Germination and seedling emergence responses of common bean and cowpea to plant extract seed treatments

The present study was initiated to investigate the effect of crude plant extracts as seed treatments on Phaseolus vulgaris (common bean) and Vigna unguiculata (cowpea) seed germination and emergence in the presence of Colletotrichum lindemuthianum and Colletotrichum dematium, respectively. Common bean and cowpea seeds were treated with crude water and acetone extracts of Agapanthus caulescens Spreng., Allium sativum L., Carica papaya L. and Syzygium cordatum Hochst. ex Krauss at 5 and 15 mg/ml concentrations. Seeds treated with the synthetic fungicide fludioxonil+ mefenoxam (the commercial product Celest (R) XL) represented the positive control, whereas dimethyl sulphoxide and water-soaked seeds represented negative controls. The rolled paper towel method of the International Seed Testing Association was used to investigate the effect of the treatments on seed germination. Mean emergence time (MET) was determined using seed inoculated with the respective pathogens. The changes in the ultrastructure of embryonic roots and the connecting tissues of embryo-cotyledon of common bean and cowpea treated with Syzygium acetone extracts and Agapanthus water extracts were investigated using transmission electron microscopy (TEM). High germination percentages of > 90% were observed in bean seeds from two production seasons treated with low concentrations of water extracts of Allium, Syzygium and Agapanthus and acetone extracts of Allium, Agapanthus and Carica. These treatments also recorded high emergence percentages with low MET values, which were similar to the water control. Cowpea seeds treated with Carica water extract had the highest germination and emergence. Syzygium acetone was the only extract that gave higher germination and emergence in both IT93K5132 and PAN 311 varieties. Therefore, Carica water and Syzygium acetone extracts can be considered as potential bean and cowpea seed treatments. Generally, there were inconsistencies in terms of correlations of germination with emergence percentages in both cowpea and bean seed treated with plant extracts used in the study, which could be due to differences in vigour. The TEM study of embryo-cotyledon tissue of both species revealed that Syzygium and Agapanthus extract seed treatment may accelerate metabolic processes as evidenced by the presence of vacuoles, many cristae and few lipid bodies.