A low boron condition without high temperature stress induces internal browning in Raphanus sativus L. (Japanese radish)

Brown heart, or internal browning, is a physiological disorder in crops including radish. It is characterized by brown pigmentation in the internal tissue, which significantly reduces commercial value of the products. Field studies have revealed that the appearance of symptom is correlated with several stresses, including drought, high temperature, and boron deficiency. However, fluctuating and non-reproducible environmental factors in field experiments complicate the interpretation of the direct cause of the symptom. For studying the mechanism of the symptom, experiments under more controlled conditions with less fluctuating environmental factors are desirable. Here, we established culture method to observe brown heart in lab condition, where light, temperature, and nutrient is controlled. Under our culture conditions, we successfully observed internal browning in Japanese radish by lowering boron concentrations without heat and drought stresses, indicating that boron deficiency alone can cause internal browning. Furthermore, we cultured three cultivars and observed differences in susceptibility to internal browning, establishing that our culture method is applicable for studies that utilize inter-cultivar variations, such as QTL mapping.