Efficiency and Regulation of Water Transport in Some Woody and Herbaceous Species

Abstract

The efficiency with which plants transport water is related to the water potential differences required to drive water fluxes from the soil to the leaf. A comparative study of The extent to which climatic, plant, and edaphic factors influence plant-water balance and plant-water use is both complex and of considerable adaptive significance. Analyses of water movement in the soil-plant-atmosphere continuum and of the development of plant-water stress are facilitated by interpreting plant responses to variations in environment using conceptual and mathematical models. Elfving et al. (4) described leaf water potential (ileaf) as being dependent upon three elements: soil water potential (4'soi) flux of water through the system, and resistance to flow between the soil and the leaf (r,0ji to leaf).4.teaf = 'soil -(flux)(r,oii to leaf) (1) When the soil water supply is optimal, /'soil is approximately zero and the resistance for water flow from soil to root is negligi- (17), and Barrs (1) observed both types of response with different species. These differences may be due to differences in methodology; but it is also possible that important differences exist between species in their ability to transport water. To test these possibilities we examined relationships between 4'leaf and transpirational flux for woody and herbaceous species by varying the evaporative demand with optimal soil water supply in controlled environments. Differences in water transport efficiency may be also coupled with differences in stomatal regulation of water loss from plants. Some authors (7,10,13) have presented evidence for stomatal responses to humidity gradients between leaf and air that are independent of average leaf water status, while others (11) have concluded that stomata are relatively unaffected by changes in external humidity. If stomata do close when the humidity gradient is increased, leaf resistance would also increase and transpirational flux would not respond linearly to changes in the humidity gradient (15). The influence of the humidity gradient on leaf resistance was investigated with two herbaceous species, since the controversy concerning stomatal response to humidity may also be due to differential behavior between species. MATERIALS AND METHODS Orange

Phloem Loading Strategies and Water Relations in Trees and Herbaceous Plants

“…All the tree species surveyed have low K p (Fig. 2), which is in agreement with previous work showing that, in general, hydraulic conductance is lower in trees than in herbs (Camacho-B et al, 1974;Iovi et al, 2009). Compared with herbs, trees have lower root density (Newman, 1969;Atkinson, 1980) and a much longer and more convoluted flow path (Tyree and Ewers, 1991), both of which contribute to larger wholeplant hydraulic resistance (lower K p ).…”

“…2 and 4). These active loading herbs represent most of the herbaceous plants, which are characterized by high K p and low foliar Suc concentration (Camacho-B et al, 1974;Iovi et al, 2009;Rennie and Turgeon, 2009). Indeed, the low foliar sugar concentration of herbs is one of their signature features.…”

Stomatal Response to Environment with Sesamum indicum. L.

“…Earlier work with well watered sesame plants had demonstrated that leaf resistance increased when the humidity gradient between leaf and air was increased with bulk leaf water status remaining relatively constant (3) and that leaf resistance responded more to humidity gradients at lower temperatures (12). Interactions between the effects of humidity gradients, temperature, and CO2 concentration on leaf resistance and the effects of humidity gradients on mesophyll resistance to CO are described in this paper.…”

“…caused by changes in bulk leaf water status (3,12,16,23). This response system would provide an adaptive advantage in arid environments because it could prevent the development of water stress in the metabolically active cells within the leaf (23), and because it would promote the efficient use of water by plants (12).…”

“…Negative feedback relationships between leaf water status and stomatal aperture (4,21) (1,2,6,9,15,20). It has also been proposed that stomata may respond to the humidity gradient between leaf and air independently of effects 'This work was supported by National Science Foundation Grant GB 39856 to M. R. K.caused by changes in bulk leaf water status (3,12,16,23). This response system would provide an adaptive advantage in arid environments because it could prevent the development of water stress in the metabolically active cells within the leaf (23), and because it would promote the efficient use of water by plants (12).…”