BIL 161 – Plant Form and Function Designing a Research Project

In almost all terrestrial environments, water is a limiting factor that affects the function of living organisms. All terrestrial organisms, from animals to fungi to plants, and even some unicellular organisms, exhibit various adaptations (or lack thereof) that allow them to inhabit habitats where water may be somewhat or severely limiting to their daily functions and reproduction.

Because your research project will focus on plants, here we provide a bit of background that might inspire you.

I. Water Conservation: Variation Among Species Plants exhibit tremendous variety of form and function due to natural selection pressures exerted by their environments. They have evolved different morphologies and features due to selective pressures exerted by pollinators, herbivores, climate and other environmental factors. The climate in which a plant evolves has a profound effect on its characteristics affecting water transport.

A. Modifications to Epidermis: Plant “Skin” Plant epidermis is analogous to your own skin. It is the plant body’s first line of defense against pathogens and desiccation. However, its anatomy is quite different from your skin’s. Plant epidermis • is one cell-layer thick (in most species) • consists of cells containing no choroplasts • is covered by a non-cellular, waxy cuticle secreted by the epidermal cells • may contain specialized structures such as • stomates (gas exchange structures bordered by guard cells that open and close the stomate) • trichomes (hairlike projections that can be modified to serve any number of functions)

Many adaptations for water conservation in plants can be observed at the level of the epidermis. Consider epidermal adaptations if your team decides to compare two different plant species.

B. Modifications to Plant Organs: Root, Stem, Leaf Plants are sometimes characterized on the basis of the anatomical features they have evolved in response to selection exerted by water availability. Any given plant species can be considered a mesophyte, a xerophyte, or a hydrophyte, depending on these adaptations.

1. Mesophytes A mesophyte (Figure 1) is a plant evolved to survive best in moderate conditions--neither extremely dry nor extremely wet. Most of the ornamental plants around campus are mesophytes.

Even mesophytes may show some adaptations to conserve water (e.g., closeable stomates) or to avoid damage caused by too much water (e.g., leaves with rain-shedding “drip tips”).

Figure 1. Some of the most familiar ornamental plants can be categorized as mesophytes. Shown here are the Succulent Lupine (Lupins succulentus), maple trees (Acer sp.), and common lilac (Syringa vulgaris).

2. Xerophytes A xerophyte (Figure 2) is a plant evolved to withstand extremely dry environmental conditions. Xerophytes with fleshy, water-conserving leaves are often called succulents, but there are many other characteristics that can help a plant conserve water.

Figure 2. Xerophytes. Some water-conservation adaptations include fleshy, water- storing organs (left and center) and pubescence (right).

Water conservation adaptations common in xerophytes include (but are not limited to): • Roots, stems, or leaves modified to store water • Reduced leaf size/surface area • Gas exchange structures (lenticels) on stems, instead of leaves • Modified or recessed stomates • Stomates that open only at night (C4 and CAM metabolism plants) • Thickened waxy cuticle • Reduced leaf surface area • Pubescence (fuzziness)

The many species of cacti are perhaps the most familiar xerophytes, but many other species, including those native to humid southern Florida, may exhibit anatomical features that allow them to avoid desiccation. (We do have some relatively dry habitats, such as the rapidly vanishing Pine Rocklands, and . we also have a dry season from ~ December – May).

3. Hydrophytes A hydrophyte (Figure 3) is a plant evolved to live in very wet conditions.

Figure 3. Some famliar hydrophytes. Water lilies (left) have air-filled leaves and long petioles to keep photosynthetic tissues in sunlight. Water hyacinths, native to Africa and one of the world’s most invasive exotic hydrophytes, has air-filled tissues that keep the entire plant afloat. Carnivorous plants (right) have evolved modified leaves to capture proteinaceous prey, since nitrogen is a severely limiting factor in acidic, boggy soils.

Although water is necessary for life, too much of a good thing can drive natural selection to modify anatomy accordingly. Some adaptations of hydrophytes include • Reduced waxy cuticle • Very rapid transpiration rate • Air spaces in the parenchyma and other traits that promote flotation • Stomates located on the upper, rather than the lower, leaf surface • Reduced root system • Expanded leaf surface area • Elongated petioles • Poorly developed xylem • Little or no lignin (a major component of wood)

Water lilies (there are many species) are a familiar example. Southern Florida, with its humidity and extensive wetlands, is home to many native hydrophytes such as those you can read about here:

http://plants.ifas.ufl.edu/manage/why-manage-plants/aquatic-and-wetland-plants-in-florida/

4. Research Project: Comparing Different Species of Plants? As your team discusses the results of your literature searches, consider that various species around us, even on the UM campus, have different adaptations related to water movement and conservation. How might these species be affected by climate change, either here in Florida or in their native ranges? Which species might be best adapted to “handle” rapid changes in water availabiliy?

The logical, interesting questions you might ask are limited only by your imagination.

II. Water Conservation: Variation Within a Species When most people think of plants, they imagine organisms that are stuck in the ground and not doing anything interesting. Nothing could be further from the truth. Plants may not move around of their own volition, but their sessile existence has necessitated evolutionary responses very different from those of animals that can simply get up and move to a more hospitable location.

Under natural conditions, plants are subject to many factors that can change their metabolic and/or transpiration rate. These include (but are not limited to • Air movement • light level • temperature • humidity ...to name only a few.

A. Comparing Multiple Individuals Within a Species Evolution cannot occur without heritable variation in a population. An investigator may wish to determine whether there is sufficient variation within a population (or species) that will allow it to respond to new evolutionary pressures without going extinct. But the first step in such a study would be to establish that there is heritable variation upon which natural selection can act.

Your team might consider a pilot study on a given species that might be threatened by imminent changes to its habitat. Be aware that you will need a large enough population from which to harvest leaf samples without harming the population.

B. Comparing One Individual Under Different Environmental Conditions Plants may not move quickly, but they do change in response to environmental conditions. In response to external cues, plants can modify such amazing feats as • Opening or closing their stomates • Expanding or folding their leaves or flowers • Producing nectar or other products • Flowering at the appropriate time of year • Modifying their own infrastructure over time • Shedding and re-growing leaves or other structures ...to name only a few.

Not all of the above are directly related to water movement. But your team might wish to explore the adaptations of a single individual plant in terms of its responses to short-term environmental changes. As above, you will need to select a large, healthy plant from which you can harvest leaf samples without severely damaging or killing the plant (unless you want to buy one yourself to torture and maim without repercussions).

C. Correlating Physical Characteristics With Transpiration Rate Whatever question your team decides to address, you will need to carefully document all the anatomical features of the organisms in the two groups being compared. You have already learned to create a stomate cast and to calculate leaf surface area. Any other characteristics that might account for differences in transpiration rate between your two groups must also be characterized and quantified.