Lecture 9

BIOL 3640 – Plant Biology Lecture 7 Ecological Considerations 1 © 2009 M.Olaveson-UOIT Lecture 7: Physiological and Ecological Considerations Physiological Considerations Ecological Considerations UOIT - BIOL 3640 PLANT BIOLOGY 2 © 2009 M.Olaveson-UOIT Lecture 7: Physiological and Ecological Considerations Ecological Considerations ecology = study of an organism’s relationship to its environment links life histories of plants to ability to be successful in a particular type of habitat plants anatomy can provide important clues that suggest how plants have adapted to different environmental conditions 3 © 2009 M.Olaveson-UOIT Habitat and Plant Structure adaptation = any aspect of a plant that promotes its welfare in the environment that it inhabits include: - external morphological modifications - histological changes in tissues/cells - physiological specializations any plant that is able to survive and reproduce in its environment is adapted to some degree to that environment can be specialized for a particular set of environmental conditions intense sun or shade extreme heat or cold physiologically wet or dry conditions nutrient or mineral deficiencies extremes of pH 4 © 2009 M.Olaveson-UOIT TERRESTRIAL PLANTS terrestrial plants live in environment with greater light, water, wind stresses than aquatic habitats exposed to – drying conditions damaging solar radiation high winds toxic substances attack / invasion by pathogens/insects 5 © 2009 M.Olaveson-UOIT Habitat and Plant Structure anatomical adaptations directly influence conduction of food and water rates of transpiration temperature of tissues effects of wind and humidity responses to herbivores and disease 6 © 2009 M.Olaveson-UOIT Plant Classification based on Habitat Features Mesophytes - plants that live in moderate environments require abundant soil abundant water relatively humid atmosphere average water availability moderate temperatures adapted to temperate climates includes most ‘familiar’ plants 7 © 2009 M.Olaveson-UOIT Plant Classification based on Habitat Features Hydrophytes - plants that live in aquatic environments - on water surface or submerged - no need to conserve water adaptations: to enhance light absorption to provide more efficient gas exchange / movement Xerophytes - plants that live in dry arid habitats - need to conserve water adaptations: to maximize water conservation to minimize water loss to provide more efficient gas exchange and movement 8 © 2009 M.Olaveson-UOIT Lecture 7: Physiological and Ecological Considerations Ecological Considerations (1) Leaf Anatomy (2) Stem (Wood) Anatomy (3) Root Anatomy 9 © 2009 M.Olaveson-UOIT variations in plant structure - most closely linked to environmental factors - expressed in the morphology / anatomy of leaves leaf – considered most anatomically variable organ in plants leaf adaptations – used as indicators of environmental conditions including: (1) Leaf Anatomy Light Availability Water Availability Wind Issues 10 © 2009 M.Olaveson-UOIT Plants Compete for Sunlight plants compete for light ability to use light available impacts photosynthetic rates modifications to leaves allow plants to use available light effectively - rosette-leaf growth habit prevents other leaves from growing below plant Dandelion - Taraxacum sp. 1. Leaf Adaptations to Light Availability 11 © 2009 M.Olaveson-UOIT 1. Leaf Adaptations to Light Availability level of illumination is most influential environmental factor affecting mature leaf structure leaf anatomy maximizes light absorption through a variety of adaptations leaves classified based on light exposure during development as: - sun leaves - shade leaves 12 © 2009 M.Olaveson-UOIT leaf orientation in different parts of plant alter light exposure leaves in top of canopy exposed to full sunlight tend to have steeper angles of incidence leaves deeper in canopy become more horizontal to maximize light capture 1. Leaf Adaptations to Light Availability 13 © 2009 M.Olaveson-UOIT 1. Leaf Adaptations to Light Availability {5C22544A-7EE6-4342-B048-85BDC9FD1C3A}Characteristic Sun Plants Shade Plants Leaf Blade Size smaller larger Leaf Thickness thick thin Mesophyll Tissue (per unit area) increased decreased Chlorophyll Levels reduced increased Stomata Density higher lower Number of Veins increased decreased Cuticle (waxy) thick thin Dermal Hairs abundant few or absent anatomical adaptations 14 © 2009 M.Olaveson-UOIT 1. Leaf Adaptations to Light Availability anatomical adaptations (through phenotypic plasticity) occur in leaves on the same plant depending on light: - sun leaves have more lobed blades to reduce light exposure - shade leaves have broader less-lobed blades to maximize surface area for light absorption 15 © 2009 M.Olaveson-UOIT - palisade mesophyll cells – pillar-shaped – multi-layered – sieve effect / light channeling - spongy mesophyll cells – irregular-shaped – light scattering from air spaces 1. Leaf Adaptations to Light Availability anatomical adaptations - mesophyll thickness 16 © 2009 M.Olaveson-UOIT Figure 9.1 from Taiz and Zeiger (2006) Sun-adapted Leaf has greater thickness more palisade mesophyll Shade-adapted Leaf tends to be thinner reduced palisade mesophyll 1. Leaf Adaptations to Light Availability 17 © 2009 M.Olaveson-UOIT Figure 8.8 from Beck (2005) 1. Leaf Adaptations to Light Availability anatomical adaptations - stomatal density greater density of stomata in sun leaves leads to greater transpiration rates higher transpiration can act like a ‘sweating mechanism’ that cools leaves exposed to high light 18 © 2009 M.Olaveson-UOIT cuticle - involved in light reflection, water retention, self-cleaning, prevention of infection / attack - thicker in sun leaves 1. Leaf Adaptations to Light Availability outer epidermal cell wall covered by waxy cuticle anatomical adaptations - cuticle thickness 19 © 2009 M.Olaveson-UOIT - dermal hairs (trichomes) – deflect light 1. Leaf Adaptations to Light Availability anatomical adaptations - dermal hairs (=trichomes) 20 © 2009 M.Olaveson-UOIT 2. Leaf Adaptations to Water Availability plants adapt physiologically , morphologically and anatomically to variations in water availability plants classified based on preference to different water levels as: 1. drought tolerant plants (xerophytes) 2. resurrection plants (poikilohydric plants) 3. aquatic plants (hydrophytes) 21 © 2009 M.Olaveson-UOIT 1. Drought-Tolerant Plants (Xerophytes) can be classified on basis of response to water stress as: (i) drought-escaping – alter life cycle to reduce impact of water stress (compressed into wet periods) (ii) drought-evading – have structural features (e.g. extensive root system) to reduce / compensate water loss (iii) drought-enduring – have anatomical adaptations to allow growth/survival under water stress xerophytic plants show various drought-tolerant leaf morphologies (xeromorphic leaf adaptations) 2. Leaf Adaptations to Water Availability 22 © 2009 M.Olaveson-UOIT (iii) drought-enduring – have anatomical adaptations to allow growth/survival under water stress: temporary leaf loss changes in leaf angle rolling / folding of leaf blades (involves bulliform cells) leaf modifications - succulent leaves - reduced leaves (spines) 2. Leaf Adaptations to Water Availability 23 © 2009 M.Olaveson-UOIT (a) rolling / folding of leaf blades (involves bulliform cells) under drought conditions – bulliform cells lose turgor and cause leaf to curl reducing exposure to light and wind - results in reduced water loss 2. Leaf Adaptations to Water Availability 24 © 2009 M.Olaveson-UOIT (b) ericoid leaves under drought conditions – some plants families show reduction in leaf size ultimate leaf reduction has resulted in evolution of scale-like or needle-like spines (termed ericoid leaves) typical of evergreen conifers, heather and other low-growing shrubs 2. Leaf Adaptations to Water Availability 25 © 2009 M.Olaveson-UOIT (c) succulent leaves under drought conditions – some plants (e.g. cacti) possess fleshy or succulent leaves, stems and roots allows plants to store large amounts of water cell walls of storage cells are reinforced to prevent collapse when turgor is reduced 2. Leaf Adaptations to Water Availability 26 © 2009 M.Olaveson-UOIT 2. Poikilohydric Plants (= Resurrection Plants) structure and function vary dramatically with water availability cells, tissues and organs are able to remain viable following cycles of extreme dehydration and rehydration 2. Leaf Adaptations to Water Availability structural features include: ability of leaves to shrink in size, wrinkle or curl up spiral thickenings of xylem tracheid elements allow xylem tissues to collapse ‘like an accordion’ when drying 27 © 2009 M.Olaveson-UOIT 3. Aquatic Plants (= Hydrophytes) some plants have returned to the water three categories of aquatic plants are: (a) emergent - with portions of plant above and below water surface (e.g. cattails) (b) floating – plants float on water surface (e.g. duckweed) (c) submerged – plants completely under water 2. Leaf Adaptations to Water Availability 28 © 2009 M.Olaveson-UOIT (b) floating – plants float on water surface with some leaves submerged and other leaves on/above water leaves show heterophylly where individual plants have two types of leaves: (1) aerial leaves that are flat undissected with smooth margins, a cuticle, and numerous stomata (2) submerged leaves that are highly dissected with reduced venation, no stomata and no cuticle 2. Leaf Adaptations to Water Availability Myriophyllum - water milfoil 29 © 2009 M.Olaveson-UOIT all hydrophytes are characterized by increased amounts of aerenchyma in leaves aerenchyma = tissues composed of parenchyma cells with large intercellular air spaces that increases buoyancy 2. Leaf Adaptations to Water Availability 30 © 2009 M.Olaveson-UOIT most submerged hydrophytes are also characterized by: reduced cuticle stomata on upper surface of leaves (if present) chloroplasts in epidermal cells 2. Leaf Adaptations to Water Availability these characteristics keep plants near water surface to access light for photosynthesis and maximize gas exchange no need for cuticle when water is abundant gas exchange in submerged leaves occurs by diffusion across leaf cells (no need for stomata since CO2 and O2 are dissolved in water) chloroplasts in epidermal cells to increase light access and maximize photosynthesis 31 © 2009 M.Olaveson-UOIT {5C22544A-7EE6-4342-B048-85BDC9FD1C3A}Characteristic Xerophytes Hydrophytes Morphology blade reduced Heterophyllous (more than one type of leaf morphology) Texture leathery thin Cuticle thick thin / absent Epidermis thick-walled thin-walled Stomata Sunken (so that wind will not increase transpiration) restricted to upper surface / absent Mesophyll compact aerenchymatous Water Storage present absent Vascular Tissue abundant reduced Other Features leaves roll/fold epidermal cells have chloroplasts anatomical adaptations 2. Leaf Adaptations to Water Availability 32 © 2009 M.Olaveson-UOIT 3. Leaf Adaptations to Wind Issues alpine and tundra plants are exposed to high light, low water availability and high winds many such plants show adaptations similar to xerophytes other adaptations of mountain (alpine) plants include: (i) dense hair covering - reflects light (under high light) reduces water loss through diffusion under high winds (ii) thick cuticle and epidermal cell walls to reduce water loss from high winds (iii) low growth habit – grow close to the ground to avoid wid effects 33 © 2009 M.Olaveson-UOIT leaves have received most attention in ecological anatomical investigations structure of wood in stems has received growing attention in recent years because of the influence of climate on xylem structure and function consequently the annual growth rings formed in secondary growth in long-lived trees can provide a ‘proxy’ for inferring long-term climate patterns (2) Stem (Wood) Anatomy 34 © 2009 M.Olaveson-UOIT growth rings vary in thickness from year-to-year because of effects of environment (especially temperature and water availability) when trees are mature (and producing secondary tissues on an annual basis) – there are 2 phases: period of rapid cambial activity - correlated to periods of high rainfall / warm temperatures (2) period of slow cambial activity - correlated to periods of low rainfall / cooler temperatures Use of Growth Rings in Wood (Secondary Xylem) therefore growth rings visible because of seasonal variation in abundance / characteristics of xylem cells 35 © 2009 M.Olaveson-UOIT Use of Growth Rings in Wood (Secondary Xylem) (1) during period of rapid cambial activity – tracheids, vessel elements and fibers are large with thinner secondary cell walls (form earlywood or springwood) (2) during period of slow cambial activity – tracheids, vessel elements and fibers are smaller and thick-walled (form latewood or summerwood) 36 © 2009 M.Olaveson-UOIT Use of Growth Rings in Wood (Secondary Xylem) seasonal / annual variation in abundance/characteristics of xylem cells linked to climate at time of ring growth 37 © 2009 M.Olaveson-UOIT roots – also show anatomical variable that is related to the environmental conditions where plants is growing roots – respond most to water availability classified on basis of water conditions as follows: xerophytic roots hydrophytic roots drought-stressed / flooded roots aerial roots (3) Root Anatomy 38 © 2009 M.Olaveson-UOIT 1. Xeromorphic Roots roots of xerophytic plants (with low water availability) - are widely spreading and shallow for optimal water absorption from dry arid soils – where there is little sub-surface water available some are short tuberized roots that lack root hairs some are succulent and act in water storage – in parenchyma cells of ground tissue in root cortex Root Adaptations to Water Availability Pterocactus araucanus has small aerial green stems and large taproot that is the main site of water storage 39 © 2009 M.Olaveson-UOIT 2. Hydromorphic Roots range from well-developed like roots of terrestrial plants to highly variable roots with major differences in root hair development reduced need for mechanical support structures in aquatic roots and reduced need for xylem (for water transport), vascular tissue tends to be less developed emergent aquatic plants have least root modification (compared to land plants) - water conducting tissues are still required (for above-water structures) Root Adaptations to Water Availability 40 © 2009 M.Olaveson-UOIT 2. Hydromorphic Roots submerged / floating aquatic plants have higher degree of root specialization (compared to land plants) roots of submerged aquatics (in lakes / ponds / swamps) have extensive aerenchyma – to facilitate gas exchange - prevent root ‘suffocation’ roots of submerged aquatics (in rivers / streams) have less aerenchyma – water movement ensures adequate O2 supply to roots Root Adaptations to Water Availability 41 © 2009 M.Olaveson-UOIT 3. Drought-Stressed / Flooded Roots in plants that undergo transient drought stress followed by flooding must adjust quickly to changing water conditions under drought-stress: - plants that are temporarily deprived of water show predictable pattern of cell death from root epidermis inward death process stops at the endodermis hydrophobic Casperian strip and suberin layers of endodermis prevent water movement out of vascular tissues of roots to dry soils so cells interior to the endodermis have sufficient water to survive short-term droughts Root Adaptations to Water Availability 42 © 2009 M.Olaveson-UOIT 3. Drought-Stressed / Flooded Roots in plants that undergo transient drought stress followed by flooding must adjust quickly to changing water conditions under flood conditions: - plants that are temporarily flooded show development of aerenchymatous tissue to maximize air circulation and prevent onset of anaerobic conditions plants that are temporarily flooded show develpoment of a suberized layer of cells under the epidermis called the exodermis which slows or prevents the rapid influx of water through the apoplastic route Root Adaptations to Water Availability 43 © 2009 M.Olaveson-UOIT 4. Aerial Roots arise from other aerial organs function as protective and supporting structures play role in translocation and gas exchange occur in a variety of plants but are most highly developed in tropical and sub-tropical plants (e.g. mangrove species) Root Adaptations to Water Availability 44 © 2009 M.Olaveson-UOIT 4. Aerial Roots common in aerial epiphytes such as orchids orchid roots have three layers: (1) central vascular stele (2) cortex bounded by inner endodermal layer and outer exodermal layer – to prevent water loss (3) external covering called the velamen that can absorb water vapour from moist rainforest canopy Root Adaptations to Water Availability 45 © 2009 M.Olaveson-UOIT