potassium (K), have positive association with the transpiration rate (McDonald et al.
2002; Novák and Vidovič 2003; Cernusak et al. 2011).
The transpiration rate affects leaf temperature and energy budget by evaporative
cooling. Restrained transpiration cooling under low VPD can raise leaf temperature
(Bakker 1991). On the other hand, misting or fogging is used to reduce transpiration
rate and leaf temperature to promote the production of crops in greenhouse cultivation (Suzuki et al. 2015; Leyva et al. 2015; Zhang et al. 2017b). Besides the climatic
factors (radiation, temperature, wind, VPD), water evaporation from plants depends
on soil water content, plant structure (canopy density, leaf area, hydraulic architecture), cuticle properties and stomatal conductance (Monteith 1995; Sellin and
Lubenets 2010). The water fluxes and related uptake of solutes can also be promoted
by water channel proteins, namely aquaporins, located in the plasma membranes of
root and leaf mesophyll cells (Kaldenhoff et al. 1998). The expression of aquaporincoding genes was affected by high VPD in several species such as Arabidopsis
thaliana (Levin et al. 2009) and hybrid poplar (Populus trichocarpa  deltoides)
(Laur and Hacke 2013). Larger endodermal cells, thicker root endodermis and higher
xylem exudation rate in plants grown under low VPD compared with high VPD can
reflect the increased abundance or activity of aquaporins (Claverie et al. 2016;
Kholová et al. 2016).
2.3 Growth and Development
Different plant species and genotypes vary in their responses to changing VPD.
There is evident genotypic variation in stomatal and transpiration responses to VPD
in beans (Phaseolus), maize (Zea mays) and wheat (Triticum aestivum) (Ryan et al.
2016; Claverie et al. 2016; Medina et al. 2017). The differences in water use
characteristics will eventually manifest in the growth and development of plants.
Accordingly, low VPD has been shown to affect plant growth and leaf size as well as
leaf development in several herbaceous and woody species varying from positive
effects (e.g. Leuschner 2002; Lendzion and Leuschner 2008) to negative impact
(Holder and Cockshull 1990). In some cases, high humidity may enhance the growth
rate at the beginning of the experiment (Armstrong and Kirkby 1979).
Air humidity affects the leaf morphology by modifying mesophyll structure, cell
expansion and the formation of leaf veins. Cell expansion and cell division are often
promoted under low VPD due to the high turgor of leaf cells (Leuschner 2002;
Carins Murphy et al. 2014). Expansin and extensin genes, which are related to cell
wall loosening and expansion/elongation of plant organs, are affected by changing
VPD in soybean (Glycine max) leaves (Devi et al. 2015a). Larger leaf area in
woodland herbaceous species in low than high VPD was a consequence of promoted
epidermal cell expansion (Leuschner 2002). On the other hand, promoted cell
division seemed to explain large leaf area observed in myrtle beech (Nothofagus
cunninghamii) under low VPD (Hovenden et al. 2012). Leaf expansion can decrease
the density of stomata, leaf veins and trichomes (Carins Murphy et al. 2014; Shibuya
Northern Forest Trees Under Increasing Atmospheric Humidity
321
2002; Novák and Vidovič 2003; Cernusak et al. 2011).
The transpiration rate affects leaf temperature and energy budget by evaporative
cooling. Restrained transpiration cooling under low VPD can raise leaf temperature
(Bakker 1991). On the other hand, misting or fogging is used to reduce transpiration
rate and leaf temperature to promote the production of crops in greenhouse cultivation (Suzuki et al. 2015; Leyva et al. 2015; Zhang et al. 2017b). Besides the climatic
factors (radiation, temperature, wind, VPD), water evaporation from plants depends
on soil water content, plant structure (canopy density, leaf area, hydraulic architecture), cuticle properties and stomatal conductance (Monteith 1995; Sellin and
Lubenets 2010). The water fluxes and related uptake of solutes can also be promoted
by water channel proteins, namely aquaporins, located in the plasma membranes of
root and leaf mesophyll cells (Kaldenhoff et al. 1998). The expression of aquaporincoding genes was affected by high VPD in several species such as Arabidopsis
thaliana (Levin et al. 2009) and hybrid poplar (Populus trichocarpa  deltoides)
(Laur and Hacke 2013). Larger endodermal cells, thicker root endodermis and higher
xylem exudation rate in plants grown under low VPD compared with high VPD can
reflect the increased abundance or activity of aquaporins (Claverie et al. 2016;
Kholová et al. 2016).
2.3 Growth and Development
Different plant species and genotypes vary in their responses to changing VPD.
There is evident genotypic variation in stomatal and transpiration responses to VPD
in beans (Phaseolus), maize (Zea mays) and wheat (Triticum aestivum) (Ryan et al.
2016; Claverie et al. 2016; Medina et al. 2017). The differences in water use
characteristics will eventually manifest in the growth and development of plants.
Accordingly, low VPD has been shown to affect plant growth and leaf size as well as
leaf development in several herbaceous and woody species varying from positive
effects (e.g. Leuschner 2002; Lendzion and Leuschner 2008) to negative impact
(Holder and Cockshull 1990). In some cases, high humidity may enhance the growth
rate at the beginning of the experiment (Armstrong and Kirkby 1979).
Air humidity affects the leaf morphology by modifying mesophyll structure, cell
expansion and the formation of leaf veins. Cell expansion and cell division are often
promoted under low VPD due to the high turgor of leaf cells (Leuschner 2002;
Carins Murphy et al. 2014). Expansin and extensin genes, which are related to cell
wall loosening and expansion/elongation of plant organs, are affected by changing
VPD in soybean (Glycine max) leaves (Devi et al. 2015a). Larger leaf area in
woodland herbaceous species in low than high VPD was a consequence of promoted
epidermal cell expansion (Leuschner 2002). On the other hand, promoted cell
division seemed to explain large leaf area observed in myrtle beech (Nothofagus
cunninghamii) under low VPD (Hovenden et al. 2012). Leaf expansion can decrease
the density of stomata, leaf veins and trichomes (Carins Murphy et al. 2014; Shibuya
Northern Forest Trees Under Increasing Atmospheric Humidity
321