Trichoderma and Glomus Modulate Morphological and Physiological Traits of Vegetable Seedlings Under Heat Stress

Scorching field temperatures, intensified by climate change, pose a critical threat to crop establishment. This is particularly true for transplants, which having been raised in a controlled nursery, often lack the hardiness to survive the stressful transition to the open field. Beneficial soil microorganisms, such as Trichoderma harzianum and Glomus spp., have the potential to enhance plant tolerance to abiotic stress. This study evaluates their effects of increasing temperatures (25, 30, and 35 °C) on tomato (Solanum lycopersicum), cabbage (Brassica oleracea), cucumber (Cucumis sativus), and melon (Cucumis melo) seedlings. Seedlings were inoculated with Trichoderma or Glomus, alongside a non-inoculated control, and assessed for germination, chlorophyll, total phenolic content (TPC), photosynthesis, plant height, sturdiness index, shoot weight, and leaf area. Germination was highest in melon and cucumber at 30–35 °C, while tomato reached 88% germination at 30 °C. Trichoderma increased chlorophyll content (13.4 CCI) at 35 °C and TPC in cabbage (34.84 mg GAE g− 1 d.w.). In cucumber, Glomus enhanced photosynthesis (8.30 µmol CO₂ m⁻² s⁻¹), height (+ 68%), and sturdiness index (+ 71%). Melon at 30–35 °C showed greater shoot weight (5.28 g) and leaf area (123.5 cm²). Microbial effects were crop- and trait-specific, underscoring the need for targeted applications to optimize seedling production under rising temperatures.