An overview of the Energy Balance Experiment (EBEX-2000) is given. This experiment studied the ability of state-of-the-art measurements to close the surface energy balance over a surface (a vegetative canopy with large evapotranspiration) where closure has been difficult to obtain. A flood-irrigated cotton field over uniform terrain was used, though aerial imagery and direct flux measurements showed that the surface still was inhomogeneous. All major terms of the surface energy balance were measured at nine sites to characterize the spatial variability across the field. Included in these observations was an estimate of heat storage in the plant canopy. The resultant imbalance still was 10%, which exceeds the estimated measurement error. We speculate that horizontal advection in the layer between the canopy top and our flux measurement height may cause this imbalance, though our estimates of

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The productivity, product quality and competitive ability of important agricultural and horticultural plants in many regions of the world may be adversely affected by current and anticipated concentrations of groundlevel ozone (O 3 ). Exposure to elevated O 3 typically results in suppressed photosynthesis, accelerated senescence, decreased growth and lower yields. Various approaches used to evaluate O 3 effects generally concur that current yield losses range from 5% to 15% among sensitive plants. There is, however, considerable genetic variability in plant responses to O 3 . To illustrate this, we show that ambient O 3 concentrations in the eastern United States cause substantially different levels of damage to otherwise similar snap bean cultivars. Largely undesirable effects of O 3 can also occur in seed and fruit chemistry as well as in forage nutritive value, with consequences for animal production. Ozone may alter herbicide efficacy and foster establishment of some invasive species. We conclude that current and projected levels of O 3 in many regions worldwide are toxic to sensitive plants of agricultural and horticultural significance. Plant breeding that incorporates O 3 sensitivity into selection strategies will be increasingly necessary to achieve sustainable production with changing atmospheric composition, while reductions in O 3 precursor emissions will likely benefit world food production and reduce atmospheric concentrations of an important greenhouse gas.

Abstract. Plants growing in environments differing in prevailing humidity exhibit variations in traits associated with regulation of water loss, particularly cuticular and stomatal properties. Expansive growth is also typically reduced by low humidity. Nevertheless, there is little evidence in plants for a specific sensor for humidity, analogous to the blue light or phytochrome photoreceptors. The detailed mechanism of the stomatal response to humidity remains unknown. Available data suggest mediation by fluxes of water vapour, with evaporation rate assuming the role of sensor. This implies that stomata respond to the driving force for diffusional water loss, leaf‐air vapour pressure difference. Induction of metabolic stomatal response to humidity may involve signal metabolites, such as abscisic acid, that are present in the transpiration stream. These materials may accumulate in the vicinity of guard cells according to the magnitude and location of cuticular transpiration, both of which could change with humidity. Such a mechanism remains hypothetical, but is suggested to account for feedforward responses in which transpiration decreases with increasing evaporative demand, and for the apparent insensitivity of stomatal aperture in isolated epidermis to epidermal water status. Other responses of plants to humidity may involve similar indirect response mechanisms, in the absence of specific humidity sensors.