Chapter 7 PLANT ADAPTATIONS TO THE ENVIRONMENT II:

THERMAL, MOISTURE, AND NUTRIENT ENVIRONMENTS.

PLANTS AND THE THERMAL ENVIRONMENT

Plants are constantly absorbing short-wave and long-wave radiation from the surrounding environment.

THERMAL ENERGY BALANCE

Plants maintain a thermal balance through evaporation and convection.

Leaf size, shape and the stomatal opening/closing control influence these processes.

The energy absorbed by plants per unit of time is referred to as the plant’s net radiation balance, Rn.

Plants absorb and reflect solar radiation, and absorb short-wave radiation and emit long-wave radiation. The difference between the two is the net radiation balance.

Rn = M + S + (C + λE)

  • Less than 5% of the absorbed radiation is used in photosynthesis and stored in chemical bonds, M.
  • Energy is also used in heating the plant tissues and raising the temperature of the boundary layer, S.
  • Convection (C) and evaporation (E) dissipate energy into the environment; evaporation includes transpiration and direct evaporation.
  • λ = latent heat of vaporization; energy required to transform one unit of liquid water to vapor.

Transpiration and evaporation occur in plants: evapotranspiration.

Evaporation occurs from the surface of leaves.

Heat is dissipated by evaporation and transpiration.

Air temperature directly affects evaporation and transpiration.

Transpiration is the result of stomatal conductance and vapor pressure deficit.

Convection depends on the temperature difference between the plant and the surrounding air.

Factors influencing heat loss by convection:

  • Difference between leaf and air temperatures.
  • Conductance of the boundary layer.

Wind removes the warm air of the boundary layer and increases the difference between the leaf and the atmosphere; the boundary layer conductance increases with the movement of air.

The size and shape of the leaf affects the conductance of the boundary layer.

The ratio of surface to volume affects heat convection.

  • Small, lobed leaves are more effective at heat exchange than are larger, less lobed leaves.

Plants must replace the water lost in evapotranspiration.

Air temperature and wind velocity impose limits on the ability of plants to dissipate excess heat energy.

Water lost must be replaced; therefore, precipitation patterns have a direct effect on the plant’s energy balance.

THERMAL EFFECTS ON PHOTOSYNTHESIS AND RESPIRATION

Photosynthesis and photorespiration are sensitive to temperature changes, and respond directly to variations in temperature.

High temperatures favor oxygenation over carboxylation.

Carboxylation occurs during the dark phase, Calvin cycle, of photosynthesis and is the direct result of the activity of rubisco.

Rubisco activase is an enzyme required to change rubisco from the inactive to the active form in order to carry out carboxylation.

Rubisco activity is sensitive to temperature:

“The temperature-dependent association of rubisco activase with the thylakoid membrane was due to a conformational change in the rubisco activase itself, not to heat-induced alterations in the thylakoid membrane……During a sudden and unexpected exposure of plants to heat stress, rubisco activase is likely to manifest a second role as a chaperone in association with thylakoid-bound ribosomes, possibly protecting, as a first aid, the thylakoid associated protein synthesis machinery against heat inactivation.”

Rubisco activase: an enzyme with a temperature-dependent dual function? Rokka A, Zhang L, Aro EM. Plant J. 2001 Feb;25(4):463-71.

The net photosynthetic rate is the difference between the rate of carbon uptake in photosynthesis and the rate of carbon lost in respiration.

The net photosynthetic rate varies in plants depending on the environment in which the plant lives.

Plants living in cooler climate have a lower maximum (Tmax) and minimum temperature (Tmim) in which photosynthesis approaches zero. Their temperature optimum is lower, Topt

Biochemical and physiological adaptations allow the plant to shift its optimum uptake of carbon through photosynthesis toward the prevailing temperature of the environment. This process can also be observed during the seasonal shifts of temperature: acclimation.

PEP carboxylase is found in CAM and C4 plants, and is absent in C3 plants.

C3 and C4 show consistent differences in their photosynthetic response to temperature.

  • There is no photorespiration during the initial carbon fixation by PEP carboxylase in C4 plants.
  • The Topt of PEP carboxylase is higher than rubisco and high temperatures have little effect on C4 plants.
  • The Topt for the C3 pathway approaches that of rubisco. The Topt for C4 corresponds to the range of temperatures in which the activity of both enzymes, rubisco and PEP carboxylase, is relatively high.

Additional information about the influence of temperature in the activity of rubisco:

TEMPERATURE AND PLANT GROWTH

Plants require certain amount of photosynthetic activity accumulated over time to reach certain point of development: maximum growth, flowering, ripening of seeds, etc.

Plants require a number of degree-days of growth (photosynthetic activity) to reach maturity or bloom.

The rates of photosynthesis vary with the time of the day and the season. It stops above and below certain temperatures, Tmax and Tmin. The optimum temperature occurs between these two values.

The index of degree-days is used to relate growth to variations of temperature in a single season.

The index of degree-days is the sum of the departures in temperatures above some minimum or basetemperature.

The minimum temperature is selected as the temperature at which photosynthesis approaches zero.

The mean daily temperature reflects growth and carbon accumulation.

Taken from

Ohio University Extension, Department of Horticulture and Crop Science.

Formula: GDD = (T High plus T Low) divided by 2, minus 50

The following adjustments are necessary: 1) temperatures below 50 degrees F are set at 50 degrees F, and 2) temperatures above 86 degrees F are set at 86 degrees F. This method of calculating GDD is often referred to as the (86,50) system.

Examples of GDD Calculations:

For High = 80 degrees F, Low = 60 degrees F: GDD = 80 plus 60 divided by 2 minus 50 = 20;

For High = 60 degrees F, Low = 40 degrees F: GDD = 60 plus 50(40) divided by 2 minus 50 = 5;

For High = 90 degrees F, Low = 70 degrees F: GDD = 86(90) plus 70 divided by 2 minus 50 = 28

Growing Degree Days or heat units are calculated for each day starting the day after planting.

Check this site for a good explanation of “degree-days”:

EXTREME TEMPERATURES AND PLANT SURVIVAL

Freezing temperatures can result in the formation of intra- and extracellular ice as well as phase change in membrane lipids.

Factors associated with the ability of cells to withstand freezing temperatures:

  • Increased solute concentrations.
  • Unsaturated lipids (soluble fats) increase.
  • Lipid concentration increases.
  • Amino acids are removed from proteins: depolimerization.
  • Cell membrane becomes more permeable.
  • Small cell size
  • Abscisic acid.

Abscisic acid accumulates in the leaves with dehydration. Through its effects on second messengers such calcium ions, potassium ion channels open in the guard cells causing a massive lost of potassium. This loss of ions from the guard cells, causes water to leave the cells and the subsequent loss of turgor of the guard cells closes the stomata.

Critical minimum temperature: 0° to 10°C and -15° to -40°C.

In region where the temperature falls between -15° to -40°C, the dominant vegetation consists of broad-leaf deciduous plants.

These plants lower their freezing point by supercooling, the lowering of the freezing point by increasing solute concentration.

Cells can lower the freezing point by no more than 3°C by increasing solute concentrations.

30 genes were found in Larix kaempferi that increase the supercooling capability to -60ºC. See abstract at

Water in the cell wall and middle lamella freezes first with dropping temperature and releases heat (heat of fusion or specific heat of melting), which is absorbed by the adjacent cells and helps them to remain liquid. Then, water moves out of the cells attracted to the ice crystals.

Tolerance to freezing is not uniformly distributed through a plant.

  • Roots, bulbs and rhizomes are the most sensitive to freezing (-10 to -30°C).
  • Terminal buds are less resistant than lateral buds.
  • Woody stems are more resistant than buds and leaves.

Changes in the ultrastructure of the cells have been observed: multiple small vacuoles, increase in the number of vesicles, etc. For more details see:

Annals of Botany90: 637-645, 2002, Marzanna Stefanowska, Mieczysaw Kura and Alina Kacperska

Hairs insulate by trapping air and heat.

The growth habit also helps in surviving freezing temperatures: the interior temperature of cushion and rosette plants may be 20°C higher than the surrounding air.

A high temperature of 45°C disrupts metabolic processes.

Heat shock proteins are involved but their role is not well understood.

Cacti can maintain protein synthesis as fast as proteins breakdown and, in this way, avoid ammonia poisoning.

Morphological and nactic movements allow plants to adjust to high temperatures, e.g. folding of leaves, spines, narrow leaves, photosynthesis carried out in the stem, changing the orientation of leaves to a parallel position to sun rays.

PROCESSES OTHER THAN SURVIVAL AND GROWTH

Temperature affects germination, reproduction, flower formation, flower unfolding, and ripening of fruits.

Temperatures between –3 and +13°C are needed by certain annuals and biennials to flower normally in the spring.

PLANT RESPONSE TO WATER

Open stomata allow CO2 into the leaves for photosynthesis, but also allow the water to escape, transpiration.

WATER UPTAKE AND THE SOIL-PLANT-ATMOSPHERE CONTINUUM

A water potential gradient exists between the soil, and the tissues of the roots, the stem and branches, the leaves, and the atmosphere.

This water potential gradient is responsible for the movement of water from the soil through the plant into the atmosphere.

The units used to describe the water potential are megapascals, MPas.

Water flows from areas of high water potential (ψ) to areas of low water potential. This is called osmoticpotential.

The soil has the highest water potential and the atmosphere the lowest.

  • Ψatmosphere < ψleaf ψstem < ψroots < ψsoil

The movement of water across a membrane is called osmosis.

The osmotic potential of the cells (concentration of solutes in the cytoplasm), the matric potential or tendency of water molecules to adhere to soil particles, and the pull of gravity or gravitational potential, all influence the total water potential in the body of the plant.

As plants lose water through transpiration, the solute in the cells becomes more concentrated, the water pressure drops in the cells, and water moves in from the areas of higher concentration.

As water moves from the soil into the roots, the water potential of the soil drops and becomes more negative.

The tendency of water to adhere to surfaces is called matric potential.

As the water content of the soil drops, the remaining water adheres more tightly to soil particles and the matric potential drops.

Cohesion between the water molecules also plays a role increasing surface tension in the soil pores between the clay particles and creating menisci (sing. meniscus). Adhesion to clay particles and the formation of menisci can increase the matric potential significantly and make it unavailable to plant roots.

The texture of the soil affects the matric potential. Clay provides more surface than do sand and maintains a more negative matric potential.

As the soil water potential drops, it becomes more difficult for the plant to maintain its water potential and eventually it cannot absorb more water.

At this point, the stomata close to prevent the loss of water through transpiration but this also prevents the entry of CO2 into the leaves and disrupts photosynthesis.

The value of the leaf water potential at which the stomata close varies with the species, and depends on the biochemistry, physiology and morphology of the species.

Water must overcome the pull of gravity in order to move up the vascular tissue of the plant.

The gravitational potential is a factor of the height of the plant from a reference point. It is positive above the reference point and negative below. It is important in the movement of water in tall trees.

The gravitational potential, ψg, increases by 0.01 MPa m-1 above the ground.

RESPONSE TO SHORT-TERM MOISTURE STRESS

The closing of the stomata prevents the loss of heat by transpiration but the plant continues to intercept radiation and its internal temperature rises.

An increase in internal temperature results in heat stress that interferes with protein synthesis and if prolonged, with chlorophyll synthesis.

The plant may respond by curling its leaves, wilting, and dropping the leaves prematurely. The oldest leaves are shed first. If the drought continues, the tender twigs and branches die back.

Some plants under water-stress reduce their osmotic potential by accumulating ions of Ca2+, Mg2+, K+ and Na2+, and amino acids, sugars and sugar alcohols. The lower water potential of the leaves maintains the potential gradient from plant to soil.

Conifers and evergreens may experience a browning and a dieback during the winter months due to water stress. If the temperature is high enough for water in the vascular tissue to liquefy, the trees lose their water by transpiration but the water cannot be replaced because the ground is frozen. Dehydration of the foliage occurs.

PLANT RESPONSES TO LONG-TERM VARIATIONS IN WATER AVAILABILITY

Individual plants growing under dry conditions have thicker leaves than members of the same species growing under moist conditions.

The leaves are thicker because more layers of mesophyll are produced. There is more mesophyll per unit of area.

More mesophyll layers increase photosynthesis but reduce the surface area that absorbs radiation and loses water through transpiration.

Root production increases under dry conditions by shifting the allocation of carbon from leaves to roots.

Individuals growing under contrasting environmental condition show responses that compensate for the shortage of an essential resource.

INTERPSECIFIC VARIATION IN ADAPTATIONS TO MESIC AND XERIC ENVIRONMENTS.

Xerophytes have greater water use efficiency than mesophytes, that is, a greater rate of carbon uptake per unit of water transpired.

  • Water use efficiency: rate of carbon uptake per unit of water transpired.
  • Photosynthesis/transpiration

C4 have higher water use efficiency than C3 plants.

C4 plants maintain a very low concentration of CO2 within the mesophyll of the cells by having a great rate of carboxylation. This causes a steep gradient of CO2 concentration between the inside of the leaf and the outside air.

The steeper CO2 gradient allows C4 to maintain a higher rate of photosynthesis than C3 plants for a given stomatal conductance.

The ratio of root mass (g) to leaf is (cm2) increase with decreasing water availability.

Species adapted to high and low resource availability show responses that compensate for the shortage of an essential resource:

  • Under xeric condition, there is a lower stomatal conductance and lower rate of net photosynthesis than those species living in mesic environments.
  • Lower stomatal conductance, however, results in greater water use efficiency.
  • Lower allocation of carbon to production of leaves results in greater root production that increases the plant’s access to soil water.
  • Reduced leaf surface area and reduced photosynthesis results in less carbon uptake and reduced growth.

There is a trade off between higher rates of photosynthesis and growth when water is available and survival, growth and reproduction when water is consistently in short supply.

LINK BETWEEN PLANT WATER AND ENERGY BALANCE

Plants dissipate heat through the loss of water by transpiration.

If the water potential in the soil declines, the ability to absorb water also declines.

Under mesic conditions, transpiration is the preferred means for heat dissipation.

Plants also dissipate heat by convection.

In xeric environments, plants dissipate most its heat by convection.

Leaf size decreases gradually as conditions change from mesic to xeric.

PLANTS RESPONSE TO FLOODING

Too much water around the roots causes the death of root tips due to lack of oxygen.

Root death follows due to poor absorption. Detritus is added to the vascular tissue and the xylem clogs.

High water table causes plants to develop horizontal root systems that grow along the oxygenated soil zone.

PLANT ADAPTATIONS TO FLOODING

Aerenchyma is a specialized tissue that contains air spaces that facilitate gas exchange and transport of air from shoots to roots.

The porosity of plants adapted to flooding could be as great as 60%. Plants living in mesic or xeric environments usually have a porosity of 2-7% space by volume.

Pneumatophores are adaptations to permanently flooded environments. They probably help in providing oxygen to roots.

Aerenchyma formation: Without oxygen...

  • Roots shift for aerobic to anaerobic respiration.
  • Uptake of ions is inhibited
  • The concentration oxygen, potassium, nitrogen, and phosphorus decreases.
  • Ethylene is produced and accumulates.
  • Ethylene is insoluble and does not diffuse out of the roots and oxygen uptake is prevented.
  • Ethylene causes cells next to the cortex to break and separate to form interconnected gas-filled chambers, aerenchyma tissue.

Stem and lenticel hypertrophy

  • Hypertrophy is the enlargement of an organ without an increase in the number of constituent cells. An example of this is buttressing or butt swell which is an increase in the diameter at the base of the stem.
  • The role of this seems to be to increase air space which allows for increased movement of gases.
  • Besides that, the wide base provides extra support for shallow rooted structures on a soggy substrate.

PLANT ADAPTATIONS TO SALINITY

Salts originate from the weathering of rocks, irrigation and floods, human and animal additions and fertilizers.

As salinity increases, plants have more difficulty in extracting water from the soil.