Seasonal photosynthetic gas exchange and leaf ... - Tree Physiology

1040 LETTS, PHELAN, JOHNSON AND ROOD
measurements (Fv/Fm and ΦPSII) were made on the same
leaves sampled for leaf reflectance, with an FMS2 pulse modulated
chlorophyll fluorimeter (Hansatech Instruments, U.K.).
Before Fv/Fm was determined, all leaves were dark adapted for
a minimum of 30 min, using the leaf clip. About 30 s after
measuring Fv/Fm, ΦPSII was determined on the same leaf tissue
in full natural sunlight, rather than with the actinic light
source.
Stable carbon isotope composition
A selection of dried leaves gathered on DOY 172, 205 and 237
were chosen for N and δ 13 C analyses. Individual leaf samples
were immersed in liquid nitrogen and crushed with a mortar
and pestle. To determine stable carbon isotope composition
and total N concentration (%), tissue samples (about 5 mg)
were weighed into tin cups, and combusted in an elemental analyzer
(NC2100, CE Instruments, ThermoQuest Italia, Milan,
Italy), coupled to a gas isotope ratio mass spectrometer (Optima,VG
Isotech, Cheshire, U.K.) operating in continuous
flow mode. The δ 13 C was determined from the ratio, R
( 13 CO2/ 12 CO2):
δ 13
C(‰) = ⎛ R
⎜
⎝ R
sample
std
⎞
⎟ 1000
⎠
where Rstd refers to the molar ratio of the international Pee Dee
Belemnite standard.
Photosynthetic water-use efficiency
Photosynthetic water-use efficiency was calculated from both
gas exchange measurements and from the δ 13 C composition of
plant tissue. The former is an instantaneous measure of WUE,
and was determined as:
⎛ ci
⎞
ca
⎜1
− ⎟
A ⎝ ca
⎠
WUE = =
E 16 . v
where 1.6 is the ratio of the diffusion coefficients of H2O and
CO2 and v is related to the difference between intracellular and
atmospheric H2O vapor concentrations (ei – ea; kPa) and atmospheric
pressure, P (kPa) as:
e − e
v =
P
i a
The stable carbon isotope composition of leaf tissue, δ 13 Cp,
can be used to determine an integrated measure of WUE. The
stable carbon isotope composition of leaf material is related to
that of the source atmospheric CO2 (δ 13 Ca) and to ci/ca
(Farquhar et al. 1982, 1989):
(6)
(7)
(8)
13 13
δ C δ C a b a ci
p = a − −( − ) (9)
c
a
where δ 13 Ca is assumed to be –8‰, a is the discrimination during
diffusion of CO2 in air (4.4‰), b is net discrimination during
carboxylation (27.0‰) and ca is 380 µmol mol –1 , the mean
CO2 concentration in July 2006 at Mauna Loa, Hawaii (Tans
2007). We derived WUE from Equations 7 and 9 as (Ponton et
al. 2006):
13 13 ⎛ δ Cp − δ Ca + a⎞
ca − ca⎜
⎟
A ⎝ a − b ⎠
WUE = =
E
16 . v
(10)
Irrigation treatment
To examine the effect of soil water availability on photosynthetic
gas exchange characteristics of male and female cottonwoods,
45,000 l of water was pumped from the Oldman
River to the base of two female and two male trees on DOY
241. When irrigation was complete, the four trees were surrounded
by standing water extending at least 4 m from the base
of the trunk. On DOY 247, photosynthetic gas exchange and
leaf reflectance measurements were taken on three leaves of
each of the four irrigated and four non-irrigated trees.
Statistical analysis of physiological data
Between-sex differences in leaf photosynthetic gas exchange,
reflectance and fluorescence characteristics were assessed by
mixed model repeated measures analysis of variance
(ANOVA), with DOY as the repeated measure, sex as a fixed
effect, tree identity as a random effect, sex × DOY interaction
and post hoc evaluation by Tukey’s HSD test. Repeated measures
ANOVA was also used to assess seasonal differences in
photosynthetic gas exchange, foliar δ 13 C composition and Ψl
(Phelan 2007). One-tailed Student’s t-tests were used to test
two hypotheses: (1) females have significantly more conservative
water use indicators than males, based on stable carbon
isotope composition (higher δ 13 C and WUE, but lower ci/ca);
and (2) irrigated trees have higher gas exchange rates and
lower WUE (Amax/E and Amax/gs) than non-irrigated trees.
Results
Environmental conditions
TREE PHYSIOLOGY VOLUME 28, 2008
Total precipitation at PCCG was 178.8 mm during DOY 121–
247 (Table 1), with 76% occurring between DOY 146 and 171,
and 48% occurring in a single event on DOY 165–166. Thereafter,
rainfall was light, with all daily rainfall totals below
8 mm. In response to this rainfall pattern and variation in water
table depth, θv at 0.8-m depth was high in late June and early
July, but decreased continuously from a peak of 0.40 m 3 m –3
on DOY 170 through the months of July and August. Groundwater
depth fell from –0.73 to –2.34 m from DOY 170 to 247.
Total rainfall at PCCG was 23.5% below the 30-year mean for
Fort Macleod during the study period (Table 1; Environment
Canada 2005).
Daily maximum air temperature and vapor pressure deficit
were higher in July, August and early September than in May
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