Tiller mortality and its relationship to grain yield in spring wheat
A primary determinant of grain yield in barley (Hordeum vulgare L. emm. Lam) is the number of ear-bearing tillers per plant at harvest, which depends both on the production of tillers and on their subsequent survival to form ears. This three-year field study compares tiller production and survival in relation to final grain yield in three types of barley: 2-rowed winter (2rw), 6-rowed winter (6rw) and 2-rowed spring (2rs), grown in two contrasting environments. These three types differed significantly in shoot and ear number, the winter barleys showing higher tiller production, with the maximum number of tillers ranging from 798 to 2315 m?2 in 2rw, 711 to 1527 in 6rw and 605 to 1190 in 2rs. Grain yield across environments and years was strongly correlated () with the number of ears at harvest. The maximum number of shoots produced by each type of barley was inversely related to the mean temperature during the tillering phase. Tiller mortality was inversely related to the maximum shoot production, being significantly lower in barleys with less tillering capacity, i.e. the spring type (with average values of 34.3% and 42.7% in the two environments). The highest tiller mortality occurred before anthesis and, to a lesser extent, from anthesis to maturity. These data support the hypothesis that the principal cause for tiller mortality in barley grown under Mediterranean conditions is the competition between tillers for a limited supply of resources.
Spikeless tillers of wheat (Triticum aestivum L.) affect grain yield because of less than optimum effective plant population. This study was conducted to examine the genetic variability for tiller mortality, and its relationship to grain yield in diverse wheat lines. Twenty lines were evaluated in replicated field tests in 4 years at Rampur, Nepal. The characters investigated were maximum number of tiller produced, the number of reproductive tillers, tiller mortality, and grain yield. The lines differed significantly for all characters. The tiller mortality ranged from 7 to 30%. There were substantial effects of environment on all four characters. The entry-by-year interactions were significant for all traits, primarily because of changes in the relative genotypic differences for these traits in the four years. However, certain lines consistently ranked low or high for tiller mortality. There was a significant negative correlation between front tine tiller and grain yield in 3 out of 4 years. There was a positive correlation of highest tiller number with reproductive tiller number and with tiller mortality. Grain yield showed a nonsignificant positive correlation with maximum tiller number. The reproductive tiller number was positively correlated with grain yield. Results of this study indicate that spikeless tillers contribute negatively to grain yield and that genetic variation exists for tiller mortality in spring wheat.
Vegetative growth in the form of tillers is crucial to final yield in winter wheat (Triticum aestivum L.). To understand the impact management practices have on tiller initiation, a study was conducted using two seeding rates (1.9 × 106 vs. 6.8 × 106 ha?1) and two N timing applications (single vs. split). Tillers initiated in the fall made up the majority of spikes compared to tillers initiated from 1 January to the start of jointing (GS 30). Tillers initiated in March at either seeding rate produced very few kernels spike–1, low kernel weight, and contributed little to yield. At the high seeding rate, tillers initiated prior to 1 January were responsible for more than 87% of the grain yield. Tillers produced in January– February produced 5 to 11% of the final yield, while tillers produced in March contributed less than 2%. In contrast, at the low seeding rate tillers produced in January–February made up 20 to almost 60% of the final yield. Overall, this study shows the timing and rate of leaf initiation impacts yield and yield components. Earlier tillers have an advantage in that they have shorter periods of leaf development that result in more leaf area which in turn supports more kernel spike–1 and heavier kernels, thus more grain weight per spike. Timing of N (single vs. split) application resulted in no significant impact on tiller development, spike number, kernel number, kernel weight, or grain yield.
The number of spikes ha–1 is a critical yield component of wheat yield. Two factors contribute to the total number of spikes ha–1 at harvest, number of mainstem (MS) spikes and number of tillers plant–1. The number of tillers produced per plant is controlled by the environment during the period of tiller development from three-leaf stage to jointing (GS13–GS30) (Klepper et al., 1982) and the amount of tiller mortality that occurs from jointing to anthesis (GS30–GS69) (Jewiss, 1972; Rawson, 1971). Recent research has shown that the timing of tiller initiation and management factors such as seeding rate influence the rate of leaf development on each tiller which, in turn, influences tiller size and mortality (Tilley et al., 2015). The timing of tiller initiation and management factors such as planting date (Oakes et al., 2016) that promote leaf development could also influence other yield components such as kernels spike–1 and kernel weight. An understanding of when the most spikes are formed and the management factors that promote tiller formation during this critical period would help growers improve wheat yield.
Tillers can be formed at multiple nodes on the MS, and secondary and tertiary tillers can form from nodes on the tillers themselves (Klepper et al., 1982; Evers and Vos, 2013). Under glasshouse conditions Klepper et al. (1982) found that once a tiller is initiated, leaf development on the tiller proceeded at the same rate as leaf development on the MS. However, subsequent research has found that leaf development on each tiller proceeds at a slower rate than that on the MS or even on preceding tillers (Tilley et al., 2015). This indicates that tillers initiated first will always have an advantage in growth and development compared to those initiated later. This advantage will increase as time passes resulting in more leaf area. It is likely that tillers with more leaf area will produce more kernels, heavier kernels, and will be less likely to be lost to tiller mortality.
Timing of tiller initiation can also influence tiller mortality. Charles-Edwards (1984) concluded that self-thinning within plant communities is largely due to the lack of assimilate needed to continue growth and development within the individual stem which, in turn, can lead to a decrease in plant weight and eventually a decrease in plant yield. Some works have explored the purpose of rear tine tiller and the effects it may have on the plant as a whole and concluded tillers that abort may have benefited the plant due to assimilate and nutrient accumulation (Lupton and Pinthus, 1969; Palfi and Dezsi, 1960). However, Langer and Dougherty (1976) concluded that dead tillers had a negative effect on grain yield due to competition for assimilates and nutrients (Sharma, 1995).
Management practices such seeding rate and N application timing can influence the timing and rate of leaf and tiller development (Bauer et al., 1984; Tilley et al., 2015) and grain yield. Tompkins et al. (1991) concluded that grain yields will decline as seeding rates decline. This in part is due to a decrease in spikes. However, it was determined that grain yield can decrease at high seeding rate (HSR) (Gooding et al., 2002) due to a decrease in kernels spike–1 and a decrease in kernel weight (Puckridge and Donald, 1967; Tompkins et al., 1991). Tilley et al. (2015) found that seeding rates influenced the rate of leaf development. Phyllochron intervals (PI) were shorter for each tiller at a low seed rate (LSR) compared to the same tillers at a HSR. This resulted in more leaves on each tiller, more tillers produced and fewer tillers lost to tiller mortality.
Nitrogen is recognized as a vital nutrient needed for growth and development (Miller, 1939; Wilhelm et al., 2002). Nitrogen application timing recommendations for winter wheat in North Carolina (NC) are based on the tiller density (Weisz et al., 2001, 2011). Winter split applications are encouraged if tiller density
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