Answer Key

1. Physiological plant ecology is the study of the physiological basis of ecological behavior, representing a fusion of ecology and physiology. Its core principle is that all environmental factors affect the growth and distribution of organisms solely by affecting the rates of physiological processes within those organisms.
2. The models suggest that protein and DNA molecules are surrounded by hydration shells of lattice-ordered, “ice-like” water. The integrity of these shells is crucial for maintaining the active configuration of proteins and enabling the low-energy separation of DNA strands during replication, meaning a water deficiency can disrupt all related processes.
3. In water-stressed tomato leaves, DNA amounts remain constant, indicating that cell division ceases, unlike in control leaves where it continues. Similarly, water stress prevents a net increase in RNA, not by stopping its synthesis but by increasing its rate of destruction, as evidenced by increased RNase activity.
4. Cardinal temperatures refer to the minimum, optimum, and maximum temperatures for a specific plant function. These temperatures can vary for the same function based on the plant’s stage of development, its general physiological condition, the duration of the temperature exposure, and changes in other environmental factors.
5. The breaking of dormancy appears related to a change in the balance between growth inhibitors and stimulators. Low temperatures (chilling) cause a decrease in the amount of the inhibitor abscisic acid in seeds, while treatment with the growth stimulator gibberellic acid can also effectively break dormancy, often reversing the inhibitory effects of abscisic acid.
6. Shade-adapted plants of Solidago virgaurea reach their maximum photosynthetic rate at lower light intensities and are more efficient at photosynthesis in weak light. Sun-adapted plants, conversely, are more efficient at photosynthesis in strong light and only reach their saturation point at much higher light intensities.
7. Photoperiodism is the response of an organism to the relative lengths of light and dark periods. The four classifications for flowering response are: short-day plants (flower when daylength is below a critical maximum), long-day plants (flower when daylength is above a critical minimum), day-neutral plants (flowering is not determined by daylength), and intermediate-day plants (flower only between two critical daylengths).
8. Phytochrome is a pigment that acts as the timing mechanism for photoperiodism and exists in two forms: P_r (absorbs red light at 660 mμ) and P_fr (absorbs far-red light at 730 mμ). Sunlight converts P_r to P_fr, which is inhibitory to flowering in short-day plants; in darkness, P_fr slowly reverts to P_r, allowing the flowering process to proceed.
9. Salvia leucophylla produces volatile terpenes such as camphor and cineole. These chemicals inhibit respiration in other plants by reducing oxygen uptake in mitochondria, specifically interfering with the Krebs cycle where succinate is converted to fumarate or fumarate to malate.
10. Many low-nitrogen-requiring early invader species produce inhibitors that suppress nitrogen-fixing bacteria like Azotobacter and root-nodule bacteria. This gives the inhibitor-producing plants a competitive advantage over plants with higher nitrogen needs and likely slows the rate of succession.

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