Specialized Leaves

Autumnal Leaf Changes

• Plant in areas with seasonally fluctuations exhibit two different adaptations: color change and deciduousness

Color change in leaves

• Chlorophyll disappears and secondary pigments become visible in leaves

• As day length shortens, many temperate plants dismantle chlorophyll and reuse the sub-components

• Orange and yellow pigments (types of carotenoids) are present year-round, but masked by chlorophyll

  • Beta-carotene (orange pigment) is a typical carotenoid, but lutein (yellow compound at low concentrations, but orange-red at high concentrations) are called xanthophylls.

• Temperature, light, and water supply have an influence on the degree and the duration of fall color. Rainy and/or overcast days tend to increase the intensity of fall colors. 

• Red and purple pigments (anthocyanins) are actively produced in late summer, as chlorophyll is being dismantled, which could appear to be wasteful.

• Under stress, leaf sugars are converted to anthocyanins in the blade

  • Low temperatures above freezing will favor anthocyanin formation producing bright reds in maples

  • However, early frost will weaken the brilliant red color. 

• Several hypotheses have been given to explain this function:

Allelopathy

• • Fallen leaves leach these harmful anthocyanin chemicals into the soil, which have been found to stunt the growth of nearby saplings (Frey and Eldridge 2005)

Coevolution

• • Bright red colors are linked to the amount of anti-herbivory chemicals, acting as a warning to migrating/over-wintering insects

  • Evidence shows that brighter colors in Malus correlate with lower aphid loads (Archetti 2009)

Photoprotection

• • In the absence of chlorophyll, anthocyanins protect the leaf from the effects of light in low temperatures, allowing the efficient resorption of nutrients (Lee and Gould 2002)

Deciduous leaves

• Loss of leaves correlated to times of drought, either in warm climates or water trapped in ice during winter

• Plants respond to shortening day length and begin to produce an abscission zone between stem and leaf petiole

• Abscission cells are sensitive to auxin...

  • During the growing season, auxin is produced at a constant rate, and the abscission layer remains connected

  • In autumn, or when under stress, the auxin flow from the leaf decreases or stops, triggering cellular elongation within the abscission layer

  • The elongation of these cells breaks the connection between the different cell layers, allowing the leaf to break away from the plant.

• Adaptive Value

  • Prevent water loss through leaves during dry and/or winter period

  • Avoid increased static loads from snow accumulating on leaves during winter

  • Fallen leaves are recycled into the soil

  • Leaves may serve to release allelopathic chemicals (e.g. tannins, anthocyanins)

Arid leaf adaptations 

CAM Photosynthesis

• Physical and physiological adaptations to prevent water loss in extremely arid environments

Reduced Leaf Surface Area

• Leaves and leaflets of non-succulent deserts plants are smaller than plants in wetter environments

• Leaves tend to narrow, which allows the leaves to remain below lethal tissue temperatures without substantial transpiration

Leaf Succulence (Water storage)

• Leaves with parenchyma with large vacuoles for storing water

• e.g. Crassulaceae, Agavoideae, Aizoaceae, and Aloe are leaf succulents

Stomatal density

• Succulent plants in arid regions tend to have less stomata per unit area, than plants from areas where water is plentiful, although succulent plants are only a small fraction of desert plants forms (~15%)

• In non-succulent arid plants, there is an increased density of stomata per unit area

  • This may seem counter-intuitive but "stomata patterns are mainly a design to maximize rates of CO2 uptake, not prevent water loss" (Gibson 1998)

Sunken stomata or stomatal crypts

• Sunken stomata help trap moisture and keep stomata away from dry air that can desiccate plants

• Some desert plants, and plants in cold dry habitats (e.g. Pinus) possess these features

• Once again, in desert regions, these features tend to be found in succulent plants which are a small fraction of total species, but not in non-succulent forms (Gibson 1996)

• Roth-Nebelsick et al. (2009) found that stomatal crypts have small or no effects on transpiration!

Hypodermis

• Thick-walled cells under epidermis to prevent water loss

Spiral or Curly leaves: 

• • Curled leaves increase the amount of surface area for capturing fog and dew. 

  • Leaves reduce the overall size of the leaf exposed to the air, which reduces transpiration

  • Curled leaf acts like a mini-gutter, channeling water to the base of the plant

"Window" Leaves

• Leaves of the plant are covered under the desert sand, with exception of a small apical portion of the leaves

• In the apical portion of the leaf, tightly-packed, translucent cells allow light to “shine” on mesophyll layer (under the ground)

• e.g. Fenestraria rhopalophylla in the Kalahari Desert

Spines

• Spines are sharp and pointed leaves modified for protection and other functions

• Spines formed from leaves, therefore they will have an axillary bud above, although this is difficult to see in the Cactaceae

• e.g. Cacti (Cactaceae) and Barberry (Berberis) exhibit spines

  • In the Cactaceae, the formation of spines is more complex than just the reduction of leaves; there is suppression of all leaf-cell genes and activation of genes that control the formation of wood fibers (Univ. Texas at Austin)

• Plants use spines as possible protection from large herbivores

• There is also evidence that they provide shade in the intense light of deserts and promote moisture/condensation (In Defense of Plants 2015)

Developmentally "Plastic" Leaves

• Leaves that exhibit different growth forms on the same plant depending on the micro-conditions when the leaf develops (e.g. sunny/hot, shady/cool)

• e.g. Oaks, Sassafras, and Mulberry (Morus) leaves exhibit developmental plasticity

• Some "plastic" leaves have an adaptive significance (see Oaks below), but others are less well understood (Sassafras & Morus)

Shade versus Sun Leaves on trees

• e.g. Oak (Fagus) leaves on the same tree exhibit different shapes based upon sun and shade conditions

Sun leaves

• Leaves exposed to high light levels are thicker, smaller blade surface area, and deeply lobed 

Shade leaves

• Leaves exposed to lower light levels are thinner, larger blade, and shallowly lobed