Plant Growth Regulator Effects on Balancing Vegetative and Reproductive Phases in Alfalfa Seed Yield

The use of plant growth regulators (PGRs) has opened new prospects for increased seed production in grasses and legumes, but little information is available on the effects of PGRs on alfalfa ( Medicago sativa L.) seed production. This study was conducted to evaluate the effects of applying chlormequat chloride (CCC) (2‐chloroethyl‐trimethyl‐ammonium chloride) in combination with five PGRs {naphthylacetic acid [1‐naphthylacetic acid], sodium nitrophenolate, gibberellic acid 3 [2,4a,7‐trihydroxy‐1‐methyl‐8‐methylenegibb‐3‐ene‐1,10‐carboxylic acid‐1, 4a‐lactone], triacontanol [1‐triacontanol], and brassinolide [(22R,23R,24R)‐2α,3α,22,23‐tetrahydroxy‐ β‐homo‐7‐oxa‐5α‐ergostan‐6‐one]} on seed yield, aboveground biomass, harvest index, plant height, basal stem diameter, lodging, yield components, and seed quality. Chlormequat chloride was applied annually at the stooling stage while five PGRs were applied twice each year at the stages of flower bud formation and peak flowering. We hypothesized that CCC would decrease plant height, while the five PGRs would increase flowering and seed set. In theory the combined application of CCC with a PGR would be more efficient in improving seed yield. All the PGRs increased the mean 3‐yr seed yield and harvest index by 10%, but did not affect aboveground biomass. Chlormequat chloride reduced plant height and lodging, but reduced mean seed yield. The effects of CCC on seed yield depended on climatic conditions. No interactions between CCC and PGR on seed yield were noted. Neither the PGRs nor the CCC affected seed quality. Our results suggest that these PGRs could be used in alfalfa breeding to increase seed yield while maintaining high seed quality.