Differential morphological, cytological and biochemical responses of two rice cultivars to coumarin

We investigated the differential responses of two contrasting rice cultivars (susceptible BS-2000 and less susceptible BR-41) to coumarin at the cytological, biochemical and morphological levels. Both rice cultivars, showed similar trend in the relative sensitivities of specific root parameters to coumarin: total number of root hairs > total number of lateral roots > radical length. However, the light and electron microscopic observations of seedling root and leaf cells exhibited differential responses to coumarin exposure. BR-41 showed higher ribosomal density (RD), more mitochondria in root cells, larger starch granules in leaves and more vacuoles (smaller size) than BS-2000. Coumarin also induced different antioxidant responses in two cultivars. BR-41 showed 23% higher superoxide dismutase (SOD), 16% higher catalase (CAT) and 17% higher peroxide dismutase (POD) activities than BS-2000 when exposed to 400 mu M coumarin. However, malondialdehyde (MDA) contents and electrolyte leakage were significantly higher in BS-2000, which exhibited 8 and 10 % higher MDA levels and 49 and 45% higher electrolyte leakage than BR-41, when exposed to 200 and 400 mu M coumarin, respectively. Our results suggested that biochemical and cytological responses and root morphological characteristics jointly affect the rice resistance to coumarin stress.