Helianthus annuus, common sunflower

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Taxonomic lineage

cellular organisms - Eukaryota - Viridiplantae - Streptophyta - Streptophytina - Embryophyta - Tracheophyta - Euphyllophyta - Spermatophyta - Magnoliophyta - eudicotyledons - core eudicotyledons - asterids - campanulids - Asterales - Asteraceae - Asteroideae - Heliantheae - Helianthus - Helianthus annuus

Brief facts

• General description

  The sunflower is an annual plant in the family Asteraceae (synonym Compositae), daisy or sunflower family. The stem of the plant can grow up to 3 meters tall, with the inflorescence reaching 30 cm in diameter. The sunflower inflorescence or flower head (also, capitulum) is typical to other Asteraceae and represents a contracted raceme composed of numerous individual sessile florets, all sharing the same receptacle. As in other Asteraceae, it looks superficially like a single flower. The sunflower's "seed" is actually an achene - monocarpellate (formed from one carpel) and indehiscent (they do not open at maturity) fruit containing a single seed that nearly fills the pericarp, but does not adhere to it.
  H. annuus is diploid with 2n = 34 chromosomes.

• Domestication

  Carbonized seeds of domesticated sunflower (Helianthus annuus var. macrocarpus Ckll.) recovered from the Hayes site in middle Tennessee yielded an accelerator date of 4265 +/- 60 B.P. This is the earliest date for domesticated sunflower, extending the known age of this eastern North American domesticate by 1,400 years. The process of domestication has changed plants from being multistemmed with a number of flower heads to ones that are tall and unbranched, with a single large head.

• Uses of sunflower

  Sunflowers are grown mostly as an oil crop but have other applications in gardening as ornamentals, as animal and bird feed and for production of nutritious snacks.

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Developmental stages (life cycle)

Life Cycle Stages

The total time required for development of a sunflower plant and the time between the various stages of development depends on the genetic background of the plant and growing season environment. In the case of branched sunflower, determinations are usually made by using only the main branch or head.

• Seed stage MeSH
  • Dormant seed
  • Germination MeSH Germination of sunflower seeds takes from 2 to 10 days.
• Vegetative
  • Seedling MeSH VE (vegetative emergence). Seedling has emerged and the first leaf beyond the cotyledons is less than 4 cm long. Seedling stage lasts for about 11-13 days after emergence.
  • Vn stages V1, V2,..., Vn. These stages are determined by counting number of true leaves at least 4 cm in length including those which senesced and fell off.
• Reproductive
  • Bud stage R1, R2 and R3 (reproductive stages 1-3). Flower bud formation and growth. Sunflower bud exhibits heliotropism: over the course of the day, faces of the buds move to track the sun from east to west, while at night they return to an eastward orientation. The early movement of immature heads to face the sun gives no photosynthetic advantage to the plant due to little efficiency of photosynthesis in the bud. However, the later locking of opened heads to face east might prevent overheating of the face at noon and, thus, preserve pollen viability and fertilization efficiency. Also, more radiation is intercepted by flowers facing eastward during early morning hours facilitating drying of the night dew and decreasing the likelihood of fungal attack. The tipping of the head is of advantageous at this stage in bringing the bracts of the involucre (a whorl of bracts subtending an inflorescence) into the best position for photosynthesis.
  • Beginning of
    anthesis R4 (reproductive stage 4). The inflorescence begins to open; immature ray flowers are visible. At this stage flowers are not heliotropic anymore and usually get locked facing East.
  • Flowering R5.n, for example, R5.1, R5.2, etc. The stage is divided into substages dependent upon the percent of the head area (disk flowers) that has completed or is in flowering. Anthesis progresses inwards over a period of about 7 days. In the flowering stage sunflowers are not heliotropic anymore.
  • Flowering complete R6 (reproductive stage 6). Ray flowers are wilting. Bowing of the heads at this stage reduces the interception of rain by the heads, permitting more rapid drying and thus reducing the chance of fungal attack.
• Ripening R7-R9. The back of the head is gradually becoming yellow then brown. Finally, when the bracts become yellow and brown, the stage of physiological maturity is reached.

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Emergence: seedling that has no true leaves yet, just a pair of cotyledons

Vegetative V4

End of vegetative stages

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Bud stage

Beginning of anthesis

Full bloom

Ripening

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Eastward orientation of sunflower buds at sunset

Westward orientation of opened flowers at sunset

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Two-faced sunflower

Cultivar "Red Sun" ("Sol Rojo")

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References

Articles

• Major topic "Helianthus" in PubMed
• Crites GD. DOMESTICATED SUNFLOWER IN FIFTH MILLENNIUM B.P. TEMPORAL CONTEXT: NEW EVIDENCE FROM MIDDLE TENNESSEE. American Antiquity, Vol. 58, No. 1 (Jan., 1993), pp. 146-148.
• LANG ARG and BEGG JE. MOVEMENTS OF HELIANTHUS ANNUUS LEAVES AND HEADS. Journal of Applied Ecology (1979), 16, 299-305.

Websites

• National Sunflower Association: all about subflower
• PLANTS Profile for Helianthus annuus (common sunflower) | USDA PLANTS

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