PREANTHESIS PARTITIONING OF DRY-MATTER IN SUNFLOWER (HELIANTHUS-ANNUUS L) CROPS

A simple framework for describing the partitioning of biomass among th e organs (root, stem, laminae, head) of sunflower plants between emerg ence and anthesis has been developed using data from experiments in wh ich adequate water and nitrogen supplies were assured. It was hypothes ized that organ partitioning coefficients (the slope of the organ weig ht:total weight relationship) remained constant during phenologically defined phases of crop development. A non-linear optimization routine was used to define organ partitioning coefficients and the values of t otal plant weight at which changes in partitioning took place. It prov ed possible to relate these values to defined stages of plant developm ent, namely bud visible and commencement of rapid head growth. The dyn amics of partitioning to the stem could be described by two coefficien ts applying to the emergence-bud visible and bud visible-anthesis phas es, respectively. Complementary changes in the coefficients for leaf l aminae and fine roots took place at the bud visible stage. Once rapid growth of the inflorescence had become established, further changes in partitioning to roots and laminae, but not to stem, took place. Value s of the coefficients for each organ changed only slightly between exp eriments (three) and cultivars (five), in spite of substantial variati ons, between data sets, of total plant weight at anthesis and, to a le sser extent, in the timing of the anthesis and bud visible stages. A s atisfactory description of organ biomass dynamics was obtained when th is descriptive framework was tested against data obtained in an indepe ndent experiment.