Microstructure of nuñas: Andean popping beans (Phaseolus vulgaris L.)

Nuñas, popping beans (Phaseolus vulgaris L.), burst and expand when heated rapidly. Differences in seed microstructure between popping and conventional (non-popping) bean genotypes conceivably contribute to popping in ñuñas. However, the microstructural characteristics which contribute to the popping attribute and sites of expansion have not been identified. Seeds and excised cotyledons of unpopped and popped ñuñas were examined using scanning electron microscopy (SEM). Protoplasts of unpopped ñuñas were similar to protoplasts of conventional beans. Intercellular spaces of unpopped ñuñas were occuled by schizogenous cell walls. The occluded form of intercellular spaces differed distinctively from the open form in popped ñuñas and untreated conventional beans. The expansion of cotyledon mesophyll in popped ñuñas came primarily from expansion of cell walls and secondarily by expansion of the intercellular spaces. Cell wall thickness and dimensions of protoplasts were not changed during popping. Expansion of cell walls away from protoplasts created intracellular voids. SEM images indicated that starch granules (grains) in popped ñuñas were generally not altered by popping. Starch granules did not gelatinize or melt during popping as indicated by retention of birefringence. In contrast to popcorn (Zea mays L.), starch granules did not contribute to expansion of popped ñuña cotyledons