Sorghum bicolor (L.) Moench, one of the most important dryland cereal crops, is moderately tolerant of soil salinity, a rapidly increasing agricultural problem due to inappropriate irrigation management and salt water intrusion into crop lands as a result of climate change. The mechanisms for sorghum’s tolerance of high soil salinity have not been elucidated. This study tested whether sorghum plants adapt to salinity stress via stomatal regulation or osmotic adjustment. Sorghum plants were treated with one of seven concentrations of NaCl (0, 20, 40, 60, 80, or 100 mM). Leaf gas exchange (net CO₂ assimilation (A), transpiration (Tr); stomatal conductance of water vapor (gs), intrinsic water use efficiency (WUE)), and water (Ψw), osmotic (Ψo), and turgor Ψt potentials were evaluated at 40 days after the imposition of salinity treatments. Plants exhibited decreased A, gs, and Tr with increasing salinity, whereas WUE was not affected by NaCl treatment. Additionally, plants exhibited osmotic adjustment to increasing salinity. Thus, sorghum appears to adapt to high soil salinity via both osmotic adjustment and stomatal regulation.

Soil salinity directly affects plants, interfering in the emission of fluorescence, and promoting the degradation of photosynthetic pigments. Thus, estimating the damage to the photosynthetic apparatus caused by saline water is an important tool to detect abiotic stresses. For this purpose, sorghum plants were cultivated in a greenhouse and irrigated with two water sources (NaCl and salt mixture) with six levels of electrical conductivity (EC) (0, 2.5, 5, 7.5, 10, and 12.5 dS m^-1), forming a 6 × 2 factorial in randomized blocks with four repetitions. At 60 days after sowing, the photosynthetic pigments were quantified, the chlorophyll a fluorescence parameters (initial fluorescence [F₀]; variable fluorescence [F_v]; maximum fluorescence [F_m]; quantum efficiency of photosystem II - PSII [F_v/F_m]; electron flux per reaction center [ET₀/CR]; quantum energy dissipation [DI₀/CR] and quantum yield for heat dissipation [φD₀]) were evaluated. The photosynthetic pigments were decreased with an increase in the salinity of the irrigation water, and were more expressive at higher electrical conductivities. With the wear on the photosynthetic apparatus by the increase in salinity, a reduction in the emission of the chlorophyll a fluorescence was observed, pointing to a possible photoinhibition of photosystem II.

The energetic state of water in the soil-plant system is a relevant theme in ecosystems affected by abiotic stress. Thus, the present study aimed to investigate the water potential in soil and in sorghum grown in Fluvisol irrigated with water of increasing electrical conductivity and different ionic compositions. Solutions were prepared with different ionic compositions (NaCl and a salt mix) and with six increasing levels of electrical conductivity (0, 2.5, 5.0, 7.5, 10, and 12.5 dS m⁻¹), with a randomized block design and 4 repetitions. The leaf water potential and leaf osmotic potential were determined 60 days after sowing. The soil osmotic potential was measured by saturation paste and the matric potential was estimated by the soil water retention curve. The increase in electrical conductivity, regardless of the type of saline solution applied, causes interference in the energy status of the water in the soil, especially concerning the osmotic potential. The type of salt exerts an influence on the water potential in the soil. In contrast, the salt type does not influence the potentials of water in the plant. It is not recommended adopting the same values of water total potential for the soil determined in sorghum at night.