Plants maximise chloride uptake during early vegetative development to stimulate cell expansion, maturation of the photosynthetic apparatus, and growth

Despite being an essential micronutrient and its recent classification as a beneficial macronutrient, chloride (Cl^-) has traditionally been considered of limited agricultural relevance and a potentially toxic saline ion. This study provides the first comprehensive demonstration of the quantitative and qualitative importance of Cl^- during early vegetative development (EVD) of tobacco and Arabidopsis thaliana plants. During this developmental stage, these and other species (including celery, lettuce, Swiss chard, spinach, squash, tomato, chili pepper, eggplant, and perennial ryegrass) exhibit the highest demand and transport rate of this non-assimilable mineral nutrient to maximise growth of these herbaceous and also woody (such as citrus and olive) species. While Cl^- promotes cell expansion across all growth stages, its particularly pronounced stimulation of plant growth during EVD is associated with enhanced photosynthetic performance and PSII activity. This enhancement is in turn linked to a reduction in non-regulated energy dissipation in PSII and an increase in the electron transport rate, along with ultrastructural changes in chloroplasts, underscoring that Cl^- is specifically required during EVD to drive the maturation of the photosynthetic apparatus. Unlike adult plants, the growth deficiencies caused by sub-macronutrient Cl^- levels during EVD cannot be mitigated by equivalent nitrate (NO₃ ^-) supplementation. As EVD concludes, plant demand for Cl^- gradually decreases, accompanied by a reduced growth response to Cl^- and an increased reliance on NO₃ ^-, emphasising stage-specific nutrient needs. The relevance of Cl^- as a morphogenic driver during a critical stage of development has significant implications for optimizing agronomic practices, particularly by reducing dependence on nitrogen fertilisers.