Stoma | EPFL Graph Search

In botany, a stoma (from Greek στόμα, "mouth", : stomata), also called a stomate (: stomates), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange. The pore is bordered by a pair of specialized parenchyma cells known as guard cells that regulate the size of the stomatal opening. The term is usually used collectively to refer to the entire stomatal complex, consisting of the paired guard cells and the pore itself, which is referred to as the stomatal aperture. Air, containing oxygen, which is used in respiration, and carbon dioxide, which is used in photosynthesis, passes through stomata by gaseous diffusion. Water vapour diffuses through the stomata into the atmosphere as part of a process called transpiration. Stomata are present in the sporophyte generation of all land plant groups except liverworts. In vascular plants the number, size and distribution of stomata varies widely. Dicotyledons usually have more stomata on t

BIOPHYSICAL CONTROLS ON EVAPOTRANSPIRATION: NUMERICAL MODELS AND EXPERIMENTAL OBSERVATIONS

Federico Croci

Water and food are basics of human life, unfortunately climate changes and water pollution have reduced the amount of water available for human uses. In this context the study of the hydrologic cycle is becoming of growing importance in order to optimize the anthropogenic exploitation of water resource. Evapotranspiration is the term used to describe the part of the water cycle which removes liquid water from an area with vegetation into the atmosphere by the processes of both transpiration and evaporation. Evapotranspiration is an important part of the hydrologic cycle because it represents a considerable percentage of water losses in watersheds. This process is also one of the main consumers of solar energy at the Earth's surface. The estimation of evapotranspiration it is of great importance for agriculture and water resources management, for example, by knowing the rate of water loss from a region, farmers will be better placed to efficiently manage the irrigation strategies. The rate of evapotranspiration at any location on the Earth's surface is controlled by several factors such as energy availability, humidity gradient, wind speed immediately above the surface, water availability, type of vegetation, stomatal resistance and soil characteristics. The problem of estimating evapotranspiration has been widely studied. During the past several decades, different approaches have been proposed and many formulas are available in the literature to fulfill this task. The complexity of the phenomena and its great variability all over the world make difficult to find a universal and robust method for the evapotranspiration prediction and estimation. Evapotranspiration can be obtained by many estimation methods. Some of these methods need many weather parameters as inputs while others need fewer. Factors such as data availability, the intended use, and the time scale required by the problem must be considered when choosing the evapotranspiration calculation technique. The Penman-Monteith equation was derived from the classic Penman equation adding to the terms already considered in the Penman formulation (aerodynamic resistance and energy balance) the canopy resistance. This approach requires many meteorological data, such as maximum and minimum air temperatures, relative humidity, solar radiation, and wind speed. If some of these data are not available, alternative techniques have been developed to overcome this drawback. However, this estimations of meteorological data affect the quality of the result, alternatively, in such cases other simpler methods may be used. The Penman-Monteitu equation is assumed to be the most reliable because is based on physical principles and because it considers all the climatic factors, which affect evapotranspiration.

2013

Phytolith types and type-frequencies in subalpine-alpine plant species of the European Alps

Biogenic silica extracted from 21 species commonly occurring in subalpine and alpine plant communities in the central Swiss Alps were examined using light and SEM microscopes; 19 species being screened here for the first time. An inventory of phytolith types is provided and type-frequencies are assessed. Light microscope photographs and SEM micrographs provided illustrations of the types described. The monocotyledons analysed belonged to the genera Calamagrostis, Festuca, Nardus, Poa and Carex. Monocotyledons yielded mainly types of epidermal origin (short cells, rods, cork cells, silicified stomata and trichomes). Dicotyledons analysed were from five genera of the Ericaceae family (Arctostaphylos, Calluna, Loiseleuria, Rhododendron, Vaccinium) and from one genus of the Betulaceae (Alnus). In dicotyledons, silicified epidermal jigsaw cells, stomata complexes and vessels were recovered. Conifer species of the genera Abies, Juniperus, Picea, Pinus and Larix were studied. Distinctive conifer cells were mainly silicified endodermids and transfusion tissues. Phytolith taxonomic diagnostic potential was tested by cluster analysis and principal component analysis (PCA). It was shown that grass and sedge species could be easily differentiated on the basis of phytolith types. In general, Ericaceae and conifers could also be distinguished on the basis of phytoliths; however, some species yielding mainly redundant types were not unequivocally identifiable. The aim of the present work is to provide a framework for phytolith-based plaeoecological studies at the treeline in the European Alps. (C) 2004 Elsevier B.V All rights reserved.

2004

Tree effects on urban microclimate: Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects

Paolo Burlando, Matthias Roth

Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures. Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1– 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery.