 MJ for 12h) was rather constant throughout the day and similar in shape to direct beam radiation incident upon the side of a cone. Transpiration rates calculated with the gradient equation duplicated measured values when vapor density gradients (obtained from measured needle temperature), measured stomatal resistance, and characteristic width of 3 mm in aerodynamic resistance were used in the calculations. Net radiation (Q*) for the tree was calculated by replacing the direct beam component of measured net radiation with the direct beam component incident upon a cone. The sensible heat flux, calculated with needle temperatures, was large (50.8 MJ) and nearly equal to Q* (53.85). The resulting Bowen ratio was 17. Transpiration calculated with a modified Penman-Monteith type equation simulated measured values.Presented at the Eighth International Congress of Biometeorology, 9–14 September 1979, Shefayim, Israel.     

Photosynthesis, Transpiration, Leaf Stomatal and Mesophyll Resistance Measurements by the Use of a Ventilated Diffusion Porometer

A ventilated diffusion porometer was modified and adapted for simultaneous measurements of leaf resistance and photosynthesis (using ¹⁴C). The system enables measurements to be made under field and laboratory conditions with different concentrations of CO₂ and vapor pressure gradients between the evaporating surfaces inside the leaf and the external atmosphere. The leaf is subjected to the porometer's atmosphere only for short periods (up to 30 seconds) and it is assumed that stomata are not affected. Establishing the linear regression of the effect of CO₂ concentration on net photosynthesis makes it possible to extrapolate for CO₂ compensation point, to calculate the overall resistance to CO₂ and the mesophyll resistance to CO₂.     

Drought adaptations in two Californian evergreen sclerophylls

Summary A field study was initiated to determine the patterns of water stress imposition and stomatal resistance to gas exchange in representative species of 2 evergreen sclerophyllous communities. In concurrent experiments plant water potential, temperature and vapor pressure gradient were varied to determine the relative importance of morphological and physiological parameters in delaying onset of water stress during drought periods.In general, stomatal and photosynthetic responses to water stress were similiar in both species. Both were able to fix carbon even when leaf water potentials dropped as low as-25 bars. Stomatal movements were positively correlated with soil water potential rather than to leaf water potential. However, water stress developed much more rapidly in Arbutus menziesii, a plant of more northerly distribution, than in Heteromeles arbutifolia where they occur on adjacent sites. Morphological parameters were primarily responsible for the very different patterns of water stress imposition. Consequently, Arbutus is limited to areas of shorter drought duration than is Heteromeles and this is reflected in their differing distributions.     

TRANSPIRATION RESISTANCE IN VACCINIUM MYRTILLUS

Measurements of transpiration resistance of Vaccinium myrtillus at various wind speeds indicate that this quantity is relatively insensitive to variations in wind speed in the range normally experienced under field conditions. The resistance is relatively low when compared with values reported in the literature for other plants. Placed in an identical environment, V. myrtillus plants from sunny sites were found to have measurably lower resistance and lower leaf temperatures than those from shady sites. Transpiration resistance appears to be inversely related to stomatal density.