Aerosol Impacts on Water Relations of Camphor (Cinnamomum camphora)

Ambient aerosols increase stomatal transpiration and conductance of hydroponic sunflowers by extending the hydraulic system to the leaf surface

Introduction

Many atmospheric aerosols are hygroscopic and play an important role in cloud formation. Similarly, aerosols become sites of micro-condensation when they deposit to the upper and lower surfaces of leaves. Deposited salts, in particular can trigger condensation at humidities considerably below atmospheric saturation, according to their hygroscopicity and the relative humidity within the leaf boundary layer. Salt induced water potential gradients and the resulting dynamics of concentrated salt solutions can be expected to affect plant water relations.

Methods

Hydroponic sunflowers were grown in filtered (FA) and unfiltered, ambient air (AA). Sap flow was measured for 18 days and several indicators of incipient drought stress were studied.

Results

At 2% difference in mean vapor pressure deficit (D), AA sunflowers had 49% higher mean transpiration rates, lower osmotic potential, higher proline concentrations, and different tracer transport patterns in the leaf compared to FA sunflowers. Aerosols increased plant conductance particularly at low D.

Discussion

The proposed mechanism is that thin aqueous films of salt solutions from deliquescent deposited aerosols enter into stomata and cause an extension of the hydraulic system. This hydraulic connection leads - parallel to stomatal water vapor transpiration - to wick-like stomatal loss of liquid water and to a higher impact of D on plant water loss. Due to ample water supply by hydroponic cultivation, AA plants thrived as well as FA plants, but under more challenging conditions, aerosol deposits may make plants more susceptible to drought stress.

Nocturnal Transpiration May Be Associated with Foliar Nutrient Uptake

Aerosols can contribute to plant nutrition via foliar uptake. The conditions for this are best at night because the humidity is high and hygroscopic, saline deposits can deliquesce as a result. Still, stomata tend to be closed at night to avoid unproductive water loss. However, if needed, nutrients are on the leaf surface, and plants could benefit from nocturnal stomatal opening because it further increases humidity in the leaf boundary layer and allows for stomatal nutrient uptake. We tested this hypothesis on P-deficient soil by comparing the influence of ambient aerosols and additional foliar P application on nocturnal transpiration. We measured various related leaf parameters, such as the foliar water loss, minimum leaf conductance (g_min), turgor loss point, carbon isotope ratio, contact angle, specific leaf area (SLA), tissue element concentration, and stomatal and cuticular characteristics. For untreated leaves grown in filtered, aerosol-free air (FA), nocturnal transpiration consistently decreased overnight, which was not observed for leaves grown in unfiltered ambient air (AA). Foliar application of a soluble P salt increased nocturnal transpiration for AA and FA leaves. Crusts on stomatal rims were shown by scanning electron microscopy, supporting the idea of stomatal uptake of deliquescent salts. Turgor loss point and leaf moisture content indicated a higher accumulation of solutes, due to foliar uptake by AA plants than FA plants. The hypothesis that deliquescent leaf surface salts may play a role in triggering nocturnal transpiration was supported by the results. Still, further experiments are required to characterize this phenomenon better.