Ap4 porometer

Stomatal conductance and chlorophyll fluorescence in-field measurements were performed early in the morning. Stomatal conductance was determined by measuring the dynamic diffusion conductance of leaves using the portable AP4 porometer (DeltaT Devices Ltd., Cambridge, UK), in 5 replicates. Chlorophyll fluorescence was determined with the PAM2500 portable chlorophyll fluorometer (Heinz Walz GmbH, Effeltrich, Germany) equipped with LEDs for saturation pulses and blue and red actinic lights, a Leaf-Clip Holder 2030-B with an integrated micro-quantum sensor for recording the photosynthetic active radiation (PAR, in units of flux density μmol quanta/(m²·s)) and also temperature with a NiCr-Ni thermocouple. Hence, the measured PAR parameter is identical to PPFD (photosynthetic photon flux density). The effective photochemical quantum yield (Φ) of photosystem II was determined using the following equation: Φ_PSII = (F′′_m—F_t)/F′′_m, where F′′_m represents the maximal chlorophyll fluorescence yield when photosystem II reaction centers are closed by a strong light pulse, and F_t represents the continuously recorded fluorescence, according to the manufacturer. In photochemistry, the symbol Φ is reserved for quantum yield, according to IUPAC Recommendations [182 (link)], and represents the number of defined events which occur per absorbed photon.

Stomatal conductance and dry weights were determined in six plants after 1, 6 and 9 days of NaCl treatment (n = 6). Stomatal conductance was measured in fully grown mature leaves with a portable AP4 Porometer (Delta-T Devices Ltd, Cambridge) four hours after the lights were on within the growth chamber.
Leaf water potential was determined with a pressure chamber (SF-PRES-35, SolFranc Tecnologías SL, Tarragona) on six mature leaves after 1, 6 and 9 days of NaCl treatment (n = 6). Mature fully developed leaves were excised from the main shoot after the gs measurements and introduced into the pressure chamber. Pressure was applied until xylem sap was visible at the cut surface.
Plant shoot, root and total dry weights were determined in 6 plants (n = 6) per NaCl treatment at day 1, 6 and 9, after oven-dry of the plants tissues at 65°C for 48 hours.

The assessment of stomatal conductance was accomplished between the hours of 11:00 a.m. and 12:00 p.m., whereby the measurements were taken in millimoles per square meter per second (mmol m⁻² s⁻¹) from fully expanded leaves. This was performed through the use of an AP4 porometer, manufactured by Delta-T Devices, Cambridge, UK. Similarly, the maximum quantum yields of the photosystem II were evaluated by means of the Opti-Sciences 30P⁺ fluorimeter, with measurements being recorded 30 min post-application of the leaf clips.

All stomatal conductance measurements were performed on 5–6-week-old Arabidopsis plants. The stomatal conductance determined by the AP4 Porometer (Delta-T Devices) was calculated based on the mean value from 2–3 leaf recordings per plant (Figs. 3b, 4c, 5c, 6c, d and 9; Supplementary Fig. 10d, 11d, g and 14c). The time-dependent stomatal conductance, transpiration and photosynthetic rate was recorded using LI-6400XT Portable Photosynthesis System (LI-COR Biosciences) equipped with an Arabidopsis leaf chamber fluorometer (under 150  µmol m⁻² s⁻¹ light with 10% blue light, 150 mmol s⁻¹ flow rate, 400 ppm CO₂ mixer, ~50 % relative humidity at 22 °C) as indicated (Fig. 4d; Supplementary Fig. 8g, h).

Stomatal conductance was estimated on a fully expanded leaf randomly taken from the upper part of the plant using a steady-state porometer (AP4 Porometer; Delta-T Devices). Stomatal conductance (gs) was measured between 900 and 1100 h on sunny days at 8, 15, and 45 dai.

An AP4 Porometer (Delta-T Devices, Cambridge) was used to measure stomatal conductance (g_s4, mmol m⁻² s⁻¹) from 60 to 70 DAS in WW21 (three times) and IWD21 (five times) coinciding with the pre-heading period. The porometer measured the g_s4 of the abaxial side of the youngest fully expanded leaf by clipping the sensor head onto a leaf on clear sunny days between 10:00 and 12:00. On each measurement occasion, the mean of two plants was used to determine g_s4 for each plot.