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.