Flower abscission in soybean as affected by assimilate supply and hormone levels

Abstract

<p>Flower abscission of a determinate soybean genotype, with elongated racemes, was studied in the greenhouse. The response of a typical raceme situated in the middle of the plant canopy was characterized as the plant and raceme environments were altered. The plant environment was modified by increasing plant density, debranching the plant weekly or through shading from the period of raceme appearance until a week after flowering. These treatments were used to change the dry weight allocation within the plant. As for the raceme environment, modifications were performed using compounds with assimilatory or hormonal roles which were applied to the typical raceme, or to the plant as a whole;The amount of dry weight partitioned to the raceme seemed to determine the amount of flower abscission that occurred. Continual debranching of the plant plus the application of the growth regulators 2,3,5-triiodobenzoic acid (TIBA) and 6-benzylaminopurine (BAP) to the raceme during flowering increased the dry weight of the raceme and resulted in lower percentages of flower abscission. Shading the plant one to two weeks before flowering and applying ethephon to the raceme during flowering increased the percentages of flower abscission. Ethephon seemed to be involved in the formation of an abscission layer that detached the flower from the raceme. The effect of the shading treatment could be relieved by the application of BAP at flower opening to bring flower abscission levels closer to those found in the non-shaded plants;Hormone levels within the plant may determine the amounts of water, minerals, and assimilates that are channelled to the raceme, as hormone-like compounds applied to the raceme were found to change vascular development. BAP and TIBA hastened the development of a xylem network within the ovary of the flower. TIBA was effective during the period currently thought to determine flower survival, whereas BAP resulted in better xylem development 2 to 3 days after this time. BAP may have increased the rate of development of the phloem or perhaps hastened cell differentiation, thus making the ovary a larger sink for assimilate.</p>