Morphological Analysis of Tillering in Agropyron spicatum and Agropyron desertorum

The caespitose grasses Agropyron spicatum and Agropyron desertorum exhibit a striking difference in tillering response following experimental clipping treatment, with plants of A. desertorum producing up to 18 times more tillers. The two species are similar in many aspects of their phenology and physiology. Previous examination of current photosynthate production and levels of stored carbohydrates indicate only slight differences between the species. The possible role of three anatomical/morphological constraints in controlling tillering was examined. No evidence for such constraints was found. A basal cluster of buds is present on the parent tillers. The mean bud number per tiller was similar for both species and the range (3–9) was identical. Nearly all of the bud apical meristems appeared anatomically viable throughout the growing season and vascular development occurred to within 250 to 490 μm of the various bud apices of both species. Both normal fall tillers and summer tillers produced under clipping treatment originated from the largest, most distal buds of the basal cluster of buds. However, precocious, morphologically distinctive, second-order tillers occasionally grew out from the smaller, most basal buds of some elongating fall tillers.