Seminar on Floret Site UtilizationWageningen, the Netherlands 1June 10, 1985INTRODUCTIONGrass production in the Netherlands provides green fodder,green manure, persistent ground cover for amenity purposes,and most importantly stabilization of dikes. For all thesepurposes, stem growth and flowering is undesirable. Therefore,breeding and selection of new varieties are concernedmainly with the vegetative plant characteristics. As a result,seed production characteristics are more or less neglected orare not considered until the final stages of selection.The issue is complicated by the perennial nature of mostgrass species. In the reproductive phase, grass plants are anintegrated system of reproductive and vegetative tillers.Despite the complexity of the physiology, experimentswith a single variety indicate that variation in seed yielddepends upon two components--the number of inflorescencesper unit area and the proportion of the florets that produceharvestable seeds.Most previous seed production research has concentratedon increasing the number of inflorescences per unit area.Presently it is fairly well known how to enhance inflorescenceproduction in most species from northern latitudes.During the last decade fundamental and applied researchhas focused on floret site utilization as a means of increasingseed yield. Dr. Paul Hebblethwaite and co-workers at NottinghamUniversity initiated this work and proved the importanceof fertilization and seed set percentage to seed yield.Recent research in several countries on use of growth retardantsand fungicides coupled with successes in breeding forseed retention demonstrate that innovative developments ingrass seed production are at hand. A prerequisite will be abetter understanding of the physiological processes that causethe wide variation in floret site utilization. An excellent basisis available from results of studies of assimilate productionand distribution patterns at Bangor University (UK).The objectives of this seminar were:- to summarize current information and examine problemsin floret development, fertilization, seed abortion,seed filling and ripening processes in grasses- to involve plant physiologists and cytologists in thesesubjects- to stimulate interest by plant breeders in seed productionaspects in their programsThe following five articles are from this seminar.Willem MeijerOrganizertSeminar held at Wageningen, the Netherlands on June 10,1985 was organized by the Research Station for ArableFarming and Field Production of Vegetables (PAGV) atLelystad and sponsored by the National Council for AgriculturalResearch (N. R. L. 0.), the Netherlands. Professor L.'t Mannetje, Department of Field Crops and Grassland Science,Agricultural University, Wageningen, presided overthe seminar.42
Developmental and Physiological Aspects of <strong>Seed</strong> Production in <strong>Herbage</strong> GrassesC. Marsh aliiABSTRACTThe developmental and physiological factors determining thepotential seed yield of the grass inflorescence are discussed,especially in relation to the productivity of florets. Analysis ofthe difference between potential and actual seed yield of theinflorescence reveals that a large proportion of florets are unproductive;many set seed but then abort developing seeds. Thephysiological factors influencing seed abortion are not well understoodbut nutritional factors, particularly carbohydrate supply,may be important. The inflorescence is the major source ofassimilate for seed growth and development, but some evidencesuggests that vegetative tillers may compete with developingseeds for assimilate produced by the inflorescence. Such competitionmay be important in initiating, at least in part, the abortionof some developing seeds. The productivity of florets is increasedby the application of certain plant growth regulators and fungicides,and this may be related to an improvement in the supply ofassimilate to developing seeds.Additional index words: florets, seed set, seed abortion, seedgrowth, source-sink relations, 14C-assimilate, growth regulators,fungicides.---------------------------INTRODUCTIONThe reproductive potential of the grass inflorescence is setby the number of spikelets and the number of fertile floretsper spikelet. The degree to which this potential is realized interms of yield depends on the proportion of florets thatproduce seed and the size of individual seed. Both herbagegrasses and cereals show a characteristic pattern of underutilization of reproductive potential in that the number ofseed produced per inflorescence is substantially less than thenumber of fertile florets, and this is especially the case ingrasses where there may be a sevenfold difference betweenfloret and seed numbers (Hebblethwaite et al., 1980). Thisemphasises the importance of the developmental processesoccurring from anthesis onwards, namely pollination, fertilization,seed set and seed growth, and there is good evidencethat deficiencies in each of these sequential steps restrictspotential seed yield of the inflorescence (Hill, 1980). Agronomicstudies suggest that there is a close relationship betweenthe number of seeds produced per unit area and the yield ofseed, and that this is primarily related to the number of seeds'Senior Lecturer, School of Plant Biology, University College ofNorth Wales, Bangor, Gwynedd LL57 2UW, U.K.per spikelet rather than to the number of fertile tillers producedby the crop (Hebblethwaite et al., 1982; Hampton andHebblethwaite, 1983). However, in the field it is evident thatseed production falls far short of its potential due to thelodging of the crop canopy, as large increases in seed yieldresult if lodging is prevented or reduced (Hebblethwaite andIvins, 1978; Hampton and Hebblethwaite, 1985a).This paper will consider some of the phsysiological anddevelopmental factors underlying the establishment of theyield potential of the grass inflorescence, and the subsequentutilization of this potential especially with respect to the fateof fertile florets.The Components of YieldIn perennial grasses the population of flowering shoots iscomposed of tillers of varying age and origin, and thesedisplay a considerable range of yield potential. Studies on theyield components of the inflorescence of Lolium perennehave revealed relatively small differences in spikelet numberbetween inflorescences of tillers appearing during the summerand winter months, but large differences in the numberof florets formed per spikelet (Ryle, 1964; Hill and Watkin,1975; Colvill and Marshall, 1984). In this species up to 14florets may be produced per spikelet, but not all of thesebecome fertile and produce a seed. The grass inflorescencethus produces far more florets than seeds and this is evidentfrom the results shown in Fig. 1 where the inflorescences ofsuccessively appearing primary tillers display a progressivedecline in the numbers of florets and seeds, with just underthree-quarters of all florets bearing a seed. The differencebetween floret and seed numbers is less in later than earlierproduced tillers, e.g. 65% of florets yield seeds in the firsttwo primary tillers (Tl and T2) compared with almost 80%for the T7-Tll group, thus spikelets with relatively fewflorets tend to be more productive than those with a highernumber of florets.Despite these differences the seed yield of the inflorescenceof the main shoot and early primary tillers is significantlygreater than that of later appearing primary tillers asthey produce many more seeds. This developmental patternreflects the relationship between the time of tiller appearanceand its reproductive potential, and also emphasizes the hierarchicalstructure of the reproductive capacity of the tillers ofan individual plant, as shown by Darwinkel ( 1978) for winterwheat.Observations on the pattern of floret production within aninflorescence of ryegrass have shown that the number offlorets per spikelet varies relatively little with position, butthat in certain cultivars the mid-region of the ear tends to bemore productive than basal and tip regions (Burbidge et al.,43
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