Osmotic stress at seedling stage impairs reproductive development in buckwheat (Fagopyrum esculentum)

Common buckwheat (Fagopyrum esculentum Moench) is highly sensitive to drought. Many studies have investigated the impact of water stresses, applied late during plant development, on seed production by this non-Poaceae cereal but information concerning the effects of stresses occurring early during plant's life is essentially lacking. The aim of the present work was to define which yield-related parameters are more affected by a 4-day water constraint, applied at three different developmental stages, by transferring buckwheat plants of the cultivar 'La Harpe' grown in a hydroponic system to a medium supplemented with polyethylene glycol. Stresses during the vegetative phase, when the shoot apical meristem (SAM) was still initiating leaves, or later, when its activity was stopped after production of approximately 10 inflorescences, were less detrimental than an osmotic constraint applied to plants recently engaged in the reproductive phase, with SAM having initiated six to seven inflorescences. The number of cymes and flowers per inflorescence and the number of pollen grains per anther were decreased. The production of seeds in the first inflorescence was also reduced but this reduction paralleled the decrease in cyme number, suggesting that seed set, per se, was not affected. Pollen viability, female fertility and weight of seeds reaching maturity were not altered by the water deficit. The present work clearly identified two processes that, in buckwheat, are particularly sensitive to an osmotic stress: (1) the activity of reproductive meristems involved in the production of cymes and flowers and (2) male sporogenesis.