Chapter 6
Plant Adaptations to the Environment

Links for Enrichment and Further Learning

• More details about features that help plants survive in arid environments
• Arid Agriculture

Student Learning Outcomes. Lessons in this chapter are intended to help students to:

Study Questions / Quiz Prep. (Consult Required Reading and lecture notes for answers.)

1. What is the ultimate source of carbon from which all life is constructed?
2. What is an autotroph?
3. What is a heterotroph?

Photosynthesis

4. Photosynthesis is responsible not only for the _____________________ we animals consume, but also for the ______________________ we breathe.
5. The initial product of photosynthesis is not a sugar. It is a 3-carbon molecule that can be chained together to make sugar and what other kinds of molecules?

Carbon Balance

6.  “Carbon balance” is an expression for comparing the relative activities of photosynthesis and respiration. How does the plant achieve a positive carbon balance? Negative carbon balance?
7. What happens to a plant operating at a continuously negative carbon balance?

Light

8. What is the carbon balance at light levels below the light compensation point? Above the light compensation point?
9. Can plants ever get too much light? If so, what is the result?

Stomata

10. What are leaf stomata? Leaf stomata control the flow of what?
11. As CO2 diffuses into the leaf through the stomata, _____________________ diffuses out through the same opening.
12. What balance must be achieved in the operation of the stomata?

Heat

13. As the environment warms up, the leaf warms up and photosynthesis increases… to a point. What happens when the leaf gets too hot?
14. How do leaves on terrestrial plants dissipate heat to the surrounding environment?
15. How can leaf shape make a difference in the efficiency of heat loss?
16. Does heat shape influence heat loss by convection, or heat loss by evaporation, or both?

Carbon Allocation

17. Leaf tissue, stem tissue, root tissue – what are they good for?
18. Increased allocations towards leaf tissue results in increases in___________________.
19. Increased allocations towards stem and root tissues results in increases in what?
20. Why not just allocate all carbon exclusively for leaf production?

High Light vs. Low Light

21. What are the photosynthetic tradeoffs made by shade-tolerant plants as compared to the tradeoffs made by shade-intolerant plants?

Diversity of Leaf Morphology

22.  Figure 6.11. What is the explanation for differences in leaf morphology for leaves from the top canopy compared to leaves from the bottom canopy?
23. Are such differences the result of evolutionary adaptation or physiological adaptation?

Dealing with Saltiness

24. From the presentation page: How do salt grass (Distichlis), and pickleweed (Salicornia) differ in their abilities to cope with high salt environments?

Water Demand and Temperature (NOTE: Disregard all discussions involving C₃ , C₄ and CAM biochemical pathways)

25. In tropical regions with distinct wet and dry seasons, some species of plants drop their leaves at the onset of the dry season. How might this be a rewarding behavior? What about the carbon balance?
26. Plants may respond to a decrease in available soil water by increasing the allocation of carbon to the production of __________________ while decreasing the allocation of carbon to ________________.
27. How might this be a rewarding allocation pattern?

More on the Topic of water Conservation in Hot and Arid Environments

28. Some plants have leaves covered with light hairs (white sage, jojoba). How might this be a rewarding feature for plants living in hot, dry environments?
29. Some plants have leaves coated with waxes and resins (monkeyflower, black sage). How might this feature be rewarding for plants living in hot, dry environments?

Coping with Extremely Cold Winters

30. Some plants use chemical techniques for frost hardening in response to cold weather. Can you think of any plants that do this?
31. Some plants avoid the high costs of frost hardening by doing what instead?
32. We have at least one Maple tree on the Fullerton College campus. Every winter it loses its leaves – as if it lived in cold Vermont. But the winters are mild here. Why does this tree still lose its leaves in winter?

Nutrients

33. Why is nitrogen such an influential nutrient in photosynthesis and overall leaf function?
34. What are some adaptations that allow some species of plants to be successful despite low nutrient soils?
35. Tropical rain forest soils often are nutrient poor. Many tropical rain forest trees have large shallow root systems. Why would such a system be more rewarding than a deeper system of roots?

Carbon Balance

Positive Carbon Balance	Negative Carbon Balance
Summer in Vermont	Winter in Vermont
Sonoran Desert. Spring following winter rains	Mojave Desert. Late summer after many months of drought.

Light

This graph shows photosynthetic activity (CO2 uptake, Y-axis) as a function of Light intensity (X-axis). Note that when there is no light, there is no photosynthetic activity, and CO2 uptake is negative (negative carbon balance). This means that more CO2 is being given off than taken in. As light increases, CO2 uptake eventually exceeds CO2 output (positive carbon balance). Eventually, despite increased light intensity, CO2 uptake levels off (saturation).

This graph compares photosynthetic activity (CO2 uptake) between Sun-Grown plants and Shade-Grown plants. Note that at low light levels, shade-grown plants outperform sun-grown plants. But at higher light levels, sun-grown plants outperform shade-grown plants.
Bromeliad. A plant that does very well in shady environments..	Sunflower. A plant that does very well in sunny environments.

Stomata

Drawing of cross-section of a stoma ("stomata" is plural).	A scanning electron micrograph of a stoma on the underside of a leaf.

Heat

African Savanna - tropical. Hot all year long. Rainy season, followed by months of drought.

Chaparral plant community - temperate. Cool winters with about 13 inches of rain, followed by months of drought and hot summers.

Coastal Sage Scrub plant community - temperate. Cool winters with about 13 inches of rain, followed by months of drought and hot summers.

Desert Scrub plant community, Mojave Desert - temperate. Cool/Cold winters with about 7 inches of rain, followed by months of drought and extremely hot summers. Yuccas continue to photosynthesize using chlorophyll-containing stems while the leaves of surrounding plants are mostly dried up and dead. - temperate

White Sage of the Coastal Sage Scrub plant community. Light colored leaves reflect incoming sunlight, thereby reducing the plant's heat uptake.

Monkey Flower of the Coastal Sage Scrub plant community. A thick waxy coating on the leaves helps reduce water loss in this arid environment.

Black Sage of the Coastal Sage Scrub plant community. A thick waxy coating on the leaves helps reduce water loss in this arid environment.

Jojoba of the Sonoran Desert. Light colored leaves reduce heat uptake. In addition, the vertical orientation of leaves helps reduce the amount of leaf surface exposed to the sun during the hottest part of the day when the sun is directly overhead.

Cardone cactus in Baja Mexico - Sonoran Desert. Light coloration and vertical orientation help reduce heat uptake. Tall growth places the bulk of the plant above the baking desert floor, and high enough for cooler breezes. Water storage inside the body gives the cactus more control over limited water resources.

Canyon Live Oak. Leaves have small spines around their margins. The spines increase the leave's Surface Area / Volume. This effect can help the leaf more quickly dump heat to passing, cooler air.

Does shape influence the surface area-to-volume ratio?

	Volume = 8x8x1 = 64 cubic units Surface Area = 2(8x8) + 4(8x1) = 160 square units Surface Area/Volume = 160/64 = 2.5
	Volume = 5x8x1 + 6x4x1 = 64 cubic units Surface Area = 64x2 + 70 = 198 Surface Area/Volume = 198/64 = 3.1

Motorcycle engine with "cooling fins" that substantially increase the engine's surface area-to-volume ratio.

Coping with Extremely Cold Winters

Boreal / conifer forest - pine and fir trees - temperate

Pine needles harden against frost and remain active during freezing conditions.

Vermont Maple forest - temperate. Mild summers, extremely cold winters.

Instead of frost hardening, maple trees shutdown chlorophyll production and withdraw sugars and nutrients from the leaf into the branch for safekeeping during freezing winters. Then the leaf falls of the tree.

Water-rich environments -- Forests

Tropical rain forest. Hot and bright sun all year. Rain most of the year - 100 inches or more. Nutrient poor soil.

Dipteryx in the Peruvian rain forest. Massive shallow roots efficiently acquire nutrients from rapidly decomposing fallen leaves.

Salty Environments

Coastal marsh. This marsh is flooded with seawater.

Desert salina. Extremely high salt content in soil.

Pickelweed (Salicornia), found in coastal marshes and in desert salinas. Pickelweed takes in salty water, diverts the salts to the tips where the salt concentrates. Eventually, the salt kills the tip, after which the tip breaks off, taking the salt with it.

Salt grass (Distichlis) - found in coastal marshes and in desert salinas. Salt grass takes in salty water then excretes excess salt to the outside of its blades.