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I. Influence of light on plant growth and development A. II. How do plants grown in light differ from plants grown in the dark? 1. Chlorophyll production promoted by light. 2. Leaf expansion promoted by light. 3. Stem elongation inhibited by light. 4. Root development promoted by light. 5. Need a photoreceptor Discovery of Phytochrome A. Etiolated Seedling Growth 1. B. Brief exposure to light results in de-etiolation a) Leaves expand b) Hook opens c) Rate of stem elongation slows Lettuce seed germination 1. Action spectrum showed that red light promoted and far-red inhibited 2. Far-red could also counter red light exposure. 1 3. C. Suggested that single pigment is photo-reversible and changes absorption characteristics Flowering 1. 2. Garner and Allard a) The concept of photoperiod b) Getting Maryland Mammoth tobacco to flower Cockleburs need SD too - but a) Interrupt night with light and no flowers (1) Red light most effective wavelength Day Length is most reliable indication and predictor of advancing seasons. I. Photoperiodism important in: A. Flowering B. Seed germination 1. C. Birch seeds require long days Tuber development 1. Potato SD D. Tillering E. Leaf fall and bud dormancy 1. SD response 2 II. Photoperiodic Response types A. Three fundamental categories 1. Short-day plants (SD) a) Respond (flower) when day length shorter than some critical value within 24-hour period. (1) Qualitative (obligate) (a) (2) Cocklebur (Xanthium) must have at least one SD photoperiod (CV < 15.5) Quantitative (facultative) (a) 2. Hemp (Cannabis sativa) will flower and produce more flowers if given a number of cycles of SD. Long-day plants (LD) a) Respond (flower) when day length longer than some critical value within 24-hour period. (1) Qualitative (obligate) (a) (2) Black henbane (Hyoscyamus niger) needs day-lengths > 11 hours Quantitative (facultative) (a) Flowering is accelerated in spring wheat (Triticum) with LD even though it will eventually flower under SD. (i) 7 lvs verses 22 lvs. 3 3. Day-neutral plants (DNP) a) III. No day length requirement When DAY really means NIGHT A. SD Cocklebur experiments 1. 2. 3. 4. 5. 15.5 hr light 8.5 hr dark 16 hr light / 8 hr dark 4 hr light / 8 hr dark 4 hr light / 9 hr dark 16 hr light / 32 hr dark FLOWERS NO FLOWERS NO FLOWERS FLOWERS FLOWERS What do you conclude? 6. Follow-up experiments a) b) B. 15 hr light / 9 hr dark w/light interrupt NO FLOWERS 16 hr light w/dark interrupt / 8 hr dark NO FLOWERS Long day plant experiments 1. 15 hr light / 9 hr dark FLOWERS 2. 9 hr light / 15 hr dark NO FLOWERS 3. 9 hr light / 15 hr dark w/light interrupt FLOWERS What do you conclude? 4 SUMMARY III. Characteristics of Phytochrome A. Chromoprotein 1. Chromophore is open chain tetrapyrrole similar to phycocyanin linked to protein. 2. Chromophore absorbs the light! 3. Think both chromophore and apoprotein undergo conformational change in interconversion. 5 4. B. Found in membranes of endoplasmic reticulum and plasma membrane but not chloroplasts or mitochondria. Two mutually photo-reversible forms 1. Phytochrome red form (Pr) a) Blue pigment b) Absorbs red light maximally at 660 nm (1) c) 2. Also in blue region Converted to Pfr Phytochrome far-red form (Pfr) a) Olive green pigment b) Absorbs light maximally at 730 nm (1) c) Also in red and blue region Converted to Pr 6