SUNBURN IN HORTICULTURAL CROPS: CAUSES, PHYSIOLOGICAL AND MOLECULAR MECHANISMS, AND ECONOMIC IMPACTS

Abstract

Sunburn is an important abiotic disorder affecting the productivity, quality and marketability of horticultural crops. It occurs when plant tissues receive excessive solar radiation and experience elevated surface temperatures beyond their capacity for physiological protection and acclimation. The disorder affects a wide range of fruit, vegetable and ornamental crops, although its symptoms and underlying mechanisms vary with species, plant organ and environmental conditions. In fruit crops, sunburn commonly appears as browning, bleaching, necrosis or localized surface injury, whereas in vegetables it is frequently expressed as fruit sunscald, leaf scorching and tissue necrosis. In ornamental crops, excessive radiation and high temperature may cause petal bleaching, pigment degradation, marginal necrosis, bud injury and reduced flower longevity. The development of sunburn is influenced by solar radiation, ultraviolet exposure, fruit or tissue temperature, relative humidity, water availability, canopy architecture and nutritional status. At the physiological and biochemical levels, excessive radiation and heat can disrupt photosynthesis, induce photoinhibition, promote reactive oxygen species accumulation, alter membrane stability and affect pigment metabolism. Sunburn results in economic losses through reduced marketable yield, quality downgrading, postharvest deterioration and rejection in premium markets. Recent molecular studies have further improved understanding of antioxidant metabolism, stress-responsive pathways and pigment-related changes associated with sunburn development. This review discusses the occurrence, causes, physiological and molecular mechanisms, crop-wise manifestations and economic consequences of sunburn across fruit, vegetable and ornamental crops, while identifying research priorities for understanding this disorder under changing climatic conditions.