Abstract
Seed dehydration is the normal terminal event in the development of orthodox seeds and is physiologically related to the cessation of grain dry mass accumulation and crop grain yield. For a better understanding of grain dehydration, we evaluated the hypothesis that hydraulic conductance of the ear decreases during the latter stages of development and that this decrease results from disruption or occlusion of xylem conduits. Whole ear, rachis, and stem nodes hydraulic conductance and percentage loss of xylem conductivity were measured from flowering to harvest-ripeness on bread wheat (Triticum aestivum L.) cv. Récital grown under controlled environments. Flag leaf transpiration, stomatal conductance, chlorophyll content and grain and ear water potentials were also measured during grain development. We show that grain dehydration was not related with whole plant physiology and leaf senescence, but closely correlated with the hydraulic properties of the xylem conduits irrigating the grains. Indeed, there was a substantial decrease in rachis hydraulic conductance at the onset of the grain dehydration phase. This hydraulic impairment was not caused by the presence of air embolism in xylem conduits of the stem internodes or rachis but by the occlusion of the xylem lumens by polysaccharides (pectins and callose). Our results demonstrate that xylem hydraulics plays a key role during grain maturation.
Highlights
In cereals, once the final grain number and grain size potential has been established, a few days after anthesis (Yang et al, 2009), grain yield is largely determined by the duration of the grainfilling phase (Triboi and Triboi-Blondel, 2002; Borrás and Gambín, 2010)
Hydraulic Conductance of Developing Wheat Ear developing wheat and barley (Hordeum vulgare L.) grains indicated that it is unlikely that lack of assimilate is responsible for the termination of starch synthesis in the endosperm (Jenner and Rathjen, 1975, 1977; Cochrane, 1985), and wheat canopy can maintain high leaf nitrogen concentration and photosynthetic capacity until end of grain filling when nitrogen is well managed (Triboi and Triboi-Blondel, 2002)
Grain water content increased during the lag phase, remained constant during the filling phase and declined rapidly at the onset of the drying phase
Summary
Once the final grain number and grain size potential has been established, a few days after anthesis (Yang et al, 2009), grain yield is largely determined by the duration of the grainfilling phase (Triboi and Triboi-Blondel, 2002; Borrás and Gambín, 2010). The lag phase is a period of active cell division and differentiation It is characterized by a rapid increase of the mass of water per grain (hereafter grain water content) with negligible gains in dry matter (Saini and Westgate, 2000). Grain water content reaches its maximum value at the beginning of this phase and stays nearly constant until the end, while grain water concentration decreases steadily during the first two phases of grain development and is closely related to the stage of grain development. This apparent desiccation does not affect grain water relations, since osmotic volume (i.e., the cellular volume on which osmolytes are diluted) remains fairly constant (Barlow et al, 1980; Bradford, 1994). Grains achieve their maximum dry mass, commonly referred to as physiological maturity, and their water content and water and osmotic potentials decrease gradually, which results in a reduction of metabolism and the embryo enters a metabolic inactive or quiescent state (Saini and Westgate, 2000)
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