ammonium assimilation rate. Carbon was allocated to the biosynthesis of secondary
compounds through the phenylpropanoid pathway leading to increased production
of quinic acid derivatives, catechins, hemiterpenoid glycoside, quercetin glycosides
and other phenolic glycosides. The increased production of these secondary compounds indicates the activation of defence mechanisms against a stress situation,
i.e. nutrient deficiency and soil hypoxia in this case (Sellin et al. 2015). Many
secondary compounds act as antioxidants against oxidative stress under unfavourable
environmental conditions. In addition to the phenolic compounds, elevated levels of
ascorbate metabolites and alpha-tocopherol were observed in the leaves under low
VPD treatment indicating the development of antioxidative defence.
At the ecosystem level, high content of nonstructural carbohydrates and phenolics
in the leaf litter is known to slow down the decomposition rate (e.g. Tuchman et al.
2003). In 2010, tree leaf litter flux was lower under aspen stands in the humidified
plots than in the control plots (Torga et al. 2017). Therefore, increasing air humidity
may affect the carbon and nutrient cycling and eventually the primary production of
the northern forest ecosystems.
3.4 Laboratory Studies Confirming the Field Results
For the mechanistic understanding, it is necessary to compare and verify the results
from the field experiments with the more strictly controlled laboratory studies,
where the environmental ‘noise’ (e.g. change in weather conditions) is minimized.
According to the FAHM experiment in the field, high humidity promoted branching
and reduced the slenderness of silver birch plants and altered N allocation pattern
among plant organs and within crown (Lihavainen et al. 2016b). The foliar concentrations of starch, quercetin glycosides and glutamate were higher in silver birch
plants growing under low VPD than under high VPD in a short-term chamber
experiment, while the levels of shikimic acid, ribonic acid, amino acids alanine
and valine, several citric acid cycle intermediates as well as glutamine-to-glutamate
ratio were lower (Lihavainen et al. 2016b).
The chamber study revealed several differences in the responses between the
seedlings and the larger trees and/or experimental systems. For example, enhanced
growth appeared to be an initial response in the chamber experiment, accompanied
by the efficient recycling of N, and enhanced root and stem radial growth
(Lihavainen et al. 2016b), which could compensate for the reduced transpiration
flux and nutrient acquisition under high humidity. Mineral nutrient uptake under low
VPD could be enhanced by the higher number of root tips, larger absorptive root
area and transpiring leaf area (Kupper et al. 2017). In the chamber study, carbon
accumulation in the form of poly- and oligosaccharides in low VPD-grown leaves
was indicated as increased raffinose content, which may reflect impaired phloem
loading process (Lihavainen et al. 2016b). The development of antioxidative defence
under low VPD, on the other hand, was obvious only in the long-term field
experiment (Lihavainen et al. 2016a).
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E. Oksanen et al.
compounds through the phenylpropanoid pathway leading to increased production
of quinic acid derivatives, catechins, hemiterpenoid glycoside, quercetin glycosides
and other phenolic glycosides. The increased production of these secondary compounds indicates the activation of defence mechanisms against a stress situation,
i.e. nutrient deficiency and soil hypoxia in this case (Sellin et al. 2015). Many
secondary compounds act as antioxidants against oxidative stress under unfavourable
environmental conditions. In addition to the phenolic compounds, elevated levels of
ascorbate metabolites and alpha-tocopherol were observed in the leaves under low
VPD treatment indicating the development of antioxidative defence.
At the ecosystem level, high content of nonstructural carbohydrates and phenolics
in the leaf litter is known to slow down the decomposition rate (e.g. Tuchman et al.
2003). In 2010, tree leaf litter flux was lower under aspen stands in the humidified
plots than in the control plots (Torga et al. 2017). Therefore, increasing air humidity
may affect the carbon and nutrient cycling and eventually the primary production of
the northern forest ecosystems.
3.4 Laboratory Studies Confirming the Field Results
For the mechanistic understanding, it is necessary to compare and verify the results
from the field experiments with the more strictly controlled laboratory studies,
where the environmental ‘noise’ (e.g. change in weather conditions) is minimized.
According to the FAHM experiment in the field, high humidity promoted branching
and reduced the slenderness of silver birch plants and altered N allocation pattern
among plant organs and within crown (Lihavainen et al. 2016b). The foliar concentrations of starch, quercetin glycosides and glutamate were higher in silver birch
plants growing under low VPD than under high VPD in a short-term chamber
experiment, while the levels of shikimic acid, ribonic acid, amino acids alanine
and valine, several citric acid cycle intermediates as well as glutamine-to-glutamate
ratio were lower (Lihavainen et al. 2016b).
The chamber study revealed several differences in the responses between the
seedlings and the larger trees and/or experimental systems. For example, enhanced
growth appeared to be an initial response in the chamber experiment, accompanied
by the efficient recycling of N, and enhanced root and stem radial growth
(Lihavainen et al. 2016b), which could compensate for the reduced transpiration
flux and nutrient acquisition under high humidity. Mineral nutrient uptake under low
VPD could be enhanced by the higher number of root tips, larger absorptive root
area and transpiring leaf area (Kupper et al. 2017). In the chamber study, carbon
accumulation in the form of poly- and oligosaccharides in low VPD-grown leaves
was indicated as increased raffinose content, which may reflect impaired phloem
loading process (Lihavainen et al. 2016b). The development of antioxidative defence
under low VPD, on the other hand, was obvious only in the long-term field
experiment (Lihavainen et al. 2016a).
328
E. Oksanen et al.