Atom 13 C -13 C abundance; Atom 15 N -15 N abundance; Ci -intercellular CO2 concentration; Con 13 C -13 C absolute abundance; Con 15 N -15 N absolute abundance; DAT -days after temperature and nitrogen application treatment; E -transpiration rate; Fv/Fm -maximum quantum efficiency of PSII photochemistry; gs -stomatal conductance; PN -net photosynthetic rate; qP -photochemical quenching coefficient; YII -quantum efficiency of PSII; YNPQ -quantum yield of regulated energy dissipation of PSII; δ 13 C -amount of fixed 13 C assimilates; δ 15 N -amount of fixed 15 N assimilates.

In this study, water solutions with different concentrations of abscisic acid (ABA) (10, 20, and 40 μmol L-1) were sprayed on leaves of drought-stressed tobacco seedlings. The results showed that under drought stress, both water content and photosynthetic carbon assimilation ability were reduced, PSII activity was also inhibited. Drought stress also led to the outbreak of reactive oxygen species in tobacco leaves and increased the degree of membrane peroxidation. However, spraying exogenous ABA significantly alleviated the damage caused by drought stress on the leaves. The potential mechanisms by which drought resistance in tobacco is improved by spraying exogenous ABA include: (1) regulation of the stomatal limitation of leaves, which is beneficial to water retention ability and can enhance photosynthetic function; (2) reduction of the energy pressure of the PSII reaction center by increasing nonphotochemical quenching (NPQ); and (3) effective reduction of the production of reactive oxygen species in the leaves and reducing the degree of membrane peroxidation.