White Clover Morphology Changes with Stress Treatments

white clover is positively correlated with increased size of plant parts and reduced leaf density per unit area The plasticity of white clover (Trifolium repens L.) results in (Beinhart et al., 1963). changes in plant habit in response to different environmental stresses. The objective of this research was to characterize those morphological Defoliation through grazing or clipping is a managechanges associated with plasticity in white clover clones derived from ment-controlled stress that affects plant persistence and ‘Osceola’, ‘Grassland Huia’, and SRVR germplasm. Clones were exproductivity. In a pasture, grazing removes the leaf but posed to treatments in the following two-way factorial design: no the stolon is usually undisturbed (Williams and Hogclipping or clipping on 7-d intervals and barrier to stolon rooting or land, 1978). The defoliation mobilizes energy stores in no barrier. In a three-season greenhouse test, 90 clones were measured stolon and root to produce new leaves and other photofor leaf dry weight (DW), stolon DW, stolon length, root DW, and synthetic components (Kendall and Stringer, 1985). apex number. Generated parameters were herbage DW (leaf DW White clover has a remarkable phenotypic plasticity stolon DW), biomass DW (root DW herbage DW), leaf-to-stem which results in a nonheritable change in plant morpholratio (leaf DW stolon DW), and herbage-to-root ratio (herbage ogy in response to environment, season, and manageDW root DW). Plant means were significantly reduced for all traits and ratios with repeated clipping. The rooting barrier significantly ment or harvest (Brock et al., 1988; Solangaarachchi reduced apex number, root DW, and the leaf-to-stem ratio while and Harper, 1989). In mixed pastures, plant morphology increasing the stolon DW and the herbage-to-root ratio. The interchanges in response to interspecific competition from action of root barrier and repeated clipping was significant for apex Kentucky bluegrass (Poa pratensis L.) (Atwood and number, root DW, and the herbage-to-root ratio. Near zero correlaGarber, 1942), perennial ryegrass (Lolium perenne L.) tions for the same measurements made on the same clone grown with (Dijkstra and de Vos, 1972), and bermudagrass [Cynodifferent stress treatments suggest genetic differences in the magnidon dactylon (L.) Pers.] (Brink and Rowe, 1993; Rowe tude of plasticity. Path analysis further described changes in relationand Brink, 1993). The white clover changes are more ships among traits for plants exposed to different stresses. The magnicomplex than consistent decreases in vigor, viability, tude of plasticity appeared to be a clone-specific phenomenon that or reproduction of the plant or stand (Duncan, 1994). may frustrate verifying gain with selection for progenies of selections evaluated in different environments. Response to stress is more than a simple decrease in productivity. White clover is commonly characterized by its leaf size (Williams, 1987). Leaf size has been related to W clover is an important forage legume in growth habit (Eagles and Othman, 1986; Kang et al., many temperate regions of the world (Frame and 1995), field growth, or spread (Davies and Evans, 1982); Newbould, 1986), particularly in mixed species pastures. grazing responses (Brink and Pederson, 1993; Brock White clover adaptability is attributed, in part, to genetic and Tilbrook, 2000); and rate of regrowth with clipping variability in the species (Burdon, 1980; Turkington and (Wilman and Asiegbu, 1982b). Reducing the time beHarper, 1979). Perenniality of white clover depends on tween clippings has more dramatic and detrimental efrenewal of plant parts (Beinhart, 1963; Williams, 1987), fects on large leaf types of white clover than on the and a recently established stand of white clover evolves small leaf types (Wilman and Asiegbu, 1982a; Grant from tap root growth to clonal growth in 1 to 3 yr and Barthram, 1991). For the white clover seedling, (Brock and Tilbrook, 2000). Persistence of white clover reducing the clipping frequency resulted in a quadratic ecotypes is positively associated with stolon density, dry increase of plant dry weight and this increase was greater matter yield, and internode length (Piano and Annicchifor the large leaf white clover cultivars. Annual DM arico, 1995). When grazed, cyclic morphological changes yield doubled or tripled when harvest interval changed include increasing stolon length during the spring with from 7 to 28 d, but for the 48-d harvest interval the a maximum in June and July followed by a decline yields were comparable to the 7-d harvest interval during the summer to a minimal length in October (Brink, 1995). Clipping causes a greater reduction in (Brink et al., 1998). Change in stolon density has been stolon length, herbage yield (Wilman and Asiegbu, linked to plant habit so that the less branched, more 1982b), and root carbohydrate content (Kang and Brink, viney plant habit has much less cyclic change in stolon 1995) of the large leaved than for the small leaved white density (Gibson et al., 1963). The rate of spreading of clover. With defoliation, more carbohydrate is partitioned to the apex and less to the main stolon; however, H. Seker, Eastern Anatolian Agricultural Research Institute Dathe degree of partitioning between root and apex is daskent-Eruzurum, Turkey; D. Rowe, USDA-ARS, Waste Management and Forage Research Unit, 810 Highway 12 East, Mississippi cultivar dependent (Frankow-Lindberg, 1997). DefoliaState, MS 39762; G. Brink, USDA-ARS, U.S. Dairy Forage Research tion of greenhouse plants has been shown to signifiCenter, 1925 Linden Drive, Madison, WI 53706. Received 16 Aug. cantly reduce bud viability by 44% and similar reduc2002. *Corresponding author ([email protected]). tions may occur in pastures (Newton and Hay, 1996). Published in Crop Sci. 43:2218–2225 (2003).  Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DW, dry weight; SD, standard deviation.