JASP 3 -- 1985.pdf - International Herbage Seed Group

18 JOURNAL OF APPLIED SEED PRODUCTION, VOL. 3, 1985the untreated plots was a result solely of increased fertiletiller numbers, as the number of seeds per spikelet was notincreased.DISCUSSIONThe objective of applying growth retardants to perennialryegrass seed crops has been to prevent lodging(Hebblethwaite et al. , 1980) and increase seed yield byreducing seed abortion (Hampton et al., 1985). However,seed yield increases following growth retardant applicationhave also resulted, either partly or solely, from increasedfertile tiller production (Hampton and Hebblethwaite1985a). For example, PP333 is known to reduce apicaldominance and increase tillering, the response being greaterwhen the growth retardant is applied early in the phase oftiller development (Froggatt et al., 1982). Hampton (1983)suggested that growth retardant application prior to apicaldifferentiation may lead to a greater synchronous emergenceof tillers and a greater proportion of fertile tillers, as previouslydemonstrated in wheat (Hofner and Kuhn, 1982) andbarley (Matthews et al., 1982).In both 1983 and 1984, the greatest number of seeds,though not necessarily the greatest seed yield, resulted fromEL500 application prior to apical differentiation. In 1983,fertile tiller number increases were consistent with EL500application time i.e. greatest with the earliest application,and it is possible that the same pattern would have followedin 1984. However a dry April (Table 6) meant that growthretardant activity following the FI application (20 April) wasdelayed - stem length reductions were not as great as theprevious year, and the lodging pattern followed that of untreatedplots. EL500's requirements for water before soilactivation are similar to those of PP333 (Hampton andHebblethwaite, 1984a). However, once water became available(at ear emergence), the response was an increase in tillerproduction in plots already lodged, similar to that reported inthe 1982 season (Hampton and Hebblethwaite, 1984b). Althoughfertile tiller numbers were significantly increased,seed yield was reduced because of a reduction in the numberof seeds per spikelet.EL500 application at DR allowed the production of morefertile tillers, but not a greater proportion of fertile tillers assuggested by Hampton (1983). In both years, the greatestproportion offertile tillers was achieved with SI application,as fewer vegetative tillers were produced. However, tillernumber differences between DR and SI application timeswere not significant, and the proportion of fertile tillers forboth times was over 75%, compared with around 50% foruntreated plots, a result also reported for PP333 (Hamptonand Hebblethwaite, 1984b).Hofner and Kuhn (1982) suggested that in wheat, growthretardant application, by altering the balance between GA3and GA3 inhibitors, may influence apical differentiation bysynchronizing and reducing the growth rate of individualspikelets, and lead to an increase in the number of spikeletsand of seeds per spikelet. EL500 application prior to apicaldifferentiation had no effect on the number of spikelets pertiller in either year, and differences in the number of seedsTable 6. Rainfall data, February-July; longterm average, 1983and 1984, Sutton Bonington.Rainfall (mm)Month Longterm 1983 1984average-1February 41 27.0 45.4March 45 32.9 58.3April 39 87.7 7.4May 49 78.6 59.9June 48 8.4 73.4July 51 28.6 24.0IJ9!6-1982per spikelet were not significant from that of EL500 appliedat SI. While the number of seeds per spikelet for FI plots wasreduced in 1984, Hampton and Hebblethwaite (1985a) obtainedmore seeds per spikelet from FI application of PP333than from SI application. In perennial ryegrass, increases inthe number of seeds per spikelet are more likely to result fromreduced seed abortion because of reduced competition forassimilates than any 'antigibberellin' effects of growth retardants(Hampton and Hebblethwaite, 1985c).EL500 application at DR produced the greatest number ofseeds per unit area in each year, but this potential was notachieved in terms of seed yield because of reduced thousandseed weight (TSW). This suggests either that plants treated atDR could not support the number of seeds retained in eachspikelet, or that maturity was delayed even more than the 3-5days recorded for SI application (Hampton, 1983) so that DRplots were harvested before adequate seed filling had beenallowed to occur. This requires further investigation. Seedyield results did not show any significant differences betweenDR and SI application times, but the 1984 results confirm thepossible inconsistency of FI application (Hampton andHebblethwaite, 1985a) for growth retardants which requirewater for activation.ACKNOWLEDGEMENTSWe gratefully acknowledge the financial assistance of theBritish Seeds Council, the growth retardant supplied byBlanco Ltd., the technical assistance of Mr. J. Travers, Mrs.P. Tetlow, Mrs. S. Manison, and Mrs. B. Hull. J.G.H. alsogratefully acknowledges the New Zealand National ResearchAdvisory Council for the award of a fellowship to undertakethe Ph.D. studies from which part of this paper originated.REFERENCESI. Anon. 1983. Technical report on EL500. Lilly Research Laboratories,Eli Lilly and Co., Indianapolis, U.S.A.2. Batts, G.R. 1984. The effect of growth regulators on seedproduction in Lolium perenne L.: a timing investigation. B.Sc.(Hons.) dissertation, University of Nottingham, U.K.3. Froggatt, P.J., W.D. Thomas, andJ.J. Batch. 1982. The valueof lodging control in winter wheat as exemplified by the growthregulator PP333. pp. 71-87. In A.F. Hawkins and B. Jeffcoat