Plant Growth Regulator and Irrigation Effects on Physiological and Harvest Maturity of Red Clover in Relation to Seed Quality

Core Ideas Trinexapac‐ethyl, a plant growth regulator, does not affect the quality of red clover seeds if applied at a rate of up to 700 g a.i. ha–1. Seeds reach maximum quality at harvest maturity, when seed moisture content is approximately 140 g kg–1. Visual indicators of physiological maturity of red clover include flower heads contain light brown petals and brownish‐green sepals, whereas seeds are pale green to pale yellow. At harvest maturity, heads contain dark brown petals and sepals, whereas seeds range from yellow to yellow‐dark grayish purple. A single irrigation application of water at the first flowering stage increased yield by approximately 9% in a 2‐yr study. Application of trinexapac‐ethyl at a rate of 560 g a.i. haw in the stem elongation stage increased seed yield by 16% in 1 of a 2‐yr study. Seed moisture content at physiological maturity of red clover ranges from 340 to 540 gkg–1 and approximately 140 g kg–1 at harvest maturity. Plant growth regulators (PGRs) and irrigation affect seed yield of red clover (Trifolium pretense L.). However, the effects of these factors on physiological maturity (PM), harvest maturity (HM), and seed quality are unknown. Therefore, the objectives of this study were to determine the effects of trinexapac‐ethyl (TE) PGR and irrigation on PM, HM, and seed viability and vigor on the most common red clover variety in Oregon over 2 yr. Five TE rates, ranging from 0 to 700 g a.i. ha−1, were applied at stem elongation and bud emergence stages. Irrigation was applied at the first flowering stage. Physiological and visual indicators were recorded to determine PM and HM. Standard germination (SG), tetrazolium (TZ) and cold (CT) tests were conducted to evaluate seed quality at weekly intervals starting from seed formation until HM. Irrigation resulted in a 4‐d delay in PM compared to the non‐irrigated treatment, but TE applications did not affect timing of seed maturation. At PM, flower heads contained light brown petals and brownish‐green sepals, whereas seeds were pale green to pale yellow. At HM, heads contained dark brown petals and sepals, whereas seeds ranged from yellow to yellow‐dark grayish purple. Seed dry weight did not change from PM to HM. Seed moisture content at PM ranged from 340 to 540 g kg−1 and decreased to below 140 g kg−1 at HM. At HM, seeds reached maximum quality as viability ranged from 92 to 98% by TZ and SG, and vigor ranged from 90 to 94% by CT.