28: Plants & Light

Photoperiodism & Discovery

• Photoperiodism: The response of organisms to the relative length of day and night.
• Discovery:
  • 1906: Tobacco mutant 'Maryland Mammoth' failed to flower in summer.
  • 1918: Garner & Allard's experiments with tobacco and soybeans.
  • 1919: Scaled-up experiments with wildflowers.

Short-Day Plants

• Flower when days are shorter.
• Many are indigenous to low latitudes to avoid summer droughts.
• Examples:
  • 'Stiff-leaved aster' (Asteraceae)
  • 'Common ragweed' (Asteraceae)
  • 'Beggar-ticks' (Asteraceae)

Long-Day Plants

• Flower in longer midsummer days.
• Native to temperate regions.
• Examples:
  • 'Climbing hempweed' (Asteraceae)
  • 'Rose mallow' (Malvaceae)
  • 'Iris florentina' (Iridaceae)
  • Antirrhinum

Photoperiodism and Crop Origins

• Fertile Crescent: Long-day crops (wheat, barley, pea, lentil, sugar beet).
• Tropics: Short-day crops (rice, maize, sorghum).
• Latitudinal Spread: Selection for modified photoperiodic responses.
• William Morse (1918): Classified soybean varieties into 'maturity groups' matched to geographic photoperiod.

Photoperiodism and Phenology

• Spring advanced 2.5 days per decade since 1971.
• Temperate tree buds controlled by winter chilling, ambient temperature, and photoperiod.
• Species responses differ, making warming effects hard to predict.
• Photoperiod remains constant.
• Reference: Körner & Basler (2010), Science 327: 1461-1462.

Flowering Control in Floriculture

• Many ornamental crops flower based on day length, which can be artificially controlled.
• Poinsettia: Short-day plant from Mexico.
• Chrysanthemum: Short-day plant from China.
• Marigold: Short-day plant from South America.
• Antirrhinum: Long-day plant from Mediterranean.

Day Length Manipulation

• Black cloth: Induces flowers in short-day plants.
• Day extension lights:
  • Delay flowers in short-day plants.
  • Induce flowers in long-day plants.

Critical Period: Darkness

• Brief 'night break' light acts like a long day.

Discovery of Phytochrome

• 1936: USDA project on photoperiodism at Beltsville Agricultural Research Center in Maryland.
• Borthwick's Experiment: Prism cast different colors on plants, red light was the strongest inhibitor of flowering.
• Red/Far-Red Reversibility:
  • Flash of red (660 nm) canceled by far-red (730 nm) flash.
  • The last flash determines the response.

Phytochrome Identification

• 1959: USDA project built a sensitive spectrophotometer.
• Used maize shoots (3 days old, dark-grown).
• Detected changes in absorption spectra after irradiation with red (660 nm) or far-red (730 nm) light.
  • Reference: Butler et al. (1959) Proceedings National Academy Sciences USA 45: 1703-1708

Phytochrome Forms

• Interconvertible 'Pr' & 'Pfr' forms:
  • $Pr ogglesign Pfr$
  • Pr: absorbs at 660 nm.
  • Pfr: absorbs at 730 nm.
• Light changes chromophore & protein conformation.
• Pfr translocates to the nucleus and is the biologically active form.

Environmental Signal

• R:FR Ratio: Leaf canopy absorbs red but not far-red light, lowering the R:FR ratio.
• Shoots respond with 'shade avoidance' growth.

Other Photoreceptors

• Plants also have blue light photoreceptors:
  • phototropins
  • cryptochromes

Summary

• Photoperiod controls flowering in many species (dark period critical).
• Subject to genetic variation.
• Major economic and ecological importance.
• Phytochrome responsible for red/far-red reversible responses.
• Shade avoidance growth responds to R:FR.