Anatomical variation of Capsicum chinense leaves subjected to proline application and salt stress

Plants adapt to saline conditions through various strategies, including changes in their morphology and anatomy. Thus, this study aimed to investigate the influence of salt stress and proline application on the leaf anatomy of 'Biquinho' pepper ( Capsicum chinense) cultivated under salt stress conditions. Fully expanded leaves were sampled from four plants for each combination of treatments, which included five levels of irrigation water salinity (0.50, 1.30, 3.25, 5.20, and 6.00 dS m-1) and five doses of proline (0.00, 2.90, 10.00, 17.09, and 20.00 mM). The sections were hand-cut transversely using a sharp blade, cleared with 1% sodium hypochlorite, stained with 10% safranin, mounted on semi-permanent slides with glycerin, and photomicrographed under a microscope. The variables analyzed included the thickness of the adaxial cuticle, thickness of the abaxial cuticle, thickness of the adaxial epidermis, thickness of the abaxial epidermis, length of the vascular bundle, diameter of the vascular bundle, thickness of the palisade parenchyma, thickness of the spongy parenchyma, leaf thickness, palisade to spongy parenchyma ratio, stomatal density of the adaxial epidermis, and stomatal density of the abaxial epidermis. Salt stress in C. chinense caused a reduction in abaxial cuticle thickness and an increase in adaxial epidermis thickness. The vascular bundle diameter and leaf thickness were also affected. The application of proline at a concentration of 5.35 mM increased the thickness of the adaxial cuticle. Conversely, 20 mM of proline enhanced the thickness of the abaxial epidermis, palisade to spongy parenchyma ratio, and stomatal density. These results support the notion that proline improves the leaf anatomy of peppers, potentially increasing their tolerance to salt stress.