Metabolic Plasticity Versus Conservative Strategies: How Nitrogen Form and Foliar Proline Modulate Genotypic Heat Stress Responses in Tomato

This study investigates how nitrogen form (100/0 NO3− vs. 50/50 NO3−/NH4+), heat stress (43 °C), and foliar proline application interact to modulate the growth, photosynthesis, and nitrogen metabolism of two tomato cultivars (Solanum lycopersicum L.), ‘Cherry’ and ‘Tres Cantos’. Nitrogen-exclusive nutrition optimized biomass accumulation and photosynthetic efficiency. Conversely, mixed nutrition reduced photosynthetic performance but enhanced nitrogen storage and detoxification pathways, as evidenced by a shift in amino acid profiles (decreased glutamate and aspartate alongside increased glutamine and asparagine). Under heat stress, growth declined; however, plants exhibited non-stomatal photosynthetic acclimation. While exogenous proline failed to increase biomass, it significantly enhanced heat tolerance by driving transpiration and evaporative cooling. Cultivar-specific assessments revealed high metabolic plasticity in ‘Cherry’, whereas ‘Tres Cantos’ adopted a conservative strategy centered on the accumulation of protective nitrogenous compounds.