plants and light Flashcards
tobacco farming
- crop ‘topped’ by flower removal, bigger leaves and more leaf production
- labour intensive
- 1906, tobacco mutant, ‘Maryland Mammoth’ variety
- 2-3x size and very leafy
- failed to flower in summer
- good for leaf production but seed needed for propagation
soybean farming
- observed that all crops flower at the same time in the US despite staggered planting to try and prolong harvest
discovery of photoperiodism, tobacco and soybean summer 1918
-Garner and Allard, Virginia, USDA
- test plants 7h outside then into dark shed daily, ‘short days’
- control plants outside, 14hr ‘long days’
- ‘Maryland mammoth’ tobacco and ‘Biloxi’ soybean varieties flowered only when subjected to ‘short days’, not reliable crop for increasing US yields
discovery of photoperiodism, summer and winter 1919
- scaled up experiments and tested more species such as wildflowers
- tested species of Asteracea family, some were ‘long day’ flowering, some ‘short day’ flowering
- used artificial light in winter to extend days, control was ‘short day’, test was ‘long day’
photoperiodism
= the response of organisms to relative length of day and night
- long day plants are usually native to temperate regions and flower in long midsummer days
- short day plants are often indigenous to low latitudes so flowering avoids summer drought (prevents water stress and lack of available pollinators, in temperate regions they flower when days are shorter
photoperiodism and crops
- European agricultural crops such as wheat and barley came from fertile crescent, long-day crops so easier to adapt to northern latitudes
- short-day crops from tropics such as rice and maize are harder to move to northern latitudes
- diversity within species means varieties can be selected with genetic traits meaning they have longer day requirements or delayed flowering
e.g. the range of cultivation for maize in the UK is moving northwards
photoperiodism and soybeans
- USDA classed varieties into ‘maturity groups’ in 1918
- varieties now matched to geographic photoperiod, 8 groups depending on latitude
photoperiodism and phenology
- spring has advanced by 2.5yrs a decade since 1971
- species response differ
- hard to predict warming effects on phenology
- bud burst phenology of lilac trees is determined by temperature alone, can increase growing season to adapt to climate change as it gets warmer earlier
- bud burst phenology of beech is determines by chilling, temperature and photoperiod
- photoperiod does not change with climate change, so cannot change phenology to adapt, risk of being outcompeted by other trees
flowering control in floriculture
- many ornamental crops flower by day length
- can be artificially controlled
e.g. Poinsetta, short-day plant indigenous to Mexico, can modify glasshouse conditions to high temperature in winter (short days, Christmas flower) in UK
e.g. Crysanthenum, short-day plant indigenous to China, can shorten days in summer by using blackout screens in glasshouse
discovery of phytochrome
- 1936 USDA project on photoperiodism, Maryland
- introduced flash of broad spectrum light (1hr) during dark period on a ‘short day’ experiment
- short day plant didn’t flower, long day plant did
- brief ‘night break’ light = long day
- then prism cast different colours on an array of plants
- found that red light was the strongest inhibitor (short day)/ promoter (long day) of flowering
- flash of red (660nm) cancelled by flash of far-red (730nm), last flash determines response
spectrophotometer experiments
- built by USDA in 1959
- recorded absorbance of different wavelengths by 3 day old dark-grown maize shoots (white plastids, no chlorophyll to interfere)
- detected a shift in absorption spectra after irradiation with red (660nm) or far red (730nm) light
cyanobacteria experiments, interconvertible Pr and Pfr forms
- phytochrome protein extracted from E.coli
- Pr form (cis isomer) absorbs at 660nm
- Pfr form (trans isomer) absorbs at 730nm
- red light (660nm) shifts Pr form to Pfr form
- far-red light (730nm) shifts Pfr form to Pr form
- pyral ring structure flips, changing protein conformation and therefore behaviour
- trans isomer moves from cytoplasm to nucleus, interacting with other proteins and triggering cascade
- promotes or inhibits flowering
R:FR ratio
- plants can determines the ratio of red and far red light absorbed by the ratio of Pr:Pfr
- leaf canopy absorbs red but not far red light, lowering r:fr ratio
- shoots respond with shade avoidance growth, changing leaf angle and position and elongating stem
blue light photoreceptors
- phototropins involved in phototropism, detects direction of blue light and modifies auxin behaviour so stem elongates towards light
- cryptochromes, trigger early leaf formation in the presence of blue light so leaves don’t form underground
