Rooting traits of peanut genotypes with different yield responses to pre-flowering drought stress

Water stress during the vegetative development normally is not detrimental and sometimes actually increases yield of peanut (Arachis hypogaea L.). Root growth might play an important role in response to early season drought in peanut and might result in an increase in yield. Information on the response of root characters of diverse peanut genotypes to these conditions will provide useful information for explaining mechanisms and improving peanut genotypes for exploiting positive interaction for pod yield under pre-flowering drought. The aim of this study, therefore, was to investigate the root dry weight and root length density of peanut genotypes with different yield responses to pre-flowering drought stress and their relationships with pod yield. Field experiments were conducted at the Field Crop Research Station of Khon Kaen University, Khon Kaen, Thailand during February to July 2007 and during February to July 2009. A split-plot experiment in a randomized complete block design was used. Two water management treatments were assigned as the main plots, i.e. field capacity and pre-flowering stress, and six peanut genotypes as the sub-plots. Total crop dry matter, root dry weight and root length density were recorded at 25 DAE, R5 and R7. Top dry weight and pod yield were measured at harvest and pod harvest index (PHI) was computed using the data on pod yield and biomass. Peanut genotypes were categorized into three groups based on their responses to drought for pod yield, e.g. increasing, decreasing and non-responsive groups. The genotypes of each group showed a differential response for root quantity and distribution. The increasing pod yield group had more root dry weight and root length density in the deeper soil layers during pre-flowering stress compared to the non-stress treatment. The non-responsive group showed no root response under pre-flowering drought conditions compared to the non-stress treatment. A larger root system alone without considering distribution may not contribute much to pod yield but a higher RLD at deeper layers may allow plants to mine more available water in the sub-soil. However, as yield is a complex trait, several mechanisms may be involved. The increasing pod yield group also had the ability to maintain a high PHI.