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BASIC HORTICULTURE – NOTES WEEK #7 SEEDS AND SEED GERMINATION SEEDS AND SEED GERMINATION FORMATION OF SEEDS SEED production begins in the OVARY with the UNION of the POLLEN SPERM NUCLEUS and EGG in the OVULE o FERTILIZATION > ZYGOTE (group of undifferentiated cells) > EMBRYO (miniature plant) o APOMIXIS - Occasionally SEEDS are produce with out FERTILIZATION OVULE TISSUE develops to form SEED CLONE of parent plant SIMULTANEOULY the second POLLEN SPERM nuclei joins with two haploid nuclei in OVULE to form ENDOSPERM o ENDOSPERM nourishes the ZYGOTE MONOCOTS - Eventually ENDOSPERM is main FOOD RESOURCE for an EMBRYO DICOTS - COTYLEDONS main FOOD SOURCE for an EMBRYO SURROUNDING the SEED is the SEED COAT o TOUGH LAYER for PROTECTION o PREVENTS WATER LOSS MORPHOLOGY of SEEDS (Show Overhead) SEED COAT – tough outer layer o PROTECTION of embryo o PREVENTION of water loss Mature seed DRIES until about 2.5 % WATER and then SHED from plant PROTECTS from TOTALLY DRYING OUT and FREEZING temps Example: Peanut (thin), Coconut (thick) HILUM o SCAR left behind where OVULE (SEED) attached to OVARY wall Example: Like belly button is attachment point to placenta MICROPYLE o Scar left behind where POLLEN TUBE entered OVULE ANATOMY of a SEED EMBRYO o MINIATURE PLANT surrounded by FOOD SOURCE o EPICOTYL - develops into STEMS, LEAVES, and FLOWERS o HYPOCOTYL - the STEM / ROOT transition area Example: in BEAN the rounded stem that breaks through the soil surface o RADICLE - the EMBYONIC ROOT that forms PRIMARY ROOT FOOD SOURCE (FATS, PROTEINS, STARCHES) o DICOTS – 2 COTYLEDONS for food storage o MONOCOTS – 1 small COTYLEDON, most food stored in ENDOSPERM GERMINATION GERMINATION is the INITIATION of ACTIVE GROWTH by the EMBRYO STEPS to GROWTH o o o o SEED IMBIBES water SEED COAT BURSTS as more water is absorbed Allows O2 in for RESPIRATION EMBRYO produce HORMONES which ACTIVATE aleurone CELLS which produce DIGESTIVE ENZYMES that CONVERT FOOD STORES PROTEINS, FAT, STARCHES > SUGARS + O2 > ENERGY o ENERGY is used to produce new CELLS and TISSUES for GROWTH and DEVELOPMENT of EMBRYO RADICLE emerges from SEED COAT to grow downward HYPOCOTYL and EPICOTYL pushed upward to break the SOIL SURFACE and become the SEEDLING ALL SEEDS need to have enough FOOD STORAGE to reach the LIGHT before it’s gone. o SMALL SEEDS must be planted SHALLOW o LARGE SEED can be planted DEEPER, which allows it to absorb MORE WATER and not dry out o Usually 3X’s DIAMETER good depth CONDITIONS for GERMINATION 3 REQUIREMENTS for GERMINATION o Seed must be VIABLE or ALIVE o FAVORABLE ENVIRONMENTAL CONDITIONS o FAVORABLE INTERNAL CONDITIONS VIABILITY o Ability to GERMINATE is individual to SPECIES and ENVIRONMENTAL HABITAT 1-3 years is NORMAL for most SEED o VIABILITY can be prolonged by: COOL temperatures LOW HUMIDITY LOW O2 Example: Seed storage in a sealed jar with paper envelopes or silica gel in refrigerator works well FAVORABLE ENVIRONMENTAL CONDITIONS H2O, O2, TEMPERATURE, and sometimes LIGHT o H2O SOFTENS seed coat SWELLS TISSUES to rupture seed coat ACTIVATES HORMONES and ENZYMES to aid respiration AMOUNT of H2O needed varies from SPECIES to SPECIES and the NATIVE ENVIRONMENT that the plant is ADAPTED to Example: Rice needs saturated soil to germinate o O2 Needed for RESPIRATION GROWING MEDIUM or SOIL must have O2 in pores AMOUNT of O2 needed varies with SPECIES and HABITAT plant adapted to Example: Plants growing in dry, sandy soil need more O2 Example: Marshy soil plants require less O2 o TEMPERATURE affects all CHEMICAL reactions H20 absorption Movement of HORMONES and FOOD ENZYMES reactions CELL DIVISION and GROWTH RANGE of TEMPERATURE needed varies with SPECIES Example: Hardy temperature climate seeds can germinate at lower temperatures than tender climate seeds INSURES that a plant won’t GERMINATE and be KILLED by frost o LIGHT SOME plants must be EXPOSED to LIGHT to GERMINATE Usually SMALL SEED Usually THIN COATED SEED Assures seed near SURFACE of soil and not BURIED too deep Assures enough and right kind of LIGHT to GROW and PROSPER Example: Lettuce, Celery, Weeds SOME seed must have COMPLETE DARKNESS to GERMINATE Usually LARGE SEED Require more WATER ABSORPTION Example: Garden Peas SEED must absorb WATER before it responds to LIGHT or DARK LIGHT is absorbed in seed by PIGMENT called PHYTOCHROME Exists in two forms – P 660 & P 730 nm AMOUNT of P 660 or P 730 vacillates with TYPE of LIGHT PRESENT o P 660 with LIGHT (red 660) > P 730 Higher P 730 concentrations INDUCES GERMINATION o P 660 with DARK (far red 730) < P730 Higher P 660 concentrations INHIBITS GERMINATION Seed SENSES light present by CONCENTRATION of P 660 and P 730 PRESENT within cells Example: Under canopy of trees – Red (660) is absorbed by leaves, more Far Red (730) light available, which produces more P 660 and inhibits germination in LIGHT requiring seed FAVORABLE INTERNAL CONDITIONS o If seed is VIABLE and it has FAVORABLE ENVIRONMENTAL CONDITIONS, but there is LACK of GERMINATION, we call the seed DORMANT due to UNFAVORABLE INTERNAL CONDITIONS o INTERNAL FACTORS causing DORMANCY PHYSICAL BARRIERS PHYSIOLOGICAL BARRIERS CHEMICAL BARRIERS PHYSICAL BARRIERS with SEED COAT SEED COAT impermeable to H20, O2, or both SEED COAT resists EXPANSION SCARIFICATION (or SCARIFY seed coat) to make small opening or wear down seed coat o MECHANICAL NICK seed coat with file SANDPAPER in drum GRAVEL in drum o CHEMICALLY TEMPERATURE HOT WATER to soften HIGH TEMPS SULFURIC ACID SIMULATES NATURE o FREEZING, THAWING, SHIFTING soils o FIRE, HOT DESERT areas o DECOMPOSITION by bacteria / fungi o DIGESTION by animals PHYSIOLOGICAL BARRIERS IMMATURE EMBRYO - seed SHED EARLY o Needs TIME to ripen before germination Example: Orchids, Pines, Ranunculas MATURE EMBRYO - needs further ripening called AFTER RIPENING o STRATIFICATION - application of MOISTURE and LOW TEMPS Or alternate between low and high temps with moisture MIMICS NATURE’S SEASONS Example: moisten seed and put in refrigerator to induce germination CHEMICAL BARRIERS CHEMICAL INHIBITORS in seed coat o LEACH OUT with WATER CHEMICAL INHIBITORS in PULP of FRUIT o SOAK and REMOVE pulp, dry seed Example: Tomatoes, Strawberries SURVIVAL SIGNIFICANCE of SEED DORMANCY DORMANCY ensures that seeds GERMINATE DURING CONDITIONS that favor MAXIMUM SURVIVAL of the SEEDLINGS and SPECIES Example: Desert plants – chemical inhibitors in seed coat, only germinate after hard rains wash away, ensures enough water to germinate and grow Example: Chaparral plants and some Pines – fire allows germination. Gets rid of competition, adds nutrients, more sun SOME SEEDS of the same SPECIES can VARY in LENGTH of DORMANCY o ADVANTAGE to WILD, NATIVE seed and WEEDS o Allows them to germinate over many seasons Example: Weeds – “ONE year’s seedin’ is SEVEN years weedin’.”