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Transcript
Plant Adaptations Outline: •Photosynthesis and respiration •Environmental controls on photosynthesis •Plant adaptations to: –High and low light –Water limitation –Nutrient availability Readings: Chapter 6 Conditions and Resources • Conditions are physical / chemical features of the environment – E.g. Temperature, humidity, pH, etc. Not consumed by living organisms (but may still be important to them) • Resources are consumed – Once used, they are unavailable to other organisms – Plants: sunlight, water, mineral nutrients, … – Animals: prey organisms, nesting sites, … Plant Resources • Plants are autotrophs - make their own organic carbon form inorganic nutrients – Need light, ions, inorganic molecules • Plants are sessile – Grow towards nutrients PHOTOSYNTHESIS Conversion of carbon dioxide into simple sugars LIGHT 6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O Light reactions Dark reactions carboxylation Photosynthetically Active Radiation, PAR RESPIRATION C6H12O6 + 6O2 6CO2 + 6H2O + ATP Net photosynthesis = Photosynthesis - Respiration Photosynthesis involves gas exchange Controls on photosynthesis •Light •Water •Nutrients •Temperature 1. Light PAR Tradeoff • Shade plants grow better in the sun than in the shade, • but sun plants grow faster than shade plants in direct sun Shade plant Sun plant Tradeoff • Shade plants survive well in either sun or shade • Sun plants cannot tolerate shade Shade plant Sun plant • 9 tree species of Macaranga from Borneo, Malaysia Phenotypic plasticity • Most plants have the ability to alter their morphology (within limits) in response to light conditions Phenotypic plasticity • Sun and shade leaves can exist within the same tree More deeply lobed --> More rapid heat loss Sun leaf • thicker • more cell layers • more chloroplasts Shade leaf • flat • thin • larger surface area / unit weight Sun leaves Shade leaves •Leaves at many angles •High saturation point •High compensation point •Produce more RUBISCO •Horizontal leaves, single layer •Low saturation point •Low compensation point •Produce less RUBISCO •High respiration •Less chlorophyll •RUBISCO availability limits photosynthesis rate •Low respiration •More chlorophyll •Light availability limits photosynthesis rate 2. Water Transpiration For transpiration to occur atmosphere < leaf < root < soil Water potential w = p + + m p= = hydrostatic pressure = = osmotic pressure m= = matric pressure Stomata • Reduction in soil --> stomata close • Species differ in tolerance to drying soils Strategies for drought i. Avoiders • • • • Short lifespan Wet season Seeds survive drought Drought deciduous species – Leaves shed in dry season Strategies for drought ii. Tolerators • • • Leaves transpire slowly Change orientation of leaves Sunken stomata – • E.g. pines More efficient photosynthesis • • E.g. C4 --> reduces photorespiration E.g. CAM --> stomata open at night C4 photosynthesis CAM photosynthesis C4 CAM CAM % of grasses that are C4 Water absorption • Root hairs increase surface area • Structure of the root system varies between species, depending on the amt. of soil moisture in their env’t • Individual species show phenotypic plasticity • wet soil --> shallow roots near surface (greater oxygen availability) • dry soil --> deep roots 3. Nutrients • Macronutrients – needed in large amounts (e.g. C, H, O, … N, P, K, Ca, Mg, S) • Micronutrients – trace elements (e.g. Fe, Mn, B) • Micro/macro refer to the quantity needed Table 6-1 Nutrient uptake rates • Reach plateau with increasing nutrient concentration Maximum growth rate of a plant reflects N availability in its natural habitat. A. stolonifera occurs on more nitrogen-rich soils than A. canina. Evergreen leaves • Plants adapted to nutrient-poor conditions tend to have evergreen leaves 4. Effects of temperature = Condition • Increase temperature --> increase biochemical reaction rate • At high temperature, enzymes denature --> death • Gross photosynthetic rate increases up to a point with increasing temperature • Respiration rate also increases with temperature. • Net photosynthesis is maximal at a point slightly below that at which gross photosynthesis is maximal Leaf temperature • • > 95% of sunlight absorbed by a leaf becomes heat Cooling of leaves: 1. Transpiration 2. Convection (movement of cool air around a leaf) C4 plants • Have higher temperature optima than C3 Phenotypic plasticity • Individual species can modify their Topt according to the changing seasons = acclimatization Response to cold Chilling injury Freezing - near, > 0 oC - cell membranes rupture - < 0 oC - ice inside cells = death - ice outside cells = dehydration (may survive) -may kill juveniles only Saguaro cacti (S.W. United States) store large amounts of water; they can tolerate short periods of freezing temperatures CLOSER TO HOME •Freeze-tolerant plants: frost hardening •When T decreases – plants synthesize sugars, amino acids, other molecules to act as antifreeze. •Winter – deciduous plants •Lose leaves in autumn •Leaves very efficient in summer – high photosynthesis rate •Leaves can’t survive freezing •Costly in energy, nutrients to rebuild leaves •Chilling breaks seed dormancy for temperate/boreal spp. •Germinates only in spring Plants are phenotypically plastic