Temperature Regimes Modulate Growth and Nutritional Quality of Three African Leaf Vegetables

A large population in Africa, particularly West Africa, depends on leafy vegetables such as red amaranth (Amaranthus cruentus), Lagos spinach (Celosia argentea), and African eggplant (Solanum macrocarpon) as affordable and readily available sources of nutrition. These vegetables are rich sources of phenolics, minerals, vitamins, and bioactive compounds, contributing significantly to dietary nutrition and providing an important source of revenue for farmers. However, the temperature rise due to climate change threatens their availability and nutritional value. This study assessed the effects of temperature regimes (23, 30, and 40 °C) on the growth and quality of these vegetables under greenhouse conditions for 48 (A. cruentus and C. argentea) and 54 (S. macrocarpon) days after sowing by measuring biomass (leaf, stem, shoot, root dry weight, root/shoot and leaf area), photosynthetic parameters, pigments, sugars, mineral content, antioxidant activity, total phenolic compounds, total flavonoids, and free amino acids. Temperature significantly affected biomass, with A. cruentus and C. argentea showing declines of 13.5–32.2% and 5.1–27.8%, respectively, at 40 °C compared to 23 °C, indicating sensitivity to heat stress. Photosynthetic rates increased with a rise from 23 to 30 °C by 2.1–29.2% across all species. Sugar contents remained generally stable, except for notable decreases in glucose and soluble sugars by 43.3% and 40.5%, respectively, in C. argentea between 30 and 40 °C, and a 52.6% reduction in starch in S. macrocarpon from 23 to 40 °C. Mineral nutrient responses varied by species; however, they exhibited similar increases in nitrogen and phosphorus, as well as decreases in calcium and manganese, at higher temperatures. Notably, antioxidant capacity and total phenolic compounds declined significantly in C. argentea (8.1% and 8.0%) and S. macrocarpon (4.7% and 13.3%). In contrast, free amino acid contents increased by 35.2% and 28.8% in A. cruentus and S. macrocarpon, respectively. It was concluded that A. cruentus and C. argentea suffer reduced growth and nutrients at 40 °C, while S. macrocarpon maintains biomass but has some biochemical declines; antioxidant capacity and phenolics drop at high temperatures, free amino acids rise, and 30 °C is optimal for all three.