High‐Temperature Stress and Soybean Leaves: Leaf Anatomy and Photosynthesis

Soybean ( Glycine max L. Merr.) genotype K 03‐2897 was grown in a controlled environment to study the effects of high‐temperature (HT) stress on ultrastructural changes in leaves and its relationship with photosynthetic rate. The objectives of this study were to (i) quantify the effects of HT stress during flowering stage on photosynthetic rate and (ii) observe the anatomical and ultrastructural changes in leaves of soybean grown under HT. Plants were exposed to HT (38/28°C) or optimum temperature (OT; 28/18°C) for 14 d at flowering stage R2. High‐temperature stress significantly decreased the leaf photosynthetic rate and stomatal conductance by 20.2 and 12.8%, respectively, compared with those at OT. However, HT stress significantly increased the thicknesses of the palisade and spongy layers and the lower epidermis above those at OT. In addition, HT stress damaged the plasma membrane, chloroplast membrane, and thylakoid membranes. The mitochondrial membranes, cristae, and matrix were distorted under HT stress. High‐temperature stress increased the content of leaf reducing sugars by 82.6% above that at OT. Leaves under HT stress had a higher carbon isotope ratio compared with that at OT. Decreases in photosynthesis at HT stress were mediated through anatomical and structural changes in the cell and cell organelles, particularly the chloroplast and mitochondria.