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

The factors responsible for the low transpiration rates of citrus were investigated. Leaf resistance to water vapor exchange by orange seedlings {Citrtis sinensis L. cv. Koethen) including a substantial boundary layer resistance, was as low as 1 s cm"' in humid air. Leaf resistance of well watered plants increased to values as large as 5 s cm"' when the difference in absolute humidity betweenleafandairwasincreased. Leaf resistance was only slightly influenced by temperature between 20 and 30°C providing the humidity diflerence between leaf and air was kept constant. Leaf resistance increased when leaf temperature was increased between 20 and 30°C when the absolute humidity external to the leaf was kept constant. Increased humidity differences resulted in greater increases in leaf resistance during initial experiments than when the experiments were repeated with the same leaves indicating acclimation by the plant. It was concluded that the effects of humidity differences on leaf resistance are partially responsible for the low transpiration rates of 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.