Abscission of leaf laminas – An unnoticed factor in tussock grass formation

Drop of leaves at structurally defined abscission zones is a typical feature of dicotyledonous angiosperms. It is rare in monocots including the grass family (Poaceae). Some grass species have an odd, yet poorly studied pattern of leaves disarticulating regularly between the leaf sheath and the leaf lamina. Using species of three distantly related genera, such disarticulation zones were anatomically studied for the first time to clarify the structural background of shedding. The abscission zones of the three species turned out strongly different from each other in position, spatial arrangement and cellular architecture of different tissues, which is described and illustrated in detail. Breakpoints of the lamina were consistently defined by a sharp delineation of parenchymatous against sclerenchymatous tissue, irrespective of how these tissue types were arranged, e.g., sclerenchyma on the top of the sheath (Danthoniastrum), at the base of the lamina (Macrochloa), or alternating longitudinally with parenchyma (Aristida). The tussock grasses Danthoniastrum and Macrochloa had the most complicated and elaborately structured abscission zones. The results suggest that precise and efficient shedding of leaf laminas is an important factor enabling grasses to form tussocks as implicitly addressed already in 1890 by the agrostologist E. Hackel. Tussocks are a habitually conspicuous and ecologically important growth form of grasses in arid and/or alpine vegetation zones across the world. Judging from our exemplary results it must be expected that even further structural and mechanic patterns of precise leaf blade abscission will be found in tussock grasses of other grass subfamilies, such as the panicoids, chloridoids or arundinoids.