How does the VPD response of isohydric and anisohydric plants depend on leaf surface particles?

Abstract

Atmospheric vapour pressure deficit (VPD) is the driving force for plant transpiration. Plants have different strategies to respond to this 'atmospheric drought'. Deposited aerosols on leaf surfaces can interact with plant water relations and may influence VPD response. We studied transpiration and water use efficiency of pine, beech and sunflower by measuring sap flow, gas exchange and carbon isotopes, thereby addressing different time scales of plant/atmosphere interaction. Plants were grown (i) outdoors under rainfall exclusion (OD) and in ventilated greenhouses with (ii) ambient air (AA) or (iii) filtered air (FA), the latter containing <1% ambient aerosol concentrations. In addition, some AA plants were sprayed once with 25 mM salt solution of (NH4 )2 SO4 or NaNO3 . Carbon isotope values (δ(13) C) became more negative in the presence of more particles; more negative for AA compared to FA sunflower and more negative for OD Scots pine compared to other growth environments. FA beech had less negative δ(13) C than AA, OD and NaNO3 -treated beech. Anisohydric beech showed linearly increasing sap flow with increasing VPD. The slopes doubled for (NH4 )2 SO4 - and tripled for NaNO3 -sprayed beech compared to control seedlings, indicating decreased ability to resist atmospheric demand. In contrast, isohydric pine showed constant transpiration rates with increasing VPD, independent of growth environment and spray, likely caused by decreasing gs with increasing VPD. Generally, NaNO3 spray had stronger effects on water relations than (NH4 )2 SO4 spray. The results strongly support the role of leaf surface particles as an environmental factor affecting plant water use. Hygroscopic and chaotropic properties of leaf surface particles determine their ability to form wicks across stomata. Such wicks enhance unproductive water loss of anisohydric plant species and decrease CO2 uptake of isohydric plants. They become more relevant with increasing number of fine particles and increasing VPD and are thus related to air pollution and climate change. Wicks cause a deviation from the analogy between CO2 and water pathways through stomata, bringing some principal assumptions of gas exchange theory into question.

Plants and Atmospheric Aerosols

“…In anisohydric beech seedlings, sap flow increased with increasing vapor pressure deficit (VPD) and responded to simulated aerosol deposition. The slope of the response of sap flow to VPD doubled following application of (NH 4 ) 2 SO 4 and tripled following spray application of NaNO 3 solution to the foliage of beech seedlings (Burkhardt and Pariyar 2016). In contrast, sap flow of isohydric pine seedlings was not correlated with VPD, and these aerosol applications had no effect on sap flow (Burkhardt and Pariyar 2016).…”

“…The slope of the response of sap flow to VPD doubled following application of (NH 4 ) 2 SO 4 and tripled following spray application of NaNO 3 solution to the foliage of beech seedlings (Burkhardt and Pariyar 2016). In contrast, sap flow of isohydric pine seedlings was not correlated with VPD, and these aerosol applications had no effect on sap flow (Burkhardt and Pariyar 2016). Nevertheless, in both species, and in sunflower, growth in ambient aerosolcontaining air, and simulated aerosol deposition with salt solutions, decreased (more negative) the values of δ 13 C compared to filtered air.…”

“…Nevertheless, in both species, and in sunflower, growth in ambient aerosolcontaining air, and simulated aerosol deposition with salt solutions, decreased (more negative) the values of δ 13 C compared to filtered air. This suggests reduced water use efficiency, but the mechanism in this case, and whether carbon assimilation declined or stomatal conductance increased, remains to be determined (Burkhardt and Pariyar 2016). The minimum epidermal conductance (g min ) is an important ecophysiological parameter affecting the drought tolerance of plants.…”

Plants and the changing environment

“…High temperatures may increase emissions of volatile organic compounds, which react in the lower atmosphere (Goh et al . ), while particulate pollution may interfere directly with plant ability to regulate the balance of water loss and carbon acquisition (Burkhardt & Pariyar ).…”

“…Positive feedbacks between plant responses to stress and physiological responses that may further accelerate climate change are also considered. High temperatures may increase emissions of volatile organic compounds, which react in the lower atmosphere (Goh et al 2016), while particulate pollution may interfere directly with plant ability to regulate the balance of water loss and carbon acquisition (Burkhardt & Pariyar 2016).…”