161
6.3.2 Variation Between Humid and Xeric Populations
The architecture of the aerial part of trees describes the spatial location of photosynthesizing organs for light uptake, which is particularly delicate during recruitment.
Filotaxis and branching are its most important traits and reflect the habitat preference of a species. Pastorino et al. (2010a) carried out a study, under controlled
greenhouse conditions, of the genetic variation in seedling architecture, with the
objective of determining the degree of differentiation between – and the variability
within – populations contrasting in aridity (forest type). For this study, they used
840 1-year-old plants from 28 open-pollinated families corresponding to 2 xeric
(Pilcañeu Norte, Pilcañeu Sur) and 2 humid (Cerro Catedral, Río Azul) populations
of cypress (Table 6.1). Eight traits measuring size, vigor, and architectural balances
between axes were measured and analyzed using linear mixed effect models, considering the genetic effects of population and half-sib family. The main result of this
experiment was that the degree of genetic differentiation between populations was
very low (Q ST = 0.088 on average), virtually equal to the degree of neutral differentiation of the species (Pastorino et al. 2004). The additive variances and the heritability of the characters were moderate to high (h
2
= 0.15–0.53), but the genetic
variability was not associated to the forest type.
In another experiment, Pastorino et al. (2010b) analyzed the variation of tolerance to a simulated drought in 3-year-old seedlings from two humid (Cerro Catedral,
Río Azul) and six mesic (El Maitén, Cañada Molina) to xeric (Pilcañeu Norte,
Pilcañeu Sur, Leleque, Chacabuco) Argentine populations (Table 6.4). For each of
the two most arid populations (PN and PS), material from ten open- pollinated families was included in the design, in order to quantify their intra- population variance.
The seedlings were grown in pots, to which irrigation was suppressed for 43 days at
late spring. During that period, the functioning of the photosystems was evaluated
weekly through the maximum quantum yield of the photosystem-II: Fv/Fm (Krause
and Weis 1991), which indicates the degree of stress a plant is subjected to. Seedling
survival was evaluated after a recovery period of 8 months. The authors have found
significant differences between populations for Fv/Fm and survival, although these
differences were not attributable to the type of population (rainfall regimes). The
family effect within populations PN and PS was significant for survival and Fv/Fm,
but for this last trait significance was largely influenced by the effect of one family,
which showed an exceptional mean drought tolerance. The results of this experiment suggest that the regional variation of water stress tolerance (evaluated according to the Fv/Fm parameter) would not be modeled by divergent selection and that
there could be some degree of heritable variation within xeric populations.
Long-term water-use efficiency (WUE, defined as the ratio of total plant dry matter produced to total water used over the same period) is a key physiological trait in
trees, especially in semiarid Mediterranean climates. It can be estimated indirectly
via carbon isotopic discrimination (Δ). Plants with C3 metabolism discriminate
against the
13
C isotope because it is heavier than
12
C, which is then preferred by the
carboxylation enzyme (Rubisco) and also has faster diffusion in the air (Farquhar
6 Patagonian Cypress (Austrocedrus chilensis): The Cedarwood…
6.3.2 Variation Between Humid and Xeric Populations
The architecture of the aerial part of trees describes the spatial location of photosynthesizing organs for light uptake, which is particularly delicate during recruitment.
Filotaxis and branching are its most important traits and reflect the habitat preference of a species. Pastorino et al. (2010a) carried out a study, under controlled
greenhouse conditions, of the genetic variation in seedling architecture, with the
objective of determining the degree of differentiation between – and the variability
within – populations contrasting in aridity (forest type). For this study, they used
840 1-year-old plants from 28 open-pollinated families corresponding to 2 xeric
(Pilcañeu Norte, Pilcañeu Sur) and 2 humid (Cerro Catedral, Río Azul) populations
of cypress (Table 6.1). Eight traits measuring size, vigor, and architectural balances
between axes were measured and analyzed using linear mixed effect models, considering the genetic effects of population and half-sib family. The main result of this
experiment was that the degree of genetic differentiation between populations was
very low (Q ST = 0.088 on average), virtually equal to the degree of neutral differentiation of the species (Pastorino et al. 2004). The additive variances and the heritability of the characters were moderate to high (h
2
= 0.15–0.53), but the genetic
variability was not associated to the forest type.
In another experiment, Pastorino et al. (2010b) analyzed the variation of tolerance to a simulated drought in 3-year-old seedlings from two humid (Cerro Catedral,
Río Azul) and six mesic (El Maitén, Cañada Molina) to xeric (Pilcañeu Norte,
Pilcañeu Sur, Leleque, Chacabuco) Argentine populations (Table 6.4). For each of
the two most arid populations (PN and PS), material from ten open- pollinated families was included in the design, in order to quantify their intra- population variance.
The seedlings were grown in pots, to which irrigation was suppressed for 43 days at
late spring. During that period, the functioning of the photosystems was evaluated
weekly through the maximum quantum yield of the photosystem-II: Fv/Fm (Krause
and Weis 1991), which indicates the degree of stress a plant is subjected to. Seedling
survival was evaluated after a recovery period of 8 months. The authors have found
significant differences between populations for Fv/Fm and survival, although these
differences were not attributable to the type of population (rainfall regimes). The
family effect within populations PN and PS was significant for survival and Fv/Fm,
but for this last trait significance was largely influenced by the effect of one family,
which showed an exceptional mean drought tolerance. The results of this experiment suggest that the regional variation of water stress tolerance (evaluated according to the Fv/Fm parameter) would not be modeled by divergent selection and that
there could be some degree of heritable variation within xeric populations.
Long-term water-use efficiency (WUE, defined as the ratio of total plant dry matter produced to total water used over the same period) is a key physiological trait in
trees, especially in semiarid Mediterranean climates. It can be estimated indirectly
via carbon isotopic discrimination (Δ). Plants with C3 metabolism discriminate
against the
13
C isotope because it is heavier than
12
C, which is then preferred by the
carboxylation enzyme (Rubisco) and also has faster diffusion in the air (Farquhar
6 Patagonian Cypress (Austrocedrus chilensis): The Cedarwood…