Canopy Light and Tiller Mortality in Spring Barley

Mortality of tillers in barley (Hordeum vulgare L.) is an important developmental event affecting spike number and yield potential of the crop. It has traditionally been thought that increased shading of young tillers by the developing canopy initiates the premature senescence of tillers. A series of field experiments were conducted in 1982, 1983, and 1984 at St. Paul, MN on a Waukegan silt loam soil (fine-silty over sandy or sandy skeletal, mixed, Typic Hapludoll) to test this hypothesis. Degree of shading within the crop canopy was measured in relation to the position of tiller leaves during the vegetative phase of development for three genotypes. The decline in rate of leaf appearance on nonsurviving primary tillers, monitored as an early indicator of tiller senescence, was noted within 3 to 4 wk after crop emergence and before appreciable shading of tillers occurred. This indicates that tiller mortality was not initiated by lack of light. However, after main stem elongation began, tillers soon became heavily shaded suggesting the possibility that shading plays an auxiliary role in the senescence of barley tillers. We discuss the possibility that changes in light quality early in the growing season may be important for initiating the senescence of tillers. T NUMBER of tillers produced in a barley crop community is important in establishing the yield potential of the crop. However, not all initiated tillers complete development to bear fertile spikes. The proportion of tillers that survive to produce grain depends on the genotype, N fertility, water status, plant density, and plant spatial arrangement (Thorne, 1962; Cannell, 1969; Willey and Holliday, 1971; Kirby and J.G. Lauer, Dep. of Plant, Soil and Insect Sciences, Univ. of Wyoming, Laramie, WY 82071; and S.R. Simmons, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108; Minnesota Agric. Exp. Stn. Journal Series Paper no. 15,545. Received 28 Oct. 1987. *Corresponding author. Published in Crop Sci. 29:420-424 (1989). Paris, 1972; Masle-Meynard, 1981; Simmons et al., 1982; Simmons and Rasmusson, 1986). The sequence of developmental and physiological events resulting in premature death of tillers and their interrelationship with factors in the crop environment are poorly understood. Both shading of tillers and shifts in shoot photoassimilate partitioning patterns have been proposed as factors causing tiller death in grasses (Spiertz and Ellen, 1972; Kays and Harper, 1974; Ong et al., 1978; Colvill and Marshall, 1984; Lauer and Simmons, 1985). Ong and Marshall (1979) hypothesized for perennial ryegrass (Lolium perenne L.) that reductions in the supply of photoassimilate to tillers in conjunction with increased translocation of photoassimilate to the rapidly elongating stems causes a photoassimilate limitation for young, shaded tillers resulting in their death. Simmons and Lauer (1986) confirmed for barley that the proportional allocation of photoassimilate to primary tillers in barley declined at the onset of rapid main stem elongation. The degree of shading of young tillers at the time of this shift in photoassimilate partitioning has not been previously measured. The role of light in causing tiller mortality was investigated by Spiertz and Ellen (1972) who reported that light enrichment treatments reduced tiller mortality in perennial ryegrass. Kirby and Paris (1972) hypothesized that light was important for tiller survival based on observations that mutual shading of tillers occurred at approximately the same time as the appearance of chlorosis on nonsurviving tillers. However, specific measurements of the degree of canopy shading in relation to development of tillers have not been made. Recognizing that initiation of tiller senescence likely begins some time before the appearance of chlorosis has caused us to question the premise that LAUER & SIMMONS: CANOPY LIGHT AND TILLER MORTALITY IN BARLEY 421 lack of light initiates tiller mortality in barley crop communities. Knowing more precisely the degree of correspondence between the onset of tiller senescence and the development of significant tiller shading in the canopy should help to resolve this issue. Our study was conducted with the objective of ascertaining the degree of association between canopy light conditions in field barley communities and the survival of tillers. This was approached by measuring canopy light levels in relation to the development of surviving and nonsurviving tillers in spring barley genotypes differing in tiller survival characteristics. Of particular interest was estimating the correspondence between shading and senescence of primary tillers. MATERIALS AND METHODS Plot culture. Similar studies were conducted in 1982, 1983, and 1984 at St. Paul, MN on a Waukegan silt loam soil fertilized in accordance with soil test recommendations for eastern Minnesota and a grain yield goal of 3.2 Mg/ha. Plots were 3 m long and 9 or 18 rows wide, depending on the year. Row spacing was 18 cm. Experimental designs in each year were randomized complete blocks with six, three, and four replicates, respectively. Crops were sown between 17 and 26 April and emerged between 3 and 7 May. Plant populations of 330 plants m-2 were established by overseeding and thinning. No pesticides were applied to the plots and tiller survival genotype, was the third genotype grown in 1982. ’Morex’, also with intermediate survival, replaced Manker in the 1983 and 1984 studies. Measurements of tiller leaf production. Kirby and Riggs (1978) observed that the rate of leaf production declined for nonsurviving tillers in the initial stages of their senescence, even before they displayed visible signs such as chlorosis. We chose to use this characteristic in order to nondestructively estimate the onset of tiller senescence in the crop. Leaf numbers were counted on primary tillers of 12 randomly selected plants in each plot beginning at tiller emergence. The system of Haun (1973) was used for counting leaves which the newest emerging leaf is expressed as a proportion of the most recently fully expanded one. This system permits repeated observations on the same tiller. The specific tillers monitored were TI and T2 in the axils of the first and second main shoot leaves, respectively. Nonsurviving tillers were