2 High Humidity Affects Physiology, Biochemistry
and the Structure of Plants
2.1 Stomata
Stomata are key players in regulating the water and carbon fluxes of plants. For
maintaining photosynthesis, plants need to keep their stomata open to facilitate
carbon dioxide (CO 2 ) influx to the mesophyll. However, at the same time, water
vapour evaporates through the open pores during the transpiration process. Increasing RH decreases the evaporative demand by reducing the VPD gradient between
leaf interior and surrounding air. Low VPD promotes stomatal opening and facilitates CO 2 influx to the leaf mesophyll, which can enhance photosynthesis (e.g. Dai
et al. 1992) together with the relatively small loss of water.
Apart from VPD and leaf turgor, stomata respond to changes in light, CO 2
concentration and metabolic signals (Shimazaki et al. 2007; Sellin et al. 2010; Niglas
et al. 2017). Plant responses to changing VPD are tightly associated with stomatal
regulation and abscisic acid (ABA) signalling (Thioune et al. 2017). It has been
shown that ABA levels are low and stomatal development may be disturbed by low
VPD in growth chamber or greenhouse conditions: plants produce large stomata that
are unable to close in response to normal closing stimuli such as darkness, high VPD
or ABA (Torre et al. 2003; Fanourakis et al. 2013; Aliniaeifard and van Meeteren
2013). On the contrary, in field conditions, trees grown under decreased VPD exhibit
higher stomatal sensitivity to changing VPD (Li and Li 2014; Niglas et al. 2015),
indicating the complex nature of stomatal reactions. It appears to be the daily
fluctuation of VPD that maintains stomatal responsiveness to closing stimuli (Arve
et al. 2017).
2.2 Transpiration and Mineral Nutrient Uptake
Although reduced transpiration rate under low VPD can ameliorate the water status
of plants and protect them from the water deficit, it may hinder mineral nutrient
uptake by the mass flow of water (Barber 1995; Tinker and Nye 2001). The mass
flow of water brings mineral nutrients and other solutes in contact with the root
system enabling their uptake. Therefore, the influence of air humidity on plants and
mineral nutrient uptake can depend on the nutrient and water status of the soil
(Osonubi and Davies 1980).
Several experiments have demonstrated a positive relationship between the
transpiration rate and mineral nutrition of plants (McDonald et al. 2002; Cernusak
et al. 2011). The decrease of calcium (Ca) and magnesium (Mg) concentrations in
leaves and fruits is the most commonly reported effect of high humidity (Armstrong
and Kirkby 1979; Adams and Holder 1992; Suzuki et al. 2015), but also the
concentrations of important macronutrients, nitrogen (N), phosphorus (P) and
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E. Oksanen et al.
and the Structure of Plants
2.1 Stomata
Stomata are key players in regulating the water and carbon fluxes of plants. For
maintaining photosynthesis, plants need to keep their stomata open to facilitate
carbon dioxide (CO 2 ) influx to the mesophyll. However, at the same time, water
vapour evaporates through the open pores during the transpiration process. Increasing RH decreases the evaporative demand by reducing the VPD gradient between
leaf interior and surrounding air. Low VPD promotes stomatal opening and facilitates CO 2 influx to the leaf mesophyll, which can enhance photosynthesis (e.g. Dai
et al. 1992) together with the relatively small loss of water.
Apart from VPD and leaf turgor, stomata respond to changes in light, CO 2
concentration and metabolic signals (Shimazaki et al. 2007; Sellin et al. 2010; Niglas
et al. 2017). Plant responses to changing VPD are tightly associated with stomatal
regulation and abscisic acid (ABA) signalling (Thioune et al. 2017). It has been
shown that ABA levels are low and stomatal development may be disturbed by low
VPD in growth chamber or greenhouse conditions: plants produce large stomata that
are unable to close in response to normal closing stimuli such as darkness, high VPD
or ABA (Torre et al. 2003; Fanourakis et al. 2013; Aliniaeifard and van Meeteren
2013). On the contrary, in field conditions, trees grown under decreased VPD exhibit
higher stomatal sensitivity to changing VPD (Li and Li 2014; Niglas et al. 2015),
indicating the complex nature of stomatal reactions. It appears to be the daily
fluctuation of VPD that maintains stomatal responsiveness to closing stimuli (Arve
et al. 2017).
2.2 Transpiration and Mineral Nutrient Uptake
Although reduced transpiration rate under low VPD can ameliorate the water status
of plants and protect them from the water deficit, it may hinder mineral nutrient
uptake by the mass flow of water (Barber 1995; Tinker and Nye 2001). The mass
flow of water brings mineral nutrients and other solutes in contact with the root
system enabling their uptake. Therefore, the influence of air humidity on plants and
mineral nutrient uptake can depend on the nutrient and water status of the soil
(Osonubi and Davies 1980).
Several experiments have demonstrated a positive relationship between the
transpiration rate and mineral nutrition of plants (McDonald et al. 2002; Cernusak
et al. 2011). The decrease of calcium (Ca) and magnesium (Mg) concentrations in
leaves and fruits is the most commonly reported effect of high humidity (Armstrong
and Kirkby 1979; Adams and Holder 1992; Suzuki et al. 2015), but also the
concentrations of important macronutrients, nitrogen (N), phosphorus (P) and
320
E. Oksanen et al.