Leaf physiology
Pre-dawn leaf water potential (ΨPD), photosynthesis
under saturated light intensity (A area, µmol
m-2 s-1), stomatal conductance
(g s, mol m-2s-1), and the maximum and operating efficiencies of
photosystem II were measured between July 21 and August 26, 2018. All
traits were measured during 12 sessions. Each session consisted of two
steps that were executed on the same day. First, ΨPD was
measured from each of three trees growing in one mesocosm of each
treatment combination. The selected trees were evenly distributed within
a mesocosm and ΨPD from at least one diploid and one
triploid tree was measured per mesocosm. ΨPD was
measured between 0400-0530 h using a pressure chamber (PMS Instrument
Company, Albany OR, USA). In one additional session, we quantified
ΨPD and leaf physiological traits from only two
mesocosms, of which one was exposed to drought stress and defoliation
and the other was exposed to drought stress only. During the experiment,
every mesocosm was selected two or three times for measurements.
Mesocosms subjected to drought stress were measured at different stages
of soil dry down.
In a second step, leaf gas-exchange and variable chlorophyll
fluorescence were measured on fully expanded, young leaves from all
trees growing in the previously selected mesocosms using an LI-6400XT
portable photosynthesis system with a broadleaf cuvette and a pulse
amplitude modulation (PAM) fluorometer (Li-Cor Biosciences, Lincoln, NE,
USA). During measurements, leaves in the cuvette were exposed to a
photosynthetic photon flux of 1800 µmol m-2s-1. The reference CO2 partial
pressure was 40 Pa, and the cuvette temperature was set at 25 °C. We did
not control vapor pressure deficit between leaf and cuvette air, which
ranged from 1.4–1.9 kPa. Measurements were taken on sunlit foliage
between 0900-1200h. From August 18 through August 25, gas exchange was
also measured at a cuvette CO2 partial pressure
(p CO2) ranging from 7.5 to 25 Pa on sunlit
foliage from all trees of three mesocosms that were neither drought
stressed nor defoliated. The observed relationship betweenA area and intercellularp CO2 (C i) was used to
estimate V cmax (µmol CO2m-2 s-1), the maximum velocity of
RuBP carboxylation, employing a trend-fitting method that minimized the
total sums of squares for differences between observed versus predictedA (Long & Bernacchi, 2003). Estimates relied on
Michaelis–Menten constants for CO2(K c) and oxygen (K o), as
well as photosynthetic compensation p CO2 (Γ*),
derived using formulae from Long and Bernacchi (2003). Because we did
not account for the influence of mesophyll conductance on
CO2 diffusion into the chloroplast (Dillaway & Kruger,
2010), our derived V cmax values are “apparent”
(Bernacchi et al., 2013) and based on C i rather
than chloroplastic p CO2(C c).