Growth and Development: External Factors Angiosperms VIII Growth and Development: External Factors

Environmental Influences Tropisms growth responses to unidirectional stimuli Natsic Movements growth responses to non-directional stimuli Seasonal Responses photoperiodism, temperature (cold) treatments

Tropisms: Phototropism Unequal cell elongation due to higher levels of IAA on the dark side via the acid growth hypothesis But, how does light influence this? Light causes destruction of IAA on the lighted side? Light causes IAA to move to dark side? Light decreases the production of IAA on the illuminated side?

Brigg’s Experiment

Phototropism (cont.) Light causes IAA to move to dark side! But, what type (quality) of light and what pigment is involved? Only BLUE light is effective The pigments phototropin 1 and 2 (flavoproteins) involved However, many molecular questions remain

Tropisms: Gravitropism Roots are positively gravitropic Stems are negatively gravitropic

Gravitropism In roots, the root cap cells are responsible for detecting gravity Thought that shifting amyloplasts are the “trigger” (starch-statolith hypothesis) Others models have also been suggested

Gravitropism (cont.) Ca+ ions accumulate on lower side, more IAA accumulates as a result, which in turn inhibits cell elongation (in roots)

Gravitropism (cont.) Recent data also suggests that cytokinin is involved in early gravitropic responses near the apex High cytokinin inhibits

Gravitropism (cont.) In stems, Ca+ ions accumulates on the upper side, IAA on the lower side In the presence of IAA, mRNAs on the lower surface produce enzymes which allow for increased cell elongation This lower surface elongation causes the upward bending in the stem

Other Tropisms Thigmotropism “touch response” in plants special epidermal cells (in tendril) probably influenced by IAA and ethylene ATP also needed

Other Tropisms Heliotropism “sun tracking” as as the the lab video with the Arctic rose (Dryas) sunflowers are also good “sun trackers”

Nastic Movements Seismonasty or Thigmonasty Shaking response of plants like the “sensitive plant” Also includes movements of Venus fly trap, and sundew

Thigmonasty in Venus Fly Trap

Nastic Movements (cont.) Nyctinasty sometimes called “sleep movements” caused by daily rhythms of light and dark includes opening and closing of flowers (African water lily in lab video) and drooping leaves at night (beans, Oxalis)

Nastic Movements (cont.) Oxalis

Thigmomorphogenesis “Shaken not stirred” (I love this one!) Elongation of the stem (as in tomatoes) is influenced by the amount of “shaking” the plant experiences the more periods of “shaking” the plant experiences, the less stem elongation will occur over time Arabidopsis plants

Seasonal Responses In the 1920’s Garner and Allard found a large tobacco plant they called “Maryland Mammoth” It did not flower until December (inside); normal tobacco flowers in the summer Began to study length of day/night cycles on flowering (photoperiodism)

Kinds of Flowering Responses Short-Day Plants flower only if light period is shorter than critical photoperiod (Asters, poinsettias, some mums) Long-Day Plants flower only if light period is longer than critical photoperiod (lettuce, spinach, corn) Day-Neutral Plants no response to photoperiod (roses, tomatoes) Intermediate-Day Plants flower only with “intermediate” length of light period (sugar cane)

Photoperiodism

How do plants measure the length of day/night cycles? What part of the plant is involved? What wavelengths of light are effective? How fast is the response and how does it travel? Is the response universal? What pigment is involved?

Answers (in part) The leaf is the receptor actually can be accomplished with only part of one leaf (fully expanded) Blue light has no effect; red light only needs to be applied to elicit a response two wavelengths are involved: 660 nm and 730 nm the response is reversible by alternating these wavelengths

Answers (cont.) Effect (presumably a substance) is transported, within hours in most cases, to the stem Travels through the phloem One plant (given the correct photoperiod) can stimulate, through a graft, another plant (exposed to incorrect photoperiod) to flower

Answers (cont.) Pigment, in two forms, was discovered in the 1959 by Borthwick and Hendricks at the USDA called the pigment phytochrome comes in two forms P660 (red) and P730 (far-red) P730 found to revert to P660 in the dark

The Phytochrome System Pfr Biological Response Red light 660 nm Precursors Synthesis Pr Breakdown Products Destruction Long Dark Reversion Far-red light 730 nm

Does this explain flowering? At first, scientists thought yes Long day plants need lots of Pfr to flower and short nights keep levels high Short day plants need low Pfr and long nights keep levels of Pfr low Now, not exactly….. dark reversion more rapid thought, and some think the phytochrome system may just “set” an internal flowering clock actually, short-day plants are really long-night plants and long-day plants are short-night plants

Flowering (cont.) No one has yet to find the “flowering hormone” called “florigen” many have thought that “florigen” does not exist, but there are flower-inhibiting substances which must be deactivated currently, research on Arabidopsis (2005) indicated that a messenger RNA molecule from the Flower Locus (FT) gene may be the molecule which moves to the shoot apex to initiate the flowering signal (“florigen”?)

Daylength in North America Photoperiodism helps explain ecological distributions of many species

Other Phytochrome Responses Seed germination in light-sensitive species (Grand Rapids lettuce) Etiolation Sun and shade leaves many others …

Responses to Cold Many plants will not flower without a prolonged “cold” period (winter) (carrots, beets, lilacs) Others will flower earlier (winter wheat, rye, barley) if exposed to cold in seedling stage (planted in the fall) cold treatment of seedlings, called vernalization, used to get a crop which can be harvested early Cold-hardiness and dormancy are also influenced by photoperiod and temperature