et al. 2015). A study on wheat revealed that stomatal density was reduced under low
nighttime VPD only in the leaf adaxial epidermis (Claverie et al. 2016).
An anatomical study showed that the leaves of some plant species were thicker
and the fractional air space of leaf mesophyll was generally larger under low than
high VPD (Leuschner 2002) promoting the diffusion of water and CO 2 in the leaf.
Severe disorders in leaf and fruit formation have been reported, for example, in
cucumber (Cucumis sativus) plants, due to calcium deficiency elicited by high
humidity (Staub and Navazio 1993). High air humidity may enhance root growth,
as seen in studies with root and tuber crops, such as potato (Solanum tuberosum)
(Wheeler et al. 1989) and peanut (Arachis hypogaea) (Mortley et al. 2000).
Although low nighttime VPD does not affect the growth of wheat, it affects the
hydraulic properties of roots and leaves (Claverie et al. 2016).
2.4 Biochemical and Molecular Changes
Most air humidity studies have focused on growth responses, stomatal regulation,
morphological changes and plant nutrition, but very few have focused on biochemical or molecular changes (Koch et al. 2006; Devi et al. 2015b; Georgii et al. 2017).
Low VPD was found to alter the epicuticular wax layer composition in Brassica
oleracea (Koch et al. 2006), which may affect cuticular permeability to water and
leaf wettability. However, in the same experiment, two other species, Eucalyptus
gunnii and Tropaeolum majus, were unaffected. There is evidence that short-term
exposure to different VPD conditions affects the gene expression patterns both in the
roots and the shoot (Levin et al. 2009; Devi et al. 2015b) and starch and sucrose
metabolism in the leaves (Shirke and Pathre 2004). The changes in plant
metabolome, downstream from transcription and translation, are the end results of
altered transcription and enzyme activities and thus the basis of plant phenotypic
responses to changing environment.
2.5 Interaction with Abiotic Stresses
The studies on the interaction between VPD and environmental stress factors are
scarce; some recent studies have focused on VPD effects under drought, heat and salt
stresses. Low VPD increases cuticular permeability and impairs stomatal functioning, exposing the plants to excess water loss and desiccation (Fanourakis et al. 2016).
The stomatal acclimation to VPD conditions modifies the tree responses to drought
and temperature (Marchin et al. 2016). Rise in air temperature increases VPD, and
accordingly, VPD plays an important role in heat stress, especially through hormonal
signalling (Georgii et al. 2017). The high VPD during heat stress was the major
driver of metabolic responses in Arabidopsis thaliana (Georgii et al. 2017). It was
also shown that low VPD reduced the shift in the metabolite profile that occurred
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E. Oksanen et al.
nighttime VPD only in the leaf adaxial epidermis (Claverie et al. 2016).
An anatomical study showed that the leaves of some plant species were thicker
and the fractional air space of leaf mesophyll was generally larger under low than
high VPD (Leuschner 2002) promoting the diffusion of water and CO 2 in the leaf.
Severe disorders in leaf and fruit formation have been reported, for example, in
cucumber (Cucumis sativus) plants, due to calcium deficiency elicited by high
humidity (Staub and Navazio 1993). High air humidity may enhance root growth,
as seen in studies with root and tuber crops, such as potato (Solanum tuberosum)
(Wheeler et al. 1989) and peanut (Arachis hypogaea) (Mortley et al. 2000).
Although low nighttime VPD does not affect the growth of wheat, it affects the
hydraulic properties of roots and leaves (Claverie et al. 2016).
2.4 Biochemical and Molecular Changes
Most air humidity studies have focused on growth responses, stomatal regulation,
morphological changes and plant nutrition, but very few have focused on biochemical or molecular changes (Koch et al. 2006; Devi et al. 2015b; Georgii et al. 2017).
Low VPD was found to alter the epicuticular wax layer composition in Brassica
oleracea (Koch et al. 2006), which may affect cuticular permeability to water and
leaf wettability. However, in the same experiment, two other species, Eucalyptus
gunnii and Tropaeolum majus, were unaffected. There is evidence that short-term
exposure to different VPD conditions affects the gene expression patterns both in the
roots and the shoot (Levin et al. 2009; Devi et al. 2015b) and starch and sucrose
metabolism in the leaves (Shirke and Pathre 2004). The changes in plant
metabolome, downstream from transcription and translation, are the end results of
altered transcription and enzyme activities and thus the basis of plant phenotypic
responses to changing environment.
2.5 Interaction with Abiotic Stresses
The studies on the interaction between VPD and environmental stress factors are
scarce; some recent studies have focused on VPD effects under drought, heat and salt
stresses. Low VPD increases cuticular permeability and impairs stomatal functioning, exposing the plants to excess water loss and desiccation (Fanourakis et al. 2016).
The stomatal acclimation to VPD conditions modifies the tree responses to drought
and temperature (Marchin et al. 2016). Rise in air temperature increases VPD, and
accordingly, VPD plays an important role in heat stress, especially through hormonal
signalling (Georgii et al. 2017). The high VPD during heat stress was the major
driver of metabolic responses in Arabidopsis thaliana (Georgii et al. 2017). It was
also shown that low VPD reduced the shift in the metabolite profile that occurred
322
E. Oksanen et al.