Auxin and Indol Acetic Acid Oxidase Activity during Sweetpotato Storage Root Initiation

Storage root initiation in ‘Beauregard’ sweetpotato occurs within the first three to four weeks after planting and is an important factor in determining yield.  Adventitious roots with hexarch and pentarch steles possess the potential to become storage roots, but agroclimatic factors such as temperature and soil moisture influence storage root initiation and growth and therefore yield.  In addition, auxin has been suggested to influence storage root initiation, but the interaction of temperature and auxin on storage root initiation is not clear.  This study investigated the relationship of Indol Acetic Acid (IAA) content and IAAOxidase (IAAOx) activity with storage root initiation and growth under optimum (30/22 ^oC day/night) and high temperature (40/32 ^oC day/night).  IAA content was greater and IAAOx reduced in leaves grown at high temperature. In roots, IAA content was detected in adventitious roots but not in initiated storage roots regardless of temperature. IAAOx activity increased with temperature in all root types and was much less in storage roots than adventitious roots. There were no differences in the number of storage roots initiated and the storage to total roots ratio between temperature treatments, but the storage root dry matter decreased at high temperature. These results suggest that temperature influences auxin content and IAAOx activity in leaves, but their role on storage root initiation was not clearly determined.  Auxin appears to have a role in adventitious root development, but it appears not to be required for growth (bulking) once storage root has been initiated.