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External factors and plant growth – Chapter 28 Tropism A tropism is a growth response involving bending of a plant toward or away from an external stimulus Positive tropism – move towards stimulus Negative tropism – move away from stimulus Phototropism Bending in response to light Shoots exhibit positive phototropism Caused by elongation of cells on dark side of shoot Went’s experiment Cut off coleoptile tips and placed in agar Put agar blocks on tip of shoot tip Saw elongation where auxin diffused Phototropism and auxin Light can decrease sensitivity of cells to auxin on one side Light can destroy auxin Light moves auxin to shaded side of tip Briggs et al. experiments Same amount of auxin in coleoptile tip found in light and dark tip Briggs et al. experiments Same amount of auxin when coleoptile tip is split by glass Briggs et al. experiments Same amount of auxin when coleoptile tip and agar block split with glass Briggs et al. experiments Differential amount of auxin when coleoptile tip and agar block split with glass, but tip intact Gravitropism Response to gravity Shoots are negatively gravitropic Roots are positively gravitropic Gravitropism and auxin Auxin redistributes from upper side to lower side In shoots, higher concentration on lower side promotes elongation and shoot turns up In roots, higher concentration on lower side restricts elongation and shoot turns down Gravitropism and calcium Ca/Calmodulin complex Calcium diffuses to upper surface of shoots and towards the lower surface of roots Gravitropism and cytokinin Cytokinin accumulates in lower side of gravistimulated roots Gravity perception Movement of amyloplasts Starch sheath cells in shoot Columella in rootcap What next? Starch-statolith hypothesis Amyloplasts act as gravity sensors Protoplast pressure hypothesis Entire protoplast is gravity sensor Tensitegrity model Tensional integrity is structural integrity created by interactive tension between structure components Disruption of integrity increases calcium levels Thigmotropism Response to touch Tendrils Circadian rhythms Regular 24-hour cycles in plant function Biological clocks Rhythms continue even when environment is constant Rhythms are internal Maintains rhythm with temperature changes Sychronization Biological clocks are 21 to 27 hours with constant environment Biological clocks are sychronized or entrained with environment to maintain 24 hour cycle Entrainment Photoperiodism Flowering under certain daylength conditions Important for other organisms too Photoperiodism Three general patterns Short-day plants flower in early spring or fall and must have a light period shorter than a critical length Long-day plants flower in summer and must have a light period longer than a critical length Day-neutral plants flower without respect to daylength Flash of light during dark period long-day plant will flower even if daily light period is shorter than time required Flash of light period during dark period for short-day plant will inhibit flowering Photoperiodism Can be very precise Varies with species 1 day or many days Type of light during dark period for long-day plants is important Only germinate if last flash is red light Phytochrome Phytochrome is a photoreceptor in plants Two interconvertable forms Pr absorbs red light and is biologically inactive Pfr absorbs far-red light and is biologically active Phytochrome During daytime….. Since daylight contains both types of light an equilibrium of Pr and Pfr will exist in daytime (60% Pfr at noon) At night…. Levels of Pfr decline due to dark conversion and destruction Flash of FR light during night causes conversion of Pfr to Pr. Flash of R light reverses this. Etoliation Phytochrome and shade response Greater FR light under canopy Greater FR:R ratio causes increased internodal elongation Hormonal flowering control Substance from leaf called florigen stimulates flowering Florigen is probably gibberellin and anthesin Another unidentified hormone inhibits flowering Dormancy Time of arrested growth Dormant bud or embryo has specific cues for reactivation Seeds Stratification Cold requirement simulates winter Drying Prevents germination in moist fruit Abrasion Movement Gastrointestinal tract Fruit inhibitory substances Seed banks Bud dormancy Acclimation precedes dormancy Cold period may be necessary Drying Addition of gibberellin Vernalization Cold temperatures can stimulate flowering Nastic movement Plant movements that occur in response to a stimulus but whose direction is independent of the position of the origin of the stimulus Thigmonastic movements Thigmomorphogenesis Alteration of growth patterns in response to mechanical stimuli Heliotropism Solar tracking Leaves move with moving sun