Embryological background of low seed set in distylous common buckwheat (Fagopyrum esculentum Moench) with biased morph ratios, and biostimulant-induced improvement of it

The biased ratio (1 : 2.7–1 : 19) of long-styled Pin and short-styled Thrum flowers (anisoplethy) in common buckwheat (Fagopyrum esculentum) with low seed set (9.8–33.1%) is documented for the first time in two cultivars (Kora, Panda) and two strains (PA13, PA14). To establish the reasons for low grain yield we studied pollen, embryo sacs, embryos, counted stigmas with compatible pollen and with compatible pollen tubes, and recorded seed set under semi-controlled conditions with open access of pollinators. We also sought to improve seed yield via exogenous application of eight biostimulants at the beginning of flowering. Pin pollen supply to Thrum stigmas was low, due to the imbalance of flower morphs. This did not affect seed set or male success in either flower morph. The pollen of Pin or Thrum was highly viable (97.9–99.9%) in all studied cultivars and strains, germinating well on compatible stigmas. The female success of both flower types was much lower; 49–59% of the ovules exhibited signs of degeneration (whole flower buds, ovules only) or abortion (mature embryo sacs, proembryos, embryos); the highest share of mature embryo sac abortions resulted from degeneration of synergids or the whole egg apparatus. Three biostimulants (Gibberellic acid, putrescine, Asahi SL) in PA13 and six (1-Naphthaleneacetic acid, Gibberellic acid, TYTANIT, putrescine, 6-Benzylaminopurine, Asahi SL) in PA14 decreased embryo abortions (4–12 fold) and increased seed set (0.4–2.4 times), but seed set was still low and never exceeded 33% (the highest value of the untreated with biostimulants plants). Biostimulant treatments were most effective on PA14 strain increasing seed set in 7 out of 8 treatments. These were Gibberellic acid, putrescine and Asahi SL improving seed set of two among four analysed genotypes.