COMPARATIVE DROUGHT POSTPONING AND TOLERANCE POTENTIALS OF TWO TEPARY BEAN LINES IN RELATION TO SEED YIELD

Tepary bean (Phaseolus acutifolius A. Gray) is a strategic germplasm for genetic improvement of the common bean (Phaseolus vulgaris) for resistance to various stresses including drought. However, its potential for this purpose has not been evaluated. It is imperative that the potential of various genotypes of tepary bean as genetic improvement materials is evaluated. In the present study, tepary line NE#19 had higher seed yield and greater seed size than NE#5 under well-watered conditions in the field at Assiut, Egypt. Drought reduced seed yield of both lines but NE#19 had higher seed yield, and its seed size was not affected. Both lines had comparable shoot dry mass and root depth under drought. However, line NE#19 developed fibrous roots of greater mass than NE#5 in response to drought, especially, in deep soil profile (40 to 50 cm). Root/shoot ratio was high and the portion of leaf area supported by the mass unit of root was small in NE#19. Additionally, the leaves of this line exhibited a reduced stomatal conductance in response to progressive drought. However, biochemical assessment indicated that NE#5 and not NE#19 can completely recover after rehydration of severe dehydrated leaves in terms of chlorophyll (Chi) contents and Chi a fluorescence parameters (F_0, F_M and F_V/F_M), reduced tocopherol and lack of alteration in malondialdehyde release. This was due to a prominent antioxidative role of peroxidase activity during the stress period. It is concluded that the potential drought postponing traits of NE#19 enabled it to sustain normal seed filling and to produce high seed yield. Line NE#5 was regarded as tolerant to severe drought and may not suffer great reduction in seed size and yield if given supplemental water during the critical phase of seed filling. Diversity in drought postponing and tolerance of the two lines may be of further interest to bean breeders.