Photosynthetic characteristics of linze jujube in conditions of high temperature and irradiation

Linze jujube (Zizyphus jujuba Mill. var. inermis (Bunge) Rehd.) is a unique native selection with high-quality fruits. Photosynthetic capacity was evaluated under high temperature and irradiation using CO2 exchange and chlorophyll fluorescence parameters. The diurnal course of net photosynthetic rate (Pn) was bimodal. The first peak occurred around 10:00 from late June to August. The second peak occurred around 17:00 during late June to late July with high temperature and intense sunlight, but around 16:00 in late August with lower temperature and irradiation. A clear midday depression of Pn lasted from 12:00 to 16:00 from late June to late July, but it was less clear in late August. The light compensation point and light saturation point of Pn were 28 and 1000 μmol m−2 s−1, respectively, and the CO2 compensation point and CO2 saturation point of Pn were 64.3 and 677.5 μmol mol−1, respectively in late July. Under the conditions of high temperature (Ta > 40 °C) and irradiation (PFD > 2000 μmol mol−1), photochemical efficiency of PS II (Fv/Fm) changed little between 0.78 and 0.84 all day, without photochemical damage. It was concluded that under the conditions of high temperature and irradiation, the midday reduction of Pn should be mainly attributed to stomatal conductance, which mainly resulted from the increase in vapor pressure deficit.