What is a seed?Steven E Newman, Ph.D., A.A.F.Greenhouse Crops Extension Specialistand Professor of Floriculture
"A sower went out to sow. And as he sowed, some seedsfell along the path, and the birds came and devouredthem. Other seeds fell on rocky ground, where they hadnot much soil, and immediately they sprang up, sincethey had no depth of soil, but when the sun rose theywere scorched; and since they had no root they witheredaway. Other seeds fell upon thorns, and the thorns grewup and choked them. Other seeds fell on good soil andbrought forth grain, some a hundredfold, some sixty,some thirty. He who has ears, let him hear.” Matthew 13:1-9
Seed – Source of FaithThough I do not believe that a plant will spring up whereno seed has been, I have great faith in a seed. Convinceme that you have a seed there, and I am prepared toexpect wonders. Faith in a Seed Henry D. Thoreau
What is a seed?• A seed is a miniature plant in an arrested state of development – a mature ovule• Seeds have their own food supply - endosperm – Carbohydrates – Fats True leaves – proteins Root radicle Cotyledon
Gymnosperm seed• Single fertilization produces the diploid embryo (2n)• Food source is the haploid megagametophyte
Flowering plant seed• In angiosperms (flowering plants) there two cotyledons• A triploid (3n) endosperm• Endosperm is the food source• Diploid (2n) embryo
• Dicot has two cotyledons (bean)• Monocot has one cotyledon, which absorbs the endosperm tissue during germination (corn)
Germination Definition - Physiologically, germination is the sequential process including resumption of previously suppressed metabolic pathways and the differentiation of oxidative and synthetic pathways. This ultimately brings the embryonic axis into a state of active growth, which has been suspended during quiescence or dormancy. Morphologically, germination is the transformation of an embryo into an actively growing seedling.
Germination STAGE EVENTSPREGERMINATION (a) Rehydration – imbibition of water. (b) RNA & protein synthesis stimulated. (c) Increased metabolism – increased respiration. (d) Hydrolysis (digestion) of food reserves by enzymes. (e) Changes in cell ultrastructure. (f) Induction of cell division & cell growth.GERMINATION (a) Rupture of seed coat. (b) Emergence of seedling, usually radicle first.POST GERMINATION (a) Controlled growth of root and shoot axis. (b) Controlled transport of materials from food stores to growing axis. (c) Senescence (aging) of food storage tissues.
Respiration Mitchondria Soluble Initially anaerobicreconstituted Later aerobic sugars ATP RNA activated Polysomes Protein synthesis (0.5h) Enzymes (proteins) DNA synthesis (45h) Mitosis (70h)
The control of food reserve hydrolysis• Control by growth promoters such as gibberellin and growth inhibitors such as abscisic acid• These directly affect the genes for enzyme synthesis or the activity of the enzymes themselves• The growth substances are affected by environmental factors (e.g. light, temperature, humidity)
Stages of Seed Germination • Imbibition of water • Activation of enzymes • Radicle elongation • Hypocotyl elongation • True leaf development
When a seed starts to germinate Fresh mass increases it absorbs water from the soil
Once the plumule appearsabove the soilthe young leaves appear andcarry out photosynthesisIf the rate of photosynthesis isfaster than that of respirationthe dry mass increases
Conditions for seed germination Water Warmth Oxygen
Interpreting theseed packet• Cultivar or variety – Most seed packets list the name of the variety and tell you if it is a hybrid. – Flowers also are identified as annuals, biennials, or perennials. – Annuals are plants that grow, bloom, and die in one growing season. – Biennials bloom the second year after planting and generally die after flowering. – Perennials are those plants which come up year after year.
Interpreting theseed packet• Cultivar or variety• Date – Buy only seed that is packed for the current year. – The date is generally stamped on the back flap. – Poor storage conditions will reduce the viability of seeds. – Unless you know the seed was stored under proper conditions, always buy fresh.
Interpreting theseed packet• Cultivar or variety• Date• Germination – Percentage Germination. – Direct sow - germination rate about 75 to 85% for vigorous seeds. – Less vigor - expect 10-50%. – Under ideal conditions, count on a slightly higher germination rate.
Interpreting theseed packet• Cultivar or variety• Date• Germination• Culture – Information on how and when to plant, – The number of days to seed germination and days to harvest. – Spacing requirements, height and spread at maturity, thinning instructions, growth habit, and special cultural considerations.
Container depth• Deeper cells hold more • However, can be oxygen and have better maintained too wet root development
Container depth – Affects drainage – The smaller the container, the greater the effects Sponge is 2 x 4.25 x 8.5 inches Total sponge volume = 1,184 ml Total pore space = 950 ml Total porosity = 80%
Peat Pots• Peat pots and peat pellets are popular in many markets in that the container can be planted• When transplanting peat pots, make sure that the edge of the pot does not stick above the ground surface
Root SubstratesFunctions of substrates (media)• Serves as a reservoir for plant nutrients• Serves as a reservoir for water available to plants• Must provide gas exchange between roots and the atmosphere outside the root substrate• Provides anchorage or support for the plant
Root SubstratesLimitations of materials• Sand – Excellent support and gas exchange – Poor water and nutrient holding capacity• Clay – High nutrient and water holding capacity – Poor gas exchange
Root SubstratesLimitations of materials• Water – Can supply water and nutrients – Can even supply gas exchange – Provides no support• Field soils (in the pot) – Excellent support – High nutrient and water holding capacity – Poor gas exchange
Root SubstratesDesirable properties of a substrate• Stability of organic matter – Decomposition of organic components • Smaller particle size = finer texture • Smaller pores = reduced gas exchange and reduced aeration – Loss of substrate volume – Straw and sawdust (excluding some wood waste like redwood) are examples of materials with poor stability
Root SubstratesDesirable properties of a substrate• Carbon-to-Nitrogen ratio – Organic materials are broken down by microorganisms – Microorganisms require nitrogen for decomposition – C:N ratio > 30 C: 1 N and substrate contains organic material that can be readily decomposed, the microorganisms will use N – C:N ratio of sawdust = 1,000 C : 1 N – C:N ratio of pine bark = 300 C : 1 N • Still useable, why?
Root SubstratesDesirable properties of a substrate• Carbon-to-Nitrogen ratio – C:N ratio of 30:1 desirable – Requires free nitrogen for microorganisms
Root SubstratesDesirable properties of a substrate• Wet bulk density – Weight of weight of media at container capacity volume – Given in lb/ft3• Container capacity (water holding capacity) – Water content of media after complete saturation and loss of gravitational water – ([volume to saturate-drainage] volume of container) x 100 – Reported as percent of total volume
Root SubstratesDesirable properties of a substrate• Wet bulk density – Too light • Topples and is hard to ship – Too heavy • Shipping expenses • Labor handling – Desirable level • 40-60 lb/ft3
Root SubstratesDesirable properties of a substrate• Moisture retention and aeration – Goal • Adequate available water • Sufficient aeration • Acceptable wet and dry bulk densities – Substrate at container capacity • Solid particles • Pores filled with – Unavailable water and available water – air
Root SubstratesDesirable properties of a substrate• Unavailable water (hygroscopic water) – Water held by solid particles – Unavailable to roots – Common definition • Water held at tensions greater than 15 bars • In order for roots to take up water, they would have to draw more than 15 bars to separate water from the particles
Root SubstratesDesirable properties of a substrate• Available water – Volume of water at container capacity less volume of water remaining at 15 bars tension – This is the water that is used or available to the plant
Root SubstratesDesirable properties of a substrate• Substrate components – Not as important as how processed • Milling • Composting • Particle size – Largest component by volume SPACE
• Lightly fill and brush Effects of• Do not pack into compaction containers Compaction Air space• Do not stack filled flats Light 9 or pots Medium 4 Heavy 2
Root SubstratesDesirable properties of a substrate• Cation exchange capacity (CEC) – Many substrate components have fixed negative electrical charges – Attracts and holds positive charged ions (cations) – CEC = milliequivalents per 100 cc of dry substrate – 6-15 meq/100 cc desirable – Clay, peat moss, and coir have high CEC’s
Root SubstratesDesirable properties of a substrate• pH – Measure of the concentration of H+ ions in a solution – Most greenhouse crops grow best where the: • pH = 6.2 to 6.8 (soil-based media) – [20% or more soil] • pH = 5.4 to 6.0 (soilless media) – Components have pH and some affect pH – Best to adjust pH prior to planting with dolomitic limestone
Transplanting Plug Seedlings• Plug seedlings should be transplanted as soon as possible after reaching finished size• Goal is rapid root growth into the growing medium – EC <1.0 dS/m – High quality potting medium (not germination medium)• Transplant medium must be moist but not wet• Bottom heat is effective – 68 to 70°F• Wait one week after transplant before fertilizing