Anatomical insights into the vascular lay-out of the barley rachis: implications for transport and spikelet connection.

Abstract

Vascular patterning is intimately related to plant form and function. Here, using barley (Hordeum vulgare) as a model, we studied the vascular anatomy of the spike-type inflorescence. The main aim of the present work was to clarify the relationship between rachis (spike axis) vasculature and spike size, define vascular dynamics, the implications for transport capacity and its interaction with the spikelets. We employed serial transversal internode sections to determine the internode area, vascular area, and vein number along the rachis of several barley lines. Internode area and total vascular area show a clear positive correlation with spike size, whereas vascular number is only weakly correlated. While the lateral periphery of the rachis contains large mature veins of constant size, the central part is occupied by small immature veins. Spikelet-derived veins entering the rachis often merge with the immature rachis veins but never merge with the mature veins. An increase in floret fertility through the conversion of a two-rowed barley into an isogenic six-rowed line, as well as a decrease in floret fertility due to enhanced pre-anthesis tip degeneration caused by the mutation tip sterile 2.b (tst2.b) significantly affected vein size, but had limited to no effects on vein number or internode area. The rachis vasculature is the result of a two-step process involving an initial layoutfollowed by size adjustment according to floret fertility/spike size. The restriction of large mature vessels to the periphery and that of small immature vessels to the center of the rachis suggests that long distance transport and local supply to spikelets are spatially separated processes.The identification of spikelet-derived veins entering the rachis without fusing with its vasculature indicates that a vascular continuity between rachis and spikelets may be non-essential.