Aerosol Impacts on Water Relations of Camphor (Cinnamomum camphora)

Abstract

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.

Nocturnal Transpiration May Be Associated with Foliar Nutrient Uptake

“…Clones from the same species were used to minimize the impact of genetic variability. The combination of aerosol exclusion (the AA/FA approach) and the additional amendment of aerosols were implemented to induce different amounts of leaf surface deposited material, thus combining two previously successful approaches for testing the ecophysiological impact of aerosols [ 14 , 60 ].…”

“…High concentrations of macronutrients were not detected, but Fe, Zn, and Cu concentrations were significantly higher in AA than in FA leaves and might indicate higher foliar nutrient uptake ( Table 3 ). Aerosol composition in the same two greenhouses was previously studied [ 14 ], and it was shown in their results that the aerosol load is mainly composed of nitrate, sulfate, and chloride. At the same time, Na, K, Mg, and ammonium are present in lower amounts.…”

“…Regarding minimum leaf conductance (g min ), it has been shown in previous studies that aerosols may cause leaky stomata and increase g min [ 13 ]. However, the effect on g min might depend on the nature of the aerosol and the species analyzed [ 14 , 82 , 83 ]. Here, g min did not differ significantly with aerosol load ( Table 1 ).…”

“…Between 25% and 50% of European aerosols are ionic and hygroscopic [ 9 ], meaning they can deliquesce and form concentrated salt solutions at humidities below saturation. Previously, the indirect effect of aerosols on the water cycle [ 10 ], radiation [ 11 ], nutrient cycles [ 12 ], and their direct effect on leaf surfaces [ 13 , 14 ] have been assessed.…”

“…On the other hand, HAS may also contribute to stomatal nutrient uptake. However, the results regarding HAS are controversial [ 14 , 16 ], and more studies are needed to understand this process better.…”