Desert Ecosystems: Environment and Producers

Attempts to correlate values of stomatal conductance and leaf water potential with particular environmental variables in the field are generally of only limited success because they are simultaneously affected by a number of environmental variables. For example, correlations between leaf water potential and either flux of radiant energy or vapour pressure deficit show a diurnal hysteresis which leads to a scatter diagram if many values are plotted. However, a simple model may be adequate to relate leaf water potential to the flow of water through the plant. The stomatal conductance of illuminated leaves is a function of current levels of temperature, vapour pressure deficit, leaf water potential (really turgor pressure) and ambient CO $_2$ concentration. Consequently, when plotted against any one of these variables a scatter diagram results. Physiological knowledge of stomatal functioning is not adequate to provide a mechanistic model linking stomatal conductance to all these variables. None the less, the parameters describing the relationships with the variables can be conveniently estimated from field data by a technique of non-linear least squares, for predictive purposes and to describe variations in response from season to season and plant to plant.

The Interpretation of the Variations in Leaf Water Potential and Stomatal Conductance Found in Canopies in the Field

INTRODUCTION 492 ECOLOGICAL ASPECTS OF PHOTOSYNTHETIC TEMPERATURE ADAPTATION 493 Photosynthetic Temperature Dependence in Thermally Contrasting Climates ........ 493 Photosynthetic Temperature Acclimation 497 Seasonal acclimation in natural habitats ...... ....... 497 Studies in controlled environments 499 THE MECHANISTIC BASIS FOR PHOTOSYNTHETIC RESPONSE AND ADAPTATION TO TEMPERATURE 504 Reversible Temperature Respon.ses 505 Stomata! effec� o� the . temJH!.rature response 0/ photo.rynthesis ......... ....... 505 Interacttons with /lght mtenslty 507 C, photo.rynthesis (lM photorespiration 507 C, photo.rynthesis 515 Comparison 0/ plants from contrasting thermol regimes ...... ...... 517 ["eversible Temperature Respon.ses 519 Low temperature sensitMty ....... 519 High temperature sensitivity 524 Adoptive responses in the heat stability 0/ the photosynthetic apparatus 530 CONCLUDING REMARKS 532

Photosynthetic Response and Adaptation to Temperature in Higher Plants

Publisher Summary The study of leaf anatomy and of the mechanisms of the opening and closing of stomatal guard cells leads one to suppose that the stomata constitute the main or even the sole regulating system in leaf transpiration. Meteorologists have developed a wide variety of formulae for estimating evaporation from vegetation that are based entirely on weather variables and take no account at all of the species composition or stomatal properties of the transpiring vegetation. These “potential evaporation” formulae are widely and, to a large degree, successfully used for estimating evaporation from vegetation that is not water-stressed. Transpiration depends on stomatal conductance, net radiation receipt and upon air saturation deficit, temperature, and wind speed. Saturation deficit and wind speed vary through leaf boundary layers, through canopies, and through the atmosphere above the canopies. The sensitivity of saturation deficit to changes in stomatal conductance depends on where the saturation deficit is measured. If all of the stomata on a single leaf change aperture in unison, there may be a substantial change in saturation deficit measured at the leaf surface but a negligible change in saturation deficit measured a centimetre or two away, outside the leaf boundary layer.

Stomatal Control of Transpiration: Scaling Up from Leaf to Region

Previous studies on the Physiology of stomata in higher plants suggest that stomata influence the rate of CO2 fixation in leaf mesophyll tissue. We believe that an equally important stomatal function has not been fully recognised; that stomatal aperture is determined by the capacity of the mesophyll tissue to fix carbon. We altered the capacity of leaves to fix carbon by various means, and found invariably that the diffusive conductance of the epidermis to CO2 transfer, g, (which mainly depends on the number and dimensions of the stomata) changes in nearly the same proportion as the rate of assimilation of CO2. Thus, the intercellular concentration of CO2 (ci), calculated as ci = ca–A/g (where ca is ambient concentration of CO2, A is assimilation rate of CO2), tends to remain constant providing ca is kept constant. We used routine techniques1 to measure A and estimate g in leaves placed singly in chambers. Conductance takes account of CO2 transfer through both stomata and leaf boundary layer, the conductance of the latter being 0.5 mol m−2 s−1.

Stomatal conductance correlates with photosynthetic capacity

The stomata of plants growing in the Negev Desert, namely the stomata of the mesomorphic leaves of Prunus armeniaca, the xeromorphic stems of Hammada scoparia, and the succulent leaves of Zygophyllum dumosum, respond to changes in air humidity. Under dry air conditions diffusion resistance increases. Under moist air conditions diffusion resistance decreases. When the stomata close at low air humidity the water content of the apricot leaves increases. The stomata open at high air humidity in spite of a decrease in leaf water content. This excludes a reaction via the water potential in the leaf tissue and proves that the stomatal aperture has a direct response to the evaporative conditions in the atmosphere. In all species the response to air humidity is maintained over a period of many hours also when the soil is considerably dry. The response is higher in plants with poor water supply then in well watered plants. Thus for field conditions and for morphologically different types of photosynthesizing organs the results confirm former experiments carried out with isolated epidermal strips.

Stomatal responses to changes in humidity in plants growing in the desert.

The response of stomata to a gradual increase in temperature at increasing plant water stress was studied in a hot desert habitat (Negev, Israel) in the field, but under controlled temperature and humidity conditions. Four native species (Zygophyllum dumosum, Artemisia herba-alba, Hammada scoparia, Reaumuria negevensis) and one cultivated plant (Prunus armeniaca) were used in these studies. The stomatal response to temperature was compared with the response in well-irrigated plants of the same species.At low water stress, the diffusion resistance for water vapour decreased in response to a gradual increase in temperature. Transpiration increased accordingly. This response was reversible. All species responded in the same way. The opening of stomata with increasing temperature was apparently independent of the stomatal response regulated by atmospheric humidity. At high plant water stress, the stomatal response was reversed, i.e., the stomata closed when temperature was gradually increased. This stomatal closure was also independent of the closure regulated by atmospheric humidity. The plant water potential at which the stomatal response to temperature was reversed, differed among the species investigated.

Stomatal responses to changes in temperature at increasing water stress.

During the dry season in the Negev desert (Israel) Artemisia herbaalba in its natural habitat has a very low water content. It shows values of negative hydrostatic pressure in the xylem down to -163 bars and an extreme of osmotic potential in the leaves of -92 bars. The diurnal water stress does not decrease strongly in the night. Under these conditions Artemisia is still photosynthetically active for a few hours of the day during the whole dry period.

L. Kappen

Papers

Stomatal responses to changes in humidity in plants growing in the desert.

Stomatal responses to changes in temperature at increasing water stress.

Extreme water stress and photosynthetic activity of the desert plant Artemisia herba-alba asso.