THE ROLE OF BASAL ROOT WHORL NUMBER IN DROUGHT TOLERANCE OF COMMON BEAN

Open Access
Author:
Barlow, Kathryn Marie
Graduate Program:
Horticulture
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 05, 2011
Committee Members:
  • Jonathan Paul Lynch, Thesis Advisor
Keywords:
  • drought tolerance
  • common bean
  • Phaseolus vulgaris
  • roots
  • basal root whorl number
Abstract:
Common bean (Phaseolus vulgaris) is essential to the food security of 300 million people in the tropics, and yet 40% of the bean production areas in Africa and 73% in Latin America are affected by drought. This study evaluated the utility of the number of basal root whorls as a strategy for drought tolerance in common bean. Basal root whorls form around the base of the hypocotyl in a ring of 4 basal roots. Recombinant inbred lines (RILs) segregating for basal root whorl number (BRWN) with 1, 2 or 3 whorls were used for a terminal drought study in Rock Springs, Pennsylvania. By the mid-flower stage the drought treatment had reduced shoot biomass by 46%, leaf relative water content by 23%, and increased plant water potential by 225%. Under well watered conditions there was no difference in shoot growth among the three BRWN phenotypes, yet under drought RILs with 3 whorls had 67% greater shoot biomass than the RILs with 1 whorl. Root crown phenotyping in the field at the time of shoot harvest confirmed that the number of basal roots in the field was consistent with the BRWN phenotype, and revealed variation in the number of basal roots with significant secondary growth, or ‘basal root dominance’. The extent of basal root thickening was determined to be part of the biomass allocation to roots. Basal root thickening and shoot growth under well watered conditions showed that the number of basal root whorls was not related to plant size. Grown with sufficient water, plants with few basal roots had equivalent cross sectional area of basal roots (CSA), a measurement of secondary growth, to plants with many basal roots. This compensation with greater secondary growth among RILs with a lower number of basal root whorls is one strategy plants use to maintain the same root to shoot ratio. Grown under water stress, plants with few basal roots had less basal root CSA than those with many basal roots, indicating less secondary growth capability under drought. Greater numbers of basal roots potentially enable greater exploration for soil water, therefore improving drought resistance. Breeding for greater BRWN in common bean has the potential to improve growth of common bean in water limited environments.