Li 610 dew point generator

The CO₂/H₂O exchange was analyzed by placing a leaf segment into a temperature-controlled leaf chamber where the sample was illuminated through a fiber-optic light guide from a KL 1500LCD light source (Schott, Germany). The steady-state CO₂/H₂O exchange rates at the leaf–air interface were measured with a single-channel LI-820 infrared gas analyzer (LI-COR, United States) in the open-circuit mode. Apparent photosynthesis (A) was expressed as µmol (CO₂) m⁻² s⁻¹. The leaf transpiration (E, mmol (H₂O) m⁻² s⁻¹) was calculated from the difference in gas humidity at the inlet and outlet from the leaf chamber. In this experimental system, the humidity of gas flow at the entrance to the leaf chamber was kept constant at a known level using a LI-610 dew point generator (LI-COR). Humidity at the exit of the leaf chamber was determined with a HMP50 psychrometric sensor (Vaisala INTERCAP, Finland). Water-use efficiency (WUE) was calculated as the ratio of apparent photosynthetic assimilation to the transpiration rate (A/E). After CO₂/H₂O gas exchange measuring, the light was turned off, and after steady state, the dark respiration (Rd, µmol (CO₂) m⁻² s⁻¹) was measured.

The response of A to changes in intercellular [CO₂] (C_i) of the AD and AB surfaces of wheat plants was measured simultaneously in the cultivars Brompton and Xi19 using the split‐chamber system. Photosynthesis was first stabilized at 400 μmol mol⁻¹ and then decreased through the values 250, 150, 100, and 50 μmol mol⁻¹. It was then returned to the initial value of 400 μmol mol⁻¹ and increased through the values 550, 700, 900, 1100, 1300, and 1500 μmol mol⁻¹. Photosynthesis was measured at each [CO₂] value after c. 3 min. Saturating PPFD was kept at 1000 μmol m⁻² s⁻¹ for both the AD and AB leaf surfaces. Leaf temperature and VPD were 22°C and 1 ± 0.5 kPa respectively (the latter was maintained using a Li‐610 dew point generator; Li‐Cor).