Temperature and light as ecological factors for plants

Transcription

1 PLB/EVE 117 Plant Ecology Fall Temperature and light as ecological factors for plants I. Temperature as an environmental factor A. The influence of temperature as an environmental factor is pervasive 1. Important in setting length of growing season 2. Modulates essentially all activities a. growth b. photosynthesis c. respiration d. transpiration e. nutrient uptake 3. Causes stresses when too high or too low B. Examples where we can see a relationship between temperature and plant distributions 1. Sierra field trip 2. Temperature isotherms---campanula uniflora in Norway a. Isotherm: line on map connecting points of equal temperature 3. Relationship between freezing temperatures and the distribution of the saguaro cactus (Carnegia gigantea) C. Metabolic processes exhibit minimum,optimum and maximum temperatures 1. different for different processes even in the same species II. Temperatures and plant performance A. Cold temperatures (frost, freezing) in the spring and fall usually set the length of the growing season 1. Only days in the arctic or alpine 2. All year in the tropics B. The upper temperature limits for plant growth and survival varies greatly among species C for Cyanobacteria in hot springs C for desert plants 3. Cool season and arctic plants may not do well above C C. The lower temperature limits for survival also differ greatly among species 1. Chilling sensitive plants a. killed by C b. Tropical plants 2. Freezing sensitive (but chilling tolerant) plants a. killed when tissues freeze, which occurs at -1 to -5 C 1. Freezing tolerant plants- tissues can freeze without damage but temperatures below a threshold cause tissue death a. Some temperate species killed at -6 to -8 C b. Boreal forest conifers can tolerate -40 to -70 C c. Acclimation (sometimes called cold hardening) important 1. Seasonal cycle in tolerance-- growing tissues are intolerant dormant tissues have increased tolerance d. snow cover can protect plants from low temperatures D. Temperature affects the rate of growth and development a. above a threshold, temperature accelerates development and shortens the time required for any developmental stage b. Temperatures that are too high may cause a disruption of developmental processes III. How do plants adapt to different temperatures? A. Grow only when and where temperatures are suitable

2 PLB/EVE 117 Plant Ecology Fall Winter annuals versus summer annuals 2. Genetically determined differences in temperature response B. Adjust physiology to match prevailing seasonal temperatures--acclimation 1. resistance to high and low temperatures 2. alter physiological responses such as photosynthesis or respiration a. Example from the desert and coast: Atriplex lentiformis C. Adjust plant temperature by alteration of the energy balance IV. The energy balance of a plant A. The energy balance determines the temperature of a plant. B. The energy balance under steady-state conditions ENERGY INPUT = ENERGY LOSSES + STORAGE ENERGY INPUT= Solar radiation absorbed + thermal radiation absorbed + (convective heat) ENERGY LOSSES= Transpiration + Thermal radiation emitted + (convective heat) STORAGE = energy stored chiefly in heating water (significant in stems) + photosynthesis (very small) C. More about the individual terms 1. Solar radiation-- typically about 1000 W /m 2 on a horizontal surface at midday on a clear summer day (W=Watts) a. varies daily and seasonally because of solar angle b. 10% from sky, 10% reflected to leaf c. Green leaves absorb about 60% of incident solar radiation 1. reduced by leaf pubescence, waxes a. Encelia farinosa -- seasonal changes in pubescence 2. affected by leaf orientation b. high leaf angles reduce midday solar radiation on leaf surface 2. Thermal radiation a. All objects emit thermal radiation (long wave infra-red) as a function of their temperature and emissivity R l =εσt 4 R l = thermal radiation emitted ε = emissivity (0.95 for leaves, also equals absorptivity for long wave radiation) σ = Stefan-Boltzman constant (5.6 W m -2 x 10-8 K) T = temperature of object in K b. plants emit thermal radiation as a function of their temperature c. plants receive thermal radiation as a function of the temperature of their surroundings (sky, soil, other plants, etc.) 3. Transpiration

3 PLB/EVE 117 Plant Ecology Fall a. Evaporation of water requires large amounts of energy b. Latent heat of vaporization of water = 2490 W s g -1 (transpiration of 1 g H 2 0 consumes 2490 W m -2 ) c. Transpiration rate depends on 1. Environment a. air temperature of air surrounding leaf b. relative humidity c. wind speed -reduces boundary layer 2. Leaf conductance to water loss a. stomata b. boundary layer (size of leaf) d. Importance of transpirational cooling--an experiment by Lange Sensible heat (convection) a. when leaf is warmer than air, heat (energy) is convected away from leaf b. when leaf is cooler than air, heat is convected to leaf c. rate depends on leaf size and wind speed a. small leaves= lower boundary layer conductance and less difference between leaf and air temperature b. Large leaves can be significantly warmer than air if transpiration is low, or much cooler if transpiration is high Conditions plant characteristics Leaf-air temperature Noon, full sunlight Large leaf, reflective (pubescence) T l << T air Low wind high g s (transpiration rate) Noon full sunlight large leaf, absorptive (dark green), low g s T l >> T air Low wind (low transpiration rate) Any conditions Small leaf absorptive or reflective T l T air Low or high g s g s =stomatal conductance VI. Adaptation to different light environments A. Plants are the primary factor reducing the light available to other plants. 1. Leaf area index (LAI)-- m 2 leaf area per m 2 of ground area a. the number of layers of leaves determines that light will be intercepted b. LAI varies between communities As high as 7 in tropical forests 2. Forest understories may receive only 1% of the sunlight above canopy a. important for tree seedling growth and forest regeneration B. How do plants cope with very low light 1. sun versus shade photosynthetic response a. sun shade species and ecotypes b. sun shade acclimation 2. principles regarding coping with low light a. Increase light absorption capacity 1. more leaf area/ plant weight 2. more chlorophyll/ per unit weight b. shift investments of resources from increasing light saturated photosynthetic capacity to absorbing

4 PLB/EVE 117 Plant Ecology Fall more light c. minimize respiratory losses. Characteristic Low photosynthetic capacity Low respiration rate Large investment in chlorophyll Advantage for shade leaves low cost to construct leaf area (less enzyme needed) increased net photosynthesis in shade increased light absorption C. How do plants cope with very high light 1. Solar tracking- diheliotropism a. profitable only if photosynthetic capacity is very high 2. Solar avoidance- paraheliotropism a. prevents absorption of excessive light 3. Excessive light can lead to photoinhibition a. inhibition of photosynthesis by excessive light b. mechanisms protecting against photoinhibition 1. leaf angles 2. carotenoids of the xanthophyll cycle VII. An example of the interaction between two environmental factors--night-time temperatures, high light and the regeneration niche of Eucalyptus pauciflora. Readings: TEXT: The text does not specifically cover the concepts related to temperature adaptation. Chapter 18 does provde a good discussion of variation in climate, of which temperature is a major factor. Energy balance is well covered on pages (Chapter 3). Aspects of sun and shade adaptation are covered in Chapter 2. Osmond et al., Stress Physiology and the Distribution of Plants. This article provides a very good summary of the response of plants to stress and its role in their ecology and distribution. For this lecture, pay particular attention to pages (temperature stress) and (light stress). Study questions 1. What conditions are necessary for a leaf to be cooler than the air? What conditions lead to leaf temperatures that are higher than air temperatures? 2. In the desert we often see relatively large leaves on species growing in washes where there is a lot of soil moisture but much smaller leaves on species growing on the hillsides where there is much less soil water available. Can you explain this observation in terms of the leaf energy balance and the leaf temperatures of large and small leaves in the wash and on the hill side. 3. What is an isotherm and how could it relate to the distribution of plant species? 4. An observation made in the early 1950 s by a plant ecologist was that the phenology of spring understory plants in the eastern deciduous forest in Ohio was delayed by about 10 days on north facing

5 PLB/EVE 117 Plant Ecology Fall as compared to south-facing slopes. These plants are species that initiate growth prior to leaf development in the forest canopy and thus take advantage of the period of high light in the spring. Generally this period is about days long before canopy development begins to shade the understory. The phenology of the overstory canopy is independent of the slope. Explain what might be going on here in terms of environmental effects on development and photosynthesis. (hint: think about both the light received and the temperatures) How do you think it would influence the annual carbon balance of the plants on the north and south slopes? 5. How would you identify that a species was capable of acclimation to different light environments? Suppose that you find two populations of the same species, one growing in the forest understory and the other in an open field. How would you determine if these two populations are sun and shade ecotypes. If they are indeed sun and shade ecotypes, what differences would you expect to find between them? 6. How would a thick layer of pubescence affect the energy balance of a leaf? Compare it to a glabrous (hairless) leaf. A simulation model of leaf energy budget Go to the class web page and download the leaf energy budget simulation model energybalance.exe. It is virus free and therefore safe to download. Otherwise, see Michael or I for a copy on disk. Once the model and the accompanying description are downloaded, you can run it. The slide bars allow you to set different values for parameters such as solar radiation, air temperature, etc. The model then computes the resulting energy inputs and and losses from the leaf and the resulting leaf temperatures, transpiration rates, etc. By changing values you can get a better feeling for how various leaf and environmental characteristics influence the energy budget and leaf temperature. You can use it to help answer some of the study questions above.