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Response to Light Lecture 17 Spring 2014 Shade Avoidance • Shade avoidance response – For high-light adapted plants – Grows tall • Photomorphogenesis – How does the plant know that it is in the shade? 1 2 Photoreceptors • Photoreceptors – Pigment: Molecule that absorbs certain wavelengths of light • Blue-light receptors & phytochromes (red) 3 Absorption Spectrum 4 Action Spectrum Fig. 39.16 Photoreceptor: Phytochrome Fig. 39.18 5 6 Photoreceptor: Phytochrome • What is an isomer? • In sunlight, both red and far-red wavelength – Conversion to Pfr faster Fig. 39.19 Photopigments • In shade, red light absorbed by canopy – Far-red light passes through – Phytochrome ratio shifts to more Pr – High-light adapted plants response: grow tall 7 8 Photoreceptor: Phytochrome • Typically Pfr form switches on physiological and developmental responses in plant Fig. 39.19 9 Physiological Response • De-etiolation process Fig. 39.04 Fig. 39.02 Seed Germination • Seed size & soil depth • Light cue • Weed control 10 11 Seed Germination • Effects of red & far-red light reversible Fig. 39.17 Photoperiodism & Flowering • Physiological response to relative lengths of day and night • Seasonal awareness critical • Short-day – Flower in late summer, fall or winter • Long day plants – Flower in late spring or early summer • Day neutral – Flower when reach certain stage of maturity 12 Photoperiodism & Flowering • Critical night length Fig. 39.21 13 Photoperiodism & Flowering • Critical night length Fig. 39.21 14 15 Photoperiodism & Flowering Fig. 39.22 16 Flowering Hormone • Florigen • Flowering Locus T (FT) gene Fig. 39.23 17 Blue-light Receptors • Blue light important for photosynthesis • Phototropins – Two known genes PHOT1 and PHOT2 in Arabidopsis – Other blue light receptors • Movement of chloroplasts • Opening of stomata • Phototrophic response Blue-light Receptors & Phototropism • Auxin inhibitors Fig. 39.5 18 Biological Clocks • Circadian rhythms – ~ 24 hr cycles • Oscillation in transcription of genes • CONSTANS (CO) gene – CO protein is transcription factor – Negative feedback loops Fig. 39.20 19 Biological Clocks • Light resynchronizes internal clocks • Both phytochromes and blue-light receptors • Phytochromes – In dark enzymes destroy more Pfr than Pr – Increase in Pfr due to light resets clock 20
