Buckwheat seed set in planta and during in vitro inflorescence culture: Evaluation of temperature and water deficit stress

Common buckwheat (Fagopyrum esculentum Moench) plants produce many flowers, but fewer seeds. Seed set is highly variable among years, between plants, and during the period of flowering within a plant or raceme. Seasonal varia tions suggest that temperature and water-deficit stresses are important fac tors for seed set. The effects of mild temperature and water-deficit stress es on seed set and seed filling were determined in planta and in vitro. An in vitro method to culture matched sets of racemes from individual plants w as used for precise comparisons between experimental treatments. Buckwheat racemes form new flowers continuously during several weeks in planta. Seed set resulting in yield occurs during the first 2-3 weeks of flowering and t hen rapidly declines in planta independently of mild stresses. Plants grown at 18 degreesC have 40% increased seed set, set seeds over a longer durati on, and produce 40% more dry matter per seed than plants grown at 25 degree sC. Similar patterns occurred in vitro. A 3-day water-deficit stress during the first week of flowering reduced the number of seeds by 50% without a r eduction in seed size and dry weight, or the number of flowers formed in pl anta or in vitro. The effect of water-deficit stress continued after rewate ring and subsequently was expressed as a reduction in fertility in newly fo rmed flowers, both in planta and in vitro. Mild temperature and water-defic it stresses affected both female and mate components of seed set in common buckwheat, resulting in a persistent but non-additive reduction in sink str ength.