The predominant emphasis on harmful effects of environmental stresses on growth of woody plants has obscured some very beneficial effects of such stresses. Slowly increasing stresses may induce physiological adjustment that protects plants from the growth inhibition and/or injury that follow when environmental stresses are abruptly imposed. In addition, short exposures of woody plants to extreme environmental conditions at critical times in their development often improve growth. Furthermore, maintaining harvested seedlings and plant products at very low temperatures extends their longevity. Drought tolerance: Seedlings previously exposed to water stress often undergo less inhibition of growth and other processes following transplanting than do seedlings not previously exposed to such stress. Controlled wetting and drying cycles often promote early budset, dormancy, and drought tolerance. In many species increased drought tolerance following such cycles is associated with osmotic adjustment that ...

Acclimation and adaptive responses of woody plants to environmental stresses

The responses of leaf water potential, leaf conductance, transpiration rate and net photosynthetic rate to vapour pressure deficits varying from 10 to 30 Pa kPa-1 were followed in Helianthus annuus as the extractable soil water decreased. With a vapour pressure deficit of 25 Pa kPa-1 around the entire plant as the soil water content decreased, the leaf conductance and transpiration rate showed a strong closing response to leaf water potential at a value of-0.65 MPa, whereas with a vapour pressure deficit of 10 Pa kPa-1 around the entire plant, the rate of transpiration and leaf conductance decreased almost linearly as the leaf water potential decreased from-0.4 to-1.0 MPa. Increasing the vapour pressure deficit from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps decreased the leaf conductance by a similar proportion at all extractable soil water contents. At high soil water contents, the decrease in conductance with leaf water potential was greater when the vapour pressure deficit was increased than when it was not, indicating a direct influence of vapour pressure deficit on the stomata. The rate of net photosynthesis decreased to a smaller degree than the leaf conductance when the vapour pressure deficit around the leaf was varied. Overall, the net photosynthetic rate decreased almost linearly from 20 to 25 μmol m-2 s-1 at-0.3 MPa to 5 μmol m-2 s-1 at-1.2 MPa. As the soil water decreased, the internal carbon dioxide partial pressure was maintained between 14 and 25 Pa.No unique relationship between leaf conductance, transpiration rate or photosynthetic rate and leaf water potential was observed, but in all experiments leaf conductance and the rate of net photosynthesis decreased when about two-thirds of the extractable water in the solid had been utilized irrespective of the leaf water potential. We conclude that soil water status, not leaf water status, affects the stomatal behaviour and photosynthesis of H. annuus.

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content : II. In the mesophytic herbaceous species Helianthus annuus

. Recent data concerning the impact of elevated atmospheric CO2 upon water use efficiency (WUE) and the related measure, instantaneous transpiration efficiency (ITE), are reviewed. It is concluded from both short and long-term studies that, at the scale of the individual leaf or plant, an increase in WUE or ITE is generally observed in response to increased atmospheric CO2 levels. However, the magnitude of this increase may decline with time. The opinion that elevated CO2 may substantially decrease transpiration at the regional scale is discussed. The mechanisms by which elevated CO2 may cause a change in these measures are discussed in terms of stomatal conductance, assimilation and respiration responses to elevated CO2. Finally, recent experimental data and model outputs concerning the impact of the interaction of increased temperature with elevated CO2 on WUE, ITE and yield are reviewed. It is concluded that substantially more data is required before reliable predictions about the regional scale response of WUE and catchment hydrology can be made.

The interaction of rising CO2 and temperatures with water use efficiency

The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content : I. Species comparisons at high soil water contents.

Suboptimal N nutrition increased the water potential for stomatal closure in water stressed cotton ( Gossypium hirsutum L.) leaves. This increased sensitivity to water stress had two components, increased accumulation of abscisic acid (ABA) and increased apparent stomatal sensitivity to ABA. Low N increased the threshold water potentials for stomatal closure and ABA accumulation by about 4 bars and 2 bars, respectively. Low N also greatly increased stomatal response to low concentrations of exogenous ABA applied to excised leaves through the transpiration stream. In low N leaves, kinetin decreased stomatal response to ABA to the level observed with high N leaves. Kinetin by itself had little effect on stomata, nor did it alter stomatal response to ABA in high N leaves. The results suggest a cytokinin-ABA balance which is altered by suboptimal N nutrition to favor stomatal closure during stress. Ambient temperature and N nutrition interacted to alter stomatal response to water stress. Stress-induced ABA accumulation and apparent stomatal sensitivity to ABA were independently affected. The effects of each treatment, and their interaction, could be explained as the net result of changes in both accumulation and apparent sensitivity. Although the results document environmental control of stomatal response to ABA, either altered partitioning of ABA between active and inactive pools, or altered sensitivity of the guard cells, could account for the data.

Water Relations of Cotton Plants under Nitrogen Deficiency V. Environmental Control of Abscisic Acid Accumulation and Stomatal Sensitivity to Abscisic Acid

The dynamic response of stomata to changes in atmospheric humidity was investigated in Fragaria × ananassa Duch., Picea engelmannii Parry, and Pseudotsuga menziesii (Mirb.) Franco; and the effect of water stress on this response was determined in Pseudotsuga menziesii.

The plants were rotated through three regimes of ambient temperature and vapour pressure deficit: 35°C–3. 5kPa, 35°C–0. 5 kPa, and 20°C–1. 5kPa. Branch and leaflet conductance were measured with a steady-state porometer, first at ambient vapour pressure deficit and then at one of four treatment conditions achieved by increasing or decreasing vapour pressure within the porometer cuvette. All three species showed similar stomatal response: enhanced conductance at low vapour pressure deficit and depressed conductance at high vapour pressure deficit. Engelmann spruce was more sensitive than Douglas fir and strawberry.

Plant water status significantly altered stomatal response to vapour pressure deficit. The relationship of conductance of xylem water potential was linear under ambient conditions but became curvilinear when conductance was measured above and below ambient vapour pressure deficit. Between −0. 5 MPa and −2. 0 MPa xylem water potential, the stomata were sensitive to vapour pressure deficit, but below − 2. 0 MPa, the sensitivity decreased.

Stomatal response to vapour pressure deficit and the effect of plant water stress

Two ventilated porometers (diffusion and steady-state) were compared on four broadleaf and five coniferous species. The diffusion porometer gave consistently lower conductance values for both types of species, reflecting a direct stomatal response to low chamber humidity. At high conductance values, the porometers produced a linear and nearly equal response, but the diffusion porometer was less sensitive at low conductance values. This was due to lower air flow (20% of the velocity in the steady-state porometer) and water vapor sorption (by its acrylic plastic chamber). The broadleaf species had less variation (R2 = 0.81) than did the coniferous species (R2 = 0.61), but, with the latter, there was better correspondence between the two porometers, possibly due to sampling technique. Conductance values were clustered by species.

/pdf/two-types-of-ventilated-porometers-compared-on-broadleaf-and-1izbsgna86.pdf

Two Types of Ventilated Porometers Compared on Broadleaf and Coniferous Species

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Stomatal response to vapour pressure deficit and the effect of plant water stress

Two Types of Ventilated Porometers Compared on Broadleaf and Coniferous Species