Effect of site, harvesting stage, and genotype on environmental staining in faba bean (Vicia faba L.)

Seed discoloration due to environmental staining in faba bean leads to poor quality and reduced market price. Environmentalstaininginfababeanischaracterisedbyadarkbrown,grey,orblackdiscolorationoftheseedcoatatharvest. Its cause is unknown, but it does not appear to be caused by a pathogen. Environmental conditions during pod and seed formationand atmaturity are thought tohave a large effect on the degree of environmental staining.To test the hypothesis that seeds formed under stressful conditions will have a higher degree of staining, faba bean seeds were harvested at 2differentstagesofmaturityfromtrialslocatedinarangeofenvironmentalconditionsunderaMediterranean-typeclimate of south-western Australia over 2 seasons. Four faba bean varieties were studied (Fiord, Fiesta, Ascot, and Cairo). The majority of seeds had good colour but across the trials, 3-25% were stained up to an unacceptable level and this variedwithlocationandvariety.Seedsformedlaterinplantdevelopment(locatedontheuppernodesoftheplant)hadmore staining than seeds formed earlier (located on the lower nodes). Seeds formed on small and weak plants had more staining than seedsformed on normalsized healthy plants. Fiordshowed a greater amountof staining than Ascot, Fiesta, and Cairo whengrowninthemild,southernenvironments.Earlyharvesting(atphysiologicalmaturity)didnotreduceenvironmental seed staining compared with harvesting at full maturity. Chemicalanalysisofseedtestaandcotyledonsrevealedthattotalphenoliccontentsofthetestaandcotyledonsincreased with staining. An increase in Zn and Na and a decrease in K concentration in the testa were also associated with increased staining levels.