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

58 JOURNAL OF APPLIED SEED PRODUCTION, VOL. 3, 1985decreased (Hebblethwaite, et al., 1980). This may be aresult of earlier and more severe lodging, or to increasedcompetition for assimilates between floret site and secondaryvegetative tillering (Hebblethwaite et al., 1980; Hampton,1983; Clemence and Hebblethwaite, 1984). Vegetativetillers subtending developing fertile tillers compete forassimilates with the growing seed (Hampton, 1983; Clemenceand Hebblethwaite, 1984). In a lodged crop (cv. S.24)assimilate movement from the flag and penultimate leaveswas mostly down the plant to the subtending vegetativetillers. This was confirmed by Clemence and Hebblethwaite(1984) who also showed that during stem elongation, theear was a net exporter of assimilate and that up to 10% ofthis assimilate was recovered from subtending vegetativetillers. Transfer of assimilates from vegetative to parenttillers was found to be low as only about seven percent ofthe t4C they fixed was exported (Hampton, 1983).Delaying the application of nitrogen to ear emergencedecreases the number of florets per unit area but increasesseed set. This can be attributed to a reduction in the severityof lodging at anthesis, or a reduction in the number ofpotential seed sites per ear resulting in less competitionbetween sites. However, this delay in nitrogen applicationdecreases yield in spite of the increased seed set because ofthe reduced yield potential (Hebblethwaite et al., 1980).Late nitrogen application can also result in excess secondarytillering (Nordestgaard, 1980) particularly in wet seasonsand this could result in further competition for assimilates.High levels of nitrogen increase the severity and bringforward the time of lodging (Hebblethwaite and Ivins,1977). Lodging decreases the proportion of florets that setseed (Burbidge et al., 1978). In S.24 the percentage offlorets which set seed decreased from the base to the apex ofthe ear in a lodged crop but was more uniformly distributedin an erect crop (Burbidge et al., 1978). In S. 24 thepercentage seed set for each floret position was also foundto be higher in erect crops (Burbidge et al., 1978). Lowerseed set in lodged crops have been attributed to suppressionor inhibition of anthesis (Burbidge, 1977) because of lessfavorable micro-environmental conditions (Hampton andHebblethwaite, 1983) and decreased pollen dispersal(Wright and Hebblethwaite, 1979).The above work on nitrogen and lodging indicated thatthe prevention of lodging by chemical means could significantlyincrease seed set and yield in ryegrass seed crops. In1981 and 1982 work using the chemical paclobutrazolsprayed at spikelet initiation showed that this treatmentsignificantly increased the number of seeds per spikeletpresent at final harvest by reducing the number of seedsaborted during seed development (Hampton andHebblethwaite, 1985a) and this resulted in substantialincreases in seed yield (Hampton and Hebblethwaite, 1985b).This increase due to paclobutrazol application was due to analteration in the distribution of florets and seeds per spikelet,as both the basal and penultimate spikelets contained moreflorets and seeds than did those of untreated plants (Hamptonand Hebblethwaite, 1985a).Further work indicated that in untreated plants, assimilaterecovery was lower from the terminal section of the ear,whereas in chemically treated plants, no differences werefound between basal, intermediate or terminal sections ofthe ear. Assimilate demand at all sections of the ear werealso increased when the ear and leaves were fed with t4C0 2in paclobutrazol treated plants (Hampton and Hebblethwaite,1985a). This work shows that abortion of developing seedsis an important factor in contribution to low number ofseeds harvested.As well as the above effects this chemical was highlysuccessful in preventing lodging by increasing stern basestrength, reducing stern internode length, increasing rootdry matter and leaf area duration. All these responsesenabled large increases in seed yield (Hampton andHebblethwaite, 1985b).In the short term there is little the breeder can do to selectfor higher seed yield in known varieties. The use of growthregulators therefore has a great potential for the future.Paclobutrazol (Parlay) has been released since 1985 on alimited basis for commercial use in U.K. and U.S.A.herbage seed crops. Results to date have been promising.Further work is also being carried out on other growthregulators and results have been good (Hebblethwaite,Barrett and McGilloway, 1985). However, these productscan result in upright crops which are highly susceptible toshedding prior to harvest under adverse conditions (Hampton,1983). In the long term it is therefore important thatchemical manipulation of seed shed is given researchpriority. Work started at the University of Nottingham in1985 in this area but with no success to date (Hebblethwaiteet al., 1985).In the long term the herbage seed grower may be lookingto multiple chemical use. Work at the University of Nottingham(Hampton and Hebblethwaite, 1984) has alreadyshown that a combination of GA 3 and paclobutrazol canreduce vegetative tillering prior to ear emergence, reducestem length and lodging with a possible increase in seed setand yield. The next stage is to look for a chemical whichwill substantially increase seed set, seed retention, andtransfer of assimilates to the seed. Added to this could be achemical which would prevent shedding in an upright crop.Perhaps all these functions may be found in one or twochemicals in the future. This approach is not too far distantfrom commercial reality. The speed at which such developmentswill take place will depend on the funding of researchin the future.REFERENCES1. Anslow, R. C. 1963 . Seed formation in perennial rye grass. IAnther exsertion and seed set. J. Br. Grassl. Soc. 18:90-96.2. Burbidge, A. 1977. Lodging and its controlin Latium perennegrown for seed. Ph.D. thesis, University of Nottingham.3. Burbidge, A., P.D. Hebblethwaite, and J.D. Ivins. 1978.Lodging studies in Lolium perenne grown for seed. 2. Floretsite utilization. J. Agric. Sci., Camb. 90:269-274.4. Clemence, T.G.A., and P.D. Hebblethwaite. 1984. An appraisalof ear, leaf, and stem t4C0 2assimilation, t4C assimilatedistribution and growth in a reproductive seed crop ofamenity Lolium perenne. Ann. Appl. Bioi. 105:319-327.5. Hampton, J.G. 1983. Chemical manipulation of Lolium per-