Leaf physiological impacts during an experimental
drought
Thirty California poplar (Populus trichocarpa ) saplings were
obtained from a commercial supplier (Plants of the Wild, USA). The trees
were transferred into #2 pots (6.59 L) with Supersoil planting media
(Scotts Co., USA) and maintained for two years in the UC Berkeley Oxford
Tract greenhouse under natural lighting supplemented with LED lighting
(6:00–20:00 light period; Lumigrow 325 Pro,
USA). The thirty potted trees
reached a stem diameter (5 cm) and height (1.5 m) just prior to the
commencement of experimental measurements. A subsection (15 individuals)
of the 2-year old trees had water withheld for one week (drought
plants), while a control group (15 individuals) continued to receive
normal watering patterns. For each individual throughout the controlled
drought experiment, one mature leaf was selected for leaf gas exchange
measurements in the greenhouse using a portable Li6800 photosynthesis
system including stomatal conductance (gs , mol
m⁻² s⁻¹), net photosynthesis (A, µmol m⁻² s⁻¹), and transpiration
(E , mmol m⁻² s⁻¹) under standard environmental conditions (400
ppm reference CO2, 25 mmol mol-1reference absolute humidity, 1000 µmol m-2s-1 photosynthetically active radiation, 600 µmol
s-1 leaf chamber air flow rate, 31 °C heat exchange
block). Immediately following leaf gas exchange measurements in the
morning, leaf water potential was determined using a nitrogen pressure
chamber instrument (Model 600, PMS Inst., USA). The leaf was detached
from the tree using a razor blade, and the petiole sealed in the leaf
pressure chamber where nitrogen pressure slowly increased until liquid
water was visible from the petiole. Following the gas exchange and leaf
water potential measurements, a second mature leaf was taken from each
of the 30 trees and frozen on dry ice and stored at -80°C prior to cell
wall analysis. Leaf gas exchange and water potential measurements and
frozen leaf samples were collected from one mature leaf for each of the
15 control and 15 drought-treated individuals at time = 0, 1, 4, and 7
days.