An evaluation of source-sink relationships in three dry bean (Phaseolus vulgaris L.) cultivars.

Abstract

The effect of intensities and times of source or sink related stresses on the growth and development of a determinant (Teebus) and two indeterminant (NEP 2, Bonus) dry bean (Phaseolus vulgaris L.) cultivars was measured in a series of field experiments at Potchefstroom Research Station. Variation in stress levels was attained by defoliation, thinning, light intensity manipulation (shades and reflectors) and removal of reproductive organs. No permanent detrimental effect on vegetative or reproductive organs was observed when source was reduced or increased during the vegetative period (V1-V6f) . Flower initiation (V6f-R1) was identified as the period most sensitive to defoliation as expressed in vegetative growth and economic yield. The negative effect of shading on vegetative development was reduced by an extended growing period . Thinning during flower initiation increased the vegetative and reproductive sink. A source stress (defoliation and shading) during the flowering period (R1-R5) restricted partitioning to the reproductive organs reducing seed yield and harvest index values. Reduced interplant competition during flowering favoured partitioning to the reproductive organs. Source size had a direct relationship with economic yield during flowering. This was confirmed by the absence of a yield response to partial depodding. A lack of response to defoliation (NEP 2) and shading (Bonus) may indicate a limited sink size in these two cultivars. During seed filling (R5-R9) Bonus was very sensitive to defoliation while NEP 2 was insensitive. Bonus was less sensitive to shading than Teebus. Thus in certain cultivars the level of current photosynthesis had a significant effect on seed yield throughout seed filling. The lack of a yield response to thinning in all cultivars during this period indicated that the potential sink size was set before R5. The results provided strong evidence supporting the concepts of yield component compensation in dry beans. Pod number was most seriously affected by defoliation during flower initiation and flowering. The potential sink size was determined mainly through the number of pods per plant which was in balance with the source unless some stress factor was present. The number of seeds per pod responded to current photosynthesis as well as the previously set pod number. Seed size was the least responsive yield component and it had a consistent negative relationship with the number of pods per plant.