Major parts of anthropogenic and natural aerosols are hygroscopic and deliquesce at high humidity, particularly when depositing to leaf surfaces close to transpiring stomata. Deliquescence and subsequent salt creep may establish thin, extraordinary pathways into the stomata, which foster stomatal uptake of nutrients and water but may also cause stomatal liquid water loss by wicking. Such additional water loss is not accompanied by a wider stomatal aperture with a larger CO2 influx and hypothetically reduces water use efficiency (WUE). Here, the possible direct impacts of aerosols on physical and physiological parameters of camphor (Cinnamomum camphora) were studied (i) in a greenhouse experiment using aerosol exclusion and (ii) in a field study in Taiwan, comparing trees at two sites with different aerosol regimes. Scanning electron microscopy (SEM) images showed that leaves grown under aerosol exclusion in filtered air (FA) were lacking the amorphous, flat areas that were abundant on leaves grown in ambient air (AA), suggesting salt crusts formed from deliquescent aerosols. Increasing vapor pressure deficit (VPD) resulted in half the Ball-Berry slope and double WUE for AA compared to FA leaves. This apparent contradiction to the wicking hypothesis may be due to the independent, overcompensating effect of stomatal closure in response to VPD, which affects AA more than FA stomata. Compared to leaves in a more polluted region in the Taiwanese Southwest, NaCl aerosols dominated the leaf surface conditions on mature camphor trees in Eastern Taiwan, while the considerably lower contact angles and the 2.5 times higher minimum epidermal conductances might have come from organic surfactants. Interpretations of SEM images from leaf surface microstructures should consider amorphous areas as possible indicators of aerosol deposition and other hygroscopic material. The amount and type of the material determine the resulting impacts on plant water relations, together with the surrounding atmosphere and ecophysiological traits.

In this study two crop species, winter wheat (<i>Triticum aestivum</i>) and sugar beet (<i>Beta vulgaris</i>), were monitored over the course of five days during the entire season. We investigated the link of the main physiological leaf-level mechanisms, stomatal conductance and efficiency of photosynthetic energy conversion on canopy transpiration and photosynthetic CO₂ uptake. The physiological status of 900 leaves of different plants in a natural canopy was characterized on the leaf level using chlorophyll fluorescence. Gas exchange measurements were performed at leaves of 12 individual plants of each species. Eddy covariance flux measurements provided information on CO₂, water and energy fluxes on the field scale. The diurnal pattern of stomatal resistance on the leaf level was especially for sugar beet similar to the canopy resistance, which indicates that stomatal resistance may have a large impact on the bulk canopy resistance. The diurnal changes in canopy resistance appeared to have less effect on the evapotranspiration, which was mainly dependent on the amount of incoming radiation. The similar diurnal pattern of water use efficiency on the leaf level and on the canopy level during the day, underline the influence of physiological mechanisms of leaves on the canopy. The greatest difference between water use efficiency on leaf and canopy occurred in the morning, mainly due to an increase of stomatal resistance. Limitation of CO₂ uptake occurred in the afternoon when water vapor pressure deficit increased. Maxima of net ecosystem productivity corresponded to the highest values of photosynthetic capacity of single leaves, which occurred before solar noon. Within the course of a few hours, photosynthetic efficiency and stomatal resistance of leaves varied and these variations were the reason for diurnal variations in the carbon fluxes of the whole field. During the seasonal development, the leaf area index was the main factor driving carbon and water exchange, when both crops were still growing. During senescence of winter wheat these structural parameters did not account for changes in canopy fluxes and remaining high green leaf material of sugar beet did not present the reduction in canopy fluxes due to beginning dormancy. We thus hypothesize that the functional status of plants is also important to correctly predict carbon and water fluxes throughout the season. We propose to additionally include the physiological status of plants in carbon flux models in order to improve the quality of the simulation of diurnal patterns of carbon fluxes

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