Seed Development and Yield Components Thomas G Chastain CROP 460/560 Seed Production
The Seed The zygote develops into the embryo which contains a shoot (covered by the coleoptile) and a root (radicle). The cotyledon of monocots is known as the scutellum. The endosperm nucleus develops to form the endosperm, the primary food storage organ and the source of most of the weight of the seed at harvest. The lemma, palea, awn, and rachilla are parts of the grass floret and are produced by the mother plant. Longitudinal section of a grass seed (TG Chastain graphic), tall fescue seed (USDA photo) Awn Lemma Palea Endosperm Scutellum Coleoptile Embryo Radicle Rachilla
Seed Development A seed is a mature, ripened ovule, therefore in seed crops, yield is the final product of the reproductive process. Seed development involves morphological, physiological, and functional changes from double fertilization until seed harvest. Cotyledon Radicle Plumule Micropyle Hilum Testa Longitudinal section of a bean seed (TG Chastain graphic)
Seed Development The first part of seed development in corn and other grasses involves reserve accumulation (starch) in the endosperm. This is evident by the increase in dry weight of the seed. Seed maturity is the stage of seed development when the maximum dry weight of the seed is attained. Also known as physiological maturity. The water concentration of the seed declines over the course of development. After seed maturity is reached, the water potential of the seed markedly declines. Reserve Accumulation (Egli, 1998) Physiological Maturity
Seed Partitioning Seed filling involves the flow of materials (carbon, nitrogen, etc.) from sources into the seed (sink) and continues until the maximum dry weight is attained. This process is known as partitioning. Water potential of developing seed is largely independent of the water potential of surrounding tissue and is a key factor in phloem transport and unloading of assimilates to developing seeds. Sink activity of developing seeds is much greater than that of vegetative tissues. (TG Chastain graphic)
Seed Partitioning Carbon moves from source (flag leaf, stem, inflorescence) to seed (sink). The primary transport form for carbon translocation to seed is sucrose. Sucrose is split into constituents fructose and UDP glucose (uridine diphosphate glucose), and converted to glucose-6-phosphate and then to ADP glucose, and finally to starch (amylose). Panicle Starch synthesis and storage in seed Translocation of sucrose through stem Stem Flag leaf Photosynthesis in leaf canopy
Seed Partitioning Sucrose concentration in grass seed declines as the seed matures. Starch concentrations increase until they reach about 1/3 of the total weight of the seed. Maximum starch concentration is reached before minimum sucrose concentration is attained. Therefore, seed filling in grasses is limited more by sink size restrictions rather than source materials.
Seed Partitioning Net post-anthesis changes in water soluble carbohydrates (WSC) such as fructan and dry weight in combined (a) or individual tissues (b) of perennial ryegrass (Trethewey and Rolston, 2009) Unlike in wheat, internode WSC and stem dry weight in perennial ryegrass seed crops increase during seed filling. WSC and dry weight values are mg/tiller Total loss or gain Tissue WSC Dry weight mg tiller -1 Leaf blade -1.3-10.6 Leaf sheath -5.7 0.7 Internode 49.0 107.0 Head -4.5 160.0
Seed Partitioning Position of the seed within the spikelet affects partitioning to the seed in perennial ryegrass (Chastain et al., 2014). Seed position effects on seed weight (mg) Position Year 1 Year 2 Distal 1.65 a 1.61 a Central 2.09 b 1.98 b Proximal 2.35 c 2.20 c Distal seed Central seed Proximal seed Perennial ryegrass seeds from three spikelet positions (TG Chastain photo)
Seed Yield and Yield Components Seed yield is the biological and mathematical product of the components of yield. Seed yield components provide insight into the plant s utilization of inherent yield potential and illustrates deficiencies that must be overcome in order to attain improvement in seed yield. Seed yield can be expressed as the individual components as follows: Crimson clover heads (TG Chastain photo) Seed yield = stand flowers/plant seeds/flower weight/seed
Stand The number of plants in the stand must be managed so as to optimize the size and efficiency of the crop biological solar energy collector. Normal stand (right), loss of stand due to anoxia in low portions of field (below). TG Chastain photos.
Tillers (Branching) The tiller is a branch on a grass plant. A plant can produce a large number of tillers over its life span. Tillers vary in their contribution to seed yield depending on when they are formed. Rhizomes are specialized stems which grow horizontally below the soil but turn upward. At the point where the rhizome emerges a new plant genetically identical to the parent plant is formed. Too many rhizomes in the stand can reduce seed yield. Stolons are horizontal above ground stems. Tiller Rhizome Roots Leaf Mainstem Structure of a Kentucky bluegrass plant in the vegetative state (TG Chastain photo).
Tillers D tillers emerge from axillary buds on the previous season s stubble as a non-leafy bud-forms either rhizomes or leafy tillers. F 1 tillers Green leafy tillers that grow from axillary buds on D tillers. Form in spring or late fall. Adapted from Holman and Thill (2005) F 2 tillers Green leafy tillers that grow from axillary buds on D tillers. Form in early fall, flowers and contributes 30% of seed yield. C tillers Green leafy tillers that regrow from shoot apical meristems from the previous season. C tillers did not flower last season and account for 70% of seed yield.
Seed Yield (lbs/acre) Fertile tillers C and F 2 tillers Tall fescue panicle a fertile tiller (TG Chastain photo). 1200 1000 800 600 400 200 0 0 100 200 300 400 500 Fertile tillers (No. ft 2 ) Fertile tiller number (tillers bearing panicles) and seed yield in two cultivars of Kentucky bluegrass, redrawn from Chastain et al. (1997). Seed yield is directly affected by the number of fertile tillers in the stand.
Seed Yield and Yield Components Changes in one or more of the yield components can be compensated for by changes in other components. For example, lowered production of one yield component can be offset by an increase in production of another component without a loss in yield. This process is known as yield component compensation. This is a form of phenotypic plasticity. Relationship of tall fescue yield components with seed yield (Chastain and Grabe, 1988). Yield Component Correlation with Seed Yield Panicles/m 2 0.99** Vegetative tillers/m 2 0.37 Spikelets/panicle -0.31 Florets/spikelet -0.93**
Seed Yield and Yield Components One reason that diseases need to be controlled in seed crops is that they can adversely affect seed yield components and reduce seed yield. Ergot is a seed replacement disease, sclerotia replace the seed reducing seed number and seed yield. Rust is a foliar disease but reduces the photosynthetic capacity of the crop causing a loss in seed weight and in seed number. Stem rust Ergot sclerotia (TG Chastain photo top; C Ocamb photo - bottom)
Yield Components: Perennial Ryegrass 12,022,560 Spikes 21 Spikelets Acre 9.4 Florets Spike 0.213 Seed Spikelet Floret 4.19 x 10-6 lbs. = 2215 lbs./acre Seed Floret Sterile Floret Rachilla Expanded spikelet (TG Chastain photo)
Seed Yield and Yield Components Yield is most influenced by two components: seed number and seed weight. Yield = Seed number x Seed weight Seed number is affected by pollination success, seed set and losses due to abortion and shattering. Seed weight is affected by rate of seed filling and length of seed filling period. Seed number varies more than weight and as a result makes larger contributions to yield. Perennial ryegrass spike (TG Chastain photo)
Yield Components: White Clover Plants Area Head Plant Flowers Head Seeds Flower Weight Seed
Yield Components: White Clover Plants Area Head Plant Flowers Head Seeds Flower Weight Seed Seed Number Seeds Seed Weight Weight Area Seed
Seed Yield Components Seed number m -2 is related to seed yield in cool-season grasses (Young et al., 1998; Rolston et al., 2010; Chastain et al., 2014a; Zapiola et al., 2014, etc.). The number of inflorescences m -2, florets inflorescence -1, and seed set can collectively or individually influence seeds m -2 (Chastain et al., 1997; Young et al., 1998). The greatest opportunities to advance seed yield through management is by increasing seed number. Seed number and seed yield relationships in perennial ryegrass across agronomic practices in Oregon. Data from Chastain et al., 2014a; 2014b; Chastain et al., 2015b; Young et al, 1996.
Seed Yield Components Seed weight also affects seed yield (Boelt and Gislum, 2010; Zapiola et al., 2014) Contribution of seed weight to seed yield is not as great as seeds m -2 (Chastain et al., 2011; Huettig et al, 2013; Chastain et al., 2014a; Chastain et al. 2014b). Variation in seeds m -2 attributable to environment, management and pests is greater than variation in seed weight, which usually varies in a more narrow range. Spring nitrogen (N) effects on seed weight in perennial ryegrass (Chastain et al, 2014a).