Spirulina platensis extract maintains Lactuca sativa productivity under drought and salinity conditions by enhancing RubisCO function and the “ascorbate-glutathione” pathway

Spirulina platensis extract (SPPE) is a rich source of antioxidants, vitamins, and essential nutrients. It has been widely used to mitigate stress on plants grown under adverse conditions, thereby supporting agricultural sustainability. Three trials were conducted simultaneously to assess the effects Of sppe applied either as a foliar spray at 1.0 − 2.0% or added at 0.75−1.5% to the growth medium (GRM). The performance of lettuce plants was evaluated under both normal and double stress (drought + salinity) conditions. Compared to the normal control, double stress markedly impaired plant performance, primarily due to its detrimental effects on GRM properties. However, under both normal and double stress conditions, all SPPE treatments applied to the GRM significantly improved GRM characteristics. Specifically, electrical conductivity and soluble sodium (Sol-Na⁺) content were reduced, while enhancing soluble potassium (Sol-K⁺) content and the Sol-K⁺/Sol-Na⁺ ratio. These GRM improvements translated into significant enhancements in growth attributes, photosynthetic capacity, and RubisCO metabolism (including RubisCO content, activity, and gene expression). Moreover, the accumulation of osmoregulators, such as soluble sugars, proline, and glycine betaine, was elevated. The contents of nutrients such as nitrogen, phosphorus, potassium, and zinc were significantly increased. Additionally, the K⁺/Na⁺ ratio improved, along with the activity of the "ascorbate (AsA)-glutathione (GSH)" antioxidant pathway. This included enhanced activities of ascorbate peroxidase and glutathione reductase, as well as increased levels of AsA and GSH. Conversely, H₂O₂ and Na⁺ levels were markedly reduced. Overall, both application methods were more effective under double stress conditions than under normal growth conditions. Notably, providing GRM with SPPE was more effective than foliar application in mitigating the adverse stress effects. The superior performance of GRM-applied SPPE at 1.5%, followed by foliar application at 2.0%, highlights its potential contribution to food sustainability in lettuce production under stress-prone environments.