MATERIALS AND METHODS
Plant materials and growth conditions
Two cowpea genotypes, Yacine and 58-77, generously provided by Drs Philip Roberts and Bao-Huhyn Lam, from University of California, Riverside were used for all experiments. These lines exhibited apparent tolerance and sensitivity respectively to HL stress. One seed was sown in 1 L pot filled with Suremix potting medium (Michigan Grower Products Inc, USA) and watered with half-strength Hoagland’s nutrient solution. Plants were staged in a reach-in growth chamber with the following conditions: RH, 50%; temperature, 30/20 °C day/night; light intensity, up to 400 μmol m-2 s-1 and photoperiod of 10/14 h light/dark. Seedlings were moved to a Dynamic Environment Photosynthesis Imager (DEPI) chamber (Cruz et al., 2016) for treatments 3–4 d or 12–14 d after germination for the young leaves or mature leaves respectively for treatments and assessment of photosynthetic parameters. Where necessary, older plants were used but leaves of similar maturity were used for all measurements. The relative humidity (RH) varied from 20–40% for the HT treatments whereas it was kept at 50% for the Control treatment.
High temperature and high-light treatments 
Various HT and HL treatments are described in detail within the corresponding figure descriptions. Briefly, HT treatments were imposed by heating the air temperature in the chamber using the native environmental control embedded in the growth chamber to 45 °C. For the temperature response curves, the temperature was changed by 5 °C every 2 h (starting from GT of 30 °C) but measurements were made after exposure for at least 1 h. Illumination in the DEPI chamber was provided by LED lights with wavelength in photosynthetically active radiation (PAR) range. The light intensity treatments consisted of low light (LL — 300 μmol m-2s-1) and high-light (HL — either 1000 or 1500 μmol m-2 s-1).
Chlorophyll fluorescence and absorbance change measurements
the maximal PSII efficiency (FV/FM), plants were dark adapted for 20 min before measurements were made. Most chlorophyll fluorescence measurements were made using either the MultispeQ V1 linked to the PhotosynQ platform [www.photosynq.org , (Kuhlgert et al., 2016)] or the image-based DEPI chamber (Cruz et al., 2016). Some fluorescence measurements were also performed simultaneously with gas exchange measurements using the LI-COR 6800 (Lincoln, Nebraska, USA).
The electrochromic shift was assessed simultaneously with chlorophyll fluorescence measurements using the MultispeQ V1, thus enabling us to connect PSII function with downstream reactions such as the ATP synthase activity.  The thylakoid conductivity to protons (g H+) was determined by the dark interval relaxation kinetics (DIRK) of the electrochromic shift (ECS) at 520 nm (Thomas J Avenson, Cruz, Kanazawa, & Kramer, 2005; Takizawa, Cruz, Kanazawa, & Kramer, 2007) and was utilized as a measure for the chloroplast ATP synthase activity. The proton motive force (pmf ) was assessed as the amplitude of the first-order decay kinetics of the ECS trace in the first 300 ms [ECSt, (Kanazawa & Kramer, 2002; Kanazawa et al., 2017)].
Photoprotection (the rapidly reversible form of NPQ,q E) and photo-inhibitory quenching (the slowly relaxing form of NPQ, q I) were assessed as described in (Tietz, Hall, Cruz, & Kramer, 2017), using the MultispeQ V1 or DEPI platform. Briefly, following initial pulses to determine FO and FM, leaves were dark adapted for 2 min after which q E would have relaxed, leavingq I, enabling us to determine FM.