Because the acquisition and translocation of N through transpiration flux were
reduced in the field conditions, the chamber experiment was designed to study the
impact of N availability on the metabolism in more detail using two N fertilization
levels under low VPD conditions (Lihavainen et al. 2016b). Additional N fertilization mitigated the effects of low VPD on leaf metabolism. For example, high N
supply maintained the glutamine-to-glutamate ratio at the same levels as in the plants
under high VPD with moderate N fertilization. The chamber experiment verified
that metabolic responses to low VPD were mainly driven by the limitation in N
availability.
The chamber environment is also suitable for molecular studies. The gene
expression studies with birches indicated very complex genetic regulation system
related to N metabolism (Lihavainen et al. 2016b). The leaf mRNA levels of the key
enzymes, nitrate reductase (BpNIA) and nitrite reductase (BpNiR), were upregulated
in the low VPD conditions, while the N concentration of the leaves was reduced. The
complex regulation pathways may involve feedback inhibition of gene expressions
and the key role of enzyme activators such as molybdenum (Mo), which was at high
level in these low VPD plants. Further, enzyme levels do not always correlate
directly with the corresponding gene expression.
3.5 The Key Role of Leaf Surface
The surface of the leaves contains an epicuticular wax layer that is acting as a
protective barrier between the leaf tissue and the surrounding environment. The
chemical properties and structure of the cuticular wax layer are reactive to many
environmental and biotic factors, such as UV radiation, air pollutants, herbivores
and pathogens and air humidity. The cuticular wax layer of silver birch leaves is
characterized by a high amount of dammarane and epoxydammarane triterpenoid
aglycones, flavonoid aglycones and very long-chain aliphatic hydrocarbons, forming
a strong hydrophobic layer (Lihavainen et al. 2017). Leaves of many plant species
are covered by different kinds of trichomes. Silver birch leaves are densely covered
with glandular trichomes that store and excrete lipophilic compounds to the leaf
surface and provide chemical defence. Glandular trichomes show the highest metabolic activity in the young developing leaves (Valkama et al. 2004).
Both FAHM and laboratory studies (described above) indicated that the glandular
trichome density is smaller under low VPD (Lihavainen et al. 2017), correlating with
the amount of the major triterpenoid and flavonoid aglycones. There was some
variation in the trichome responses between the adaxial and abaxial surfaces of
the leaf, indicating differences in the function of the two leaf sides. The chamber
experiment revealed that the chemical properties of cuticular waxes are affected by
low VPD in silver birch although the total wax content may remain unaffected
(Lihavainen et al. 2017). The low content of alkanes and the low proportion of
highly hydrophobic flavonoids under the low VPD conditions reduce the hydrophobicity of the cuticular wax layer. The lower hydrophobicity of wax layer increases
Northern Forest Trees Under Increasing Atmospheric Humidity
329
reduced in the field conditions, the chamber experiment was designed to study the
impact of N availability on the metabolism in more detail using two N fertilization
levels under low VPD conditions (Lihavainen et al. 2016b). Additional N fertilization mitigated the effects of low VPD on leaf metabolism. For example, high N
supply maintained the glutamine-to-glutamate ratio at the same levels as in the plants
under high VPD with moderate N fertilization. The chamber experiment verified
that metabolic responses to low VPD were mainly driven by the limitation in N
availability.
The chamber environment is also suitable for molecular studies. The gene
expression studies with birches indicated very complex genetic regulation system
related to N metabolism (Lihavainen et al. 2016b). The leaf mRNA levels of the key
enzymes, nitrate reductase (BpNIA) and nitrite reductase (BpNiR), were upregulated
in the low VPD conditions, while the N concentration of the leaves was reduced. The
complex regulation pathways may involve feedback inhibition of gene expressions
and the key role of enzyme activators such as molybdenum (Mo), which was at high
level in these low VPD plants. Further, enzyme levels do not always correlate
directly with the corresponding gene expression.
3.5 The Key Role of Leaf Surface
The surface of the leaves contains an epicuticular wax layer that is acting as a
protective barrier between the leaf tissue and the surrounding environment. The
chemical properties and structure of the cuticular wax layer are reactive to many
environmental and biotic factors, such as UV radiation, air pollutants, herbivores
and pathogens and air humidity. The cuticular wax layer of silver birch leaves is
characterized by a high amount of dammarane and epoxydammarane triterpenoid
aglycones, flavonoid aglycones and very long-chain aliphatic hydrocarbons, forming
a strong hydrophobic layer (Lihavainen et al. 2017). Leaves of many plant species
are covered by different kinds of trichomes. Silver birch leaves are densely covered
with glandular trichomes that store and excrete lipophilic compounds to the leaf
surface and provide chemical defence. Glandular trichomes show the highest metabolic activity in the young developing leaves (Valkama et al. 2004).
Both FAHM and laboratory studies (described above) indicated that the glandular
trichome density is smaller under low VPD (Lihavainen et al. 2017), correlating with
the amount of the major triterpenoid and flavonoid aglycones. There was some
variation in the trichome responses between the adaxial and abaxial surfaces of
the leaf, indicating differences in the function of the two leaf sides. The chamber
experiment revealed that the chemical properties of cuticular waxes are affected by
low VPD in silver birch although the total wax content may remain unaffected
(Lihavainen et al. 2017). The low content of alkanes and the low proportion of
highly hydrophobic flavonoids under the low VPD conditions reduce the hydrophobicity of the cuticular wax layer. The lower hydrophobicity of wax layer increases
Northern Forest Trees Under Increasing Atmospheric Humidity
329