After maize seedlings grown in full-strength Hoagland solution for 20 days were exposed to 20% polyethylene glycol (PEG)-stimulated water deficiency for two days, plant height, shoot fresh and dry weights, and pigment contents significantly decreased, whereas malondialdehyde (MDA) content greatly increased. Using transmission electron microscopy, we observed that chloroplasts of mesophyll cells in PEG-treated maize seedlings were swollen, with a disintegrating envelope and disrupted grana thylakoid lamellae. Using two-dimensional gel electrophoresis (2-DE) method, we were able to identify 22 protein spots with significantly altered abundance in the leaves of treated seedlings in response to water deficiency, 16 of which were successfully identified. These protein species were functionally classified into signal transduction, stress defense, carbohydrate metabolism, protein metabolism, and unknown categories. The change in the abundance of the identified protein species may be closely related to the phenotypic and physiological changes due to PEG-stimulated water deficiency. Most of the identified protein species were putatively located in chloroplasts, indicating that chloroplasts may be prone to damage by PEG stimulated-water deficiency in maize seedlings. Our results help clarify the molecular mechanisms of the responses of higher plants to severe water deficiency.

Water deficit is a significant abiotic stress affecting crop growth and production. While many studies have indicated that salicylic acid (SA) plays a crucial role in mitigating the detrimental effects of environmental stress on plants, its mechanism regulating the photosynthetic adaptability of maize seedlings under water deficit is still unclear. This study aimed to investigate the impact of exogenous SA on maize seedling performance under polyethylene glycol (PEG)-induced water deficit. The results showed that PEG treatment destroyed the integrity of chloroplast and reduced chlorophyll content and photosynthesis rate (Pn), leading to growth retardation of maize seedlings with lower biomass accumulation and leaf relative water content (RWC). Moreover, chlorophyll fluorescence index, including potential photochemical activity (Fv/Fo), maximum Photosystem II (PSII) quantum yield (Fv/Fm), and energy captured by PSII reaction center for electron transfer (Eto/RC), were decreased, but energy dissipated by unit reaction center (DIo/RC) was enhanced in maize seedlings under water deficit. In addition, PEG treatment also significantly declined the activity of Rubisco and Rubisco activase (RCA) in maize seedlings. In contrast, SA treatment enhanced the content of chlorophyll, as well as the transcription level of psbA, and RCA and Rubisco small subunit (rbcS) reduced the damaging effects of PEG treatment by protecting the integrity of chloroplast and repairing the damaged PSII reaction center, thus positively regulating photosynthetic reaction and water-deficit tolerance in maize seedlings. Our data implied that SA played an important regulatory role in plant resistance to water-deficit stress, and the result will further supply the regulatory network of SA-mediated photosynthetic adaptability.