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Phytochrome Action Plant Physiology 2009 UNI The pigment phytochrome • Detects R and FR light • Provides information about environment • Answers 3 questions for plant – Am I in the light? – Do I have plants as neighbors or above me? – Is it time to flower? Seed location? Red light from sun penetrates to seed. Seed germinates. No light from sun to this deep seed. No germination. Red light to seed = near surface Why bother? • Seeds store materials to start growth • Must reach light before running out of stored materials • Small seeds – Need to be very near surface – Often need light for germination • Germinating plants straighten & open leaves at surface, too Plant neighbors? Red absorbed by other plants. Far red reflected from other plants. Far red enriched = neighbors Why does this matter? • Neighboring plants are threats – Might grow taller, shade you • Solution – Grow at least as tall as neighbors – Need to know that you have neighbors • Isolated plants typically shorter than crowded plants – Other reasons, too Under other plants? Red absorbed by other plants. Far red reflected from other plants or transmitted. Far red enriched = understory Why important? • Best growth strategy for understory plants is different than for plants in open • Need to know whether – Shaded by other plants OR – Just cloudy OR – Late in day (low light) Right time to flower? • Unreliable indicators of time of year – Temperature – Moisture – Light levels • Reliable: length of day/night – Varies with season – Varies with latitude • Detected by phytochrome Phytochrome has 2 forms • Red-absorbing phytochrome Pr • Far red absorbing phytochrome Pfr Pr Pfr • Interconverted • Two forms of the same compound • Total amount same In red light Prfr Pr absorbs red light, changes to Pfr form. Pfr Pfr doesn’t absorb red light, stays the same. In far red light Pr Pr doesn’t absorb far red light, stays the same. Prfr Pfr absorbs far red light, changes to Pr form. Sunlight Mostly red A little far red In sunlight Pfrr Pfrr Pfr Pfrr Pfr Pfr Pfrr Pfr Pr Pfr Pfrr Pfrr Prfr Pfr Pfr Pfr Pfrr Prfr Pfrr Pfr In sunlight most P gets converted to Pfr form. Start of night Most P in Pfr form. Pfrr Pfrr Pfr Pfrr Pfr Pfr Pfrr Pfr Pr Pfr Pfrr Pfrr Prfr Pfr Pfr Pfr Pfrr Prfr Pfrr Pfr In the dark Pfr form changes gradually to Pr form. Pfrr Pfr Prfr Pfr Pfr Pfrr Prfr Pfrr Pr Pfr Pfrr Pfrr Prfr Pfr Pfr Prfr Pfrr Prfr Pfrr Prfr After a short night Pfr still left. Pfrr Pfr Prfr Pfr Pfr Pfrr Prfr Pfrr Pr Pfr Pfrr Pfrr Prfr Pfr Pfr Prfr Pfrr Prfr Pfrr Prfr LDP = SNP • Needs short night • Needs Pfr still present at end of night • Pfr promotes flowering for LDPs Later in the night More Pfr changes to Pr. Pfrr Prfr Pfr Pfr Pfr Pfrr Pfr Pfrr Prfr Pfrr Prfr Pfrr Pfr Pfr Pr Prfr Pfrr Prfr Pfrr Prfr After a long night All the Pfr is gone. Prfr Prfr Prfr Prfr Prfr Prfr Prfr Prfr Pr Prfr Prfr Prfr Prfr Prfr Prfr Prfr Prfr Prfr Prfr Prfr Day dawns Pfrr Pfrr Pfr Pfrr Pfr Pfr Pfrr Pfr Pr Pfr Pfrr Pfrr Prfr Pfr Pfr Pfr Pfrr Prfr Pfrr Pfr Most P gets converted to Pfr form again. SDP = LNP • Needs long night • Needs Pfr gone at end of night • Pfr inhibits flowering for SDPs LDP SDP Long day: Pfr left at end of short night. Pfr promotes flowering for LDPs. Pfr inhibits flowering for SDPs. Short day: Pfr gone at end of long night. No Pfr to promote flowering for LDPs. No Pfr to inhibit flowering for SDPs. Waiting for the right time • Plants grow leaves until it is time to flower • LDPs wait until the day is long enough – Really night short enough – Some time before June 21 • SPDs wait until the day is short enough – Really night long enough – Some time after June 21 • Flower opening happens later Day neutral plants • Flower when mature enough • Maybe other environmental signals (temp?) • Day length (dark length) doesn’t matter Through the year May Specific times. September Julyflowers at specific June August October Phytochrome tells plants • • • • If they are near the surface About their plant neighbors Whether it is time to flower And lots more