Science Practice Challenge Questions

23.1 The Plant Body


A. The net photosynthetic production rate (NPP) is the difference between the rate of carbon fixation by photosynthesis (P) and the respiration rate (R). Each of these rates can be expressed in units of grams of carbon per day (gC/d). Vascular plants convert fixed carbon that is not released as carbon dioxide into biomass with a growth rate (G).

Draw areas within the box to represent the rates of growth (G) and respiration (R) to show the limit of each on the overall growth rate. The area of the box represents the rate of photosynthesis (P).

An empty rectangle is shown. Text above the rectangle reads P is proportional to total area.
Figure 23.48

B. When the dependences on temperature of photosynthetic and respiration rates of a vascular plant are measured, the results depend on the species but have the general form shown in the figure. In these measurements, the temperature is maintained for several hours. The plant is then returned to 25 °C for several hours before the next set of measurements is made at a slightly higher temperature.

This is a chart titled How Photosynthesis and Respiration Rates Vary with Temperature in a Vascular Plant. The chart shows Leaf temperature in degrees Celsius from 0 to 40 in increments of 5 on the x-axis and progress rate in g C per day from 0 to 2.5 in increments of 0.25 on the y-axis. The rate of photosynthesis and rate of respiration are graphed. The values for rate of photosynthesis were as follows: leaf temperature 10 and process rate of 2 to 2.15; leaf temperature 15 and process rate of 2.15 to 2.3
Figure 23.49

Evaluate these data to approximately predict the quantitative effect on the NPP and free energy availability in a deciduous forest ecosystem with a 3–5-°C increase in temperature. This is the expected temperature increase by the year 2100. Assume the current average summer temperature of the forest ecosystem is 25 °C.

C. In other experiments, rather than returning the plants to 25 °C, the plant is grown for several days at a constant higher temperature. Under these conditions, the maximum photosynthetic rate shifts towards the temperature of the new growing conditions. However, there is little change in the temperature dependence of respiration rate. This is referred to as temperature acclimation, an effect of great importance to predictions of future climate change.

Pose two scientific questions whose pursuit could lead to either an improved understanding of the mechanisms of temperature acclimation or improvements in models of atmospheric carbon dioxide concentrations that control temperature.

According to the graph, growth is predicted to increase when acclimation is taken into account and the average temperature increases of Earth’s surface increases by the expected 3-5°C. Growth enhancement may be reduced, however, if respiration increases more rapidly than photosynthesis, particularly under periods of drought and stress. Thus, climate warming may result in positive, negative, or potentially no effect on the free energy availability in forest ecosystems.

D. In the figure below, the response to temperate change in terms of the rates of photosynthesis and respiration are sketched as a function of time from the very short-term (seconds) to the longer-term (decades) changes. Acclimation in the laboratory occurs in days. Analyze the graphs; in the box bounded by a dashed line, sketch curves for responses of both processes beyond the acclimation observed in the laboratory that are consistent with a neutral effect on free energy availability and provide your reasoning.

This is a figure called Response Rates of Photosynthesis and Respiration to Changes in Temperature showing a time scale for response after change in temperature from seconds to decades on the x-axis and with rate of response from slower to faster on the y-axis. Photosynthetic response and respiration response are shown using two separate lines. The line showing photosynthesis starts about one-third of the way up from the bottom, slower side of the graph. It peaks at minutes, reaching about midway between
Figure 23.50

E. Analyze the long-term effect of a rate of respiration that exceeds the rate of photosynthesis in terms of dynamic homeostasis.

23.5 Transport of Water and Solutes in Plants


A disruption of dynamic homeostasis in the relationship between vascular plants and insects is occurring as global climate changes. The reduction in the yield of soybeans is plotted against leaf area removed by two insects, beetles and aphids. Soybean blooms begin to develop in the week of 13 July. Prior to that time, there is no effect of leaf removal on yield, even with complete loss of leaves. In the week of 18 August, plants are beginning to form seeds, and loss of leaves can be devastating.

This is a graph titled, Reduction in Yield of Soybeans Plotted Against Leaf Area Removed by Two Insects. The x-axis is labeled, Percent leaf area removed, and ranges from 0 to 100. The y-axis is labeled, Percent reduction in seed yield, and ranges from 0 to 80. Six lines representing values for different date ranges are plotted. A line indicating 9 July to 12 July runs from 10 percent leaf area removed to 100 percent leaf area removed in a straight line showing 0 percent reduction in seed yield from left
Figure 23.51

A. One observed effect of climate change is the shift toward earlier development in many insects. Quantitatively describe the worst possible consequences for yield, assuming plant developmental timing is not altered by warming temperatures, if the peak abundance of Japanese beetles is shifted from 18 July to 13 July, and 80% of leaf area is lost.

B. The expression of genes involved in seed development is temperature dependent, unlike the scenario suggested in part A. More than 90% of soybean seeds planted in 2015 in the soybean-corn ecosystem of the central United States are the herbicide-resistant, genetically modified “Roundup Ready” variety. The seed has a patented genome. It produces seeds that are sterile and must be purchased each spring from the patent holder.

Predict how the use of Roundup Ready seeds affects the selection of expression regulated in response to increasing temperature.

Roundup is an herbicide whose active chemical component is glyphosphate. This molecule disrupts the synthesis of phenylalanine, tyrosine, and tryptophan. By inserting a gene from Agrobacteria, a Roundup Ready seed can synthesize these amino acids in the presence of the herbicide.

C. Pose two scientific questions that must be considered to estimate the long-term effectiveness of this strategy for weed management.


A. By increasing the photosynthetic surface area, a plant increases the rate of capture of free energy. For every carbon atom fixed into carbohydrates, between 200 and 400 water molecules are released through stomata to the atmosphere. A simple geometric model can be used to estimate the minimum number of leaves on a tree, as shown.

This is a figure showing a leaf on the left and a tree on the right. The leaf is positioned with the main vein running through the middle from left to right. There is a double-headed horizontal arrow running along the top of the leaf on the left and labeled 2 L. There is a double-headed vertical arrow running along the left side of the leaf and labeled 2 W. An equation written below the leaf reads: Area of the leaf is approximately one-eighth times 2 L times 2 W. There is a double-headed horizontal arrow
Figure 23.52

Identify and justify the data needed to describe the relationship between the free energy captured and the water transpired by a tree with dimensions D, L, and W. Use these data to construct a mathematical model of the relationship between transpiration rate and the rate of free energy captured when a single carbon atom is fixed.

B. The diversity of vascular plants decreases with increasing latitude. Equatorial ecosystems have greater plant diversity than do ecosystems further south or north. One of several explanations offered to account for this observation is the energy-equivalence model—as free energy increases, population size increases. As population size increases, mutations increase. One bit of evidence for the energy-equivalence model is the correlation of family-level diversity with actual evapotranspiration, the sum of water transferred by both transpiration and evaporation of surface water. This property is reported in mm of water per square meter of surface area.

C. Explain the relationship between free energy exchange and latitude that is the basis of the energy-equivalence model.

Shared ancestry is indicated by taxonomic classification in which a family of organisms contains many genera, and within each genus there are many species. A survey of tree flora (Latham and Ricklefs, Oikos, 67, 1993) at comparable latitudes in a temperate eastern Asia forest ecosystem (729 species in 177 genera and 67 families) and an eastern North America forest ecosystem (253 species in 90 genera and 46 families) had no species in common, but there were 20 common genera and 40 common families. Actual evapotranspiration for the two ecosystems are 850 ± 200 mm (eastern North America) and 730 ± 160 mm (eastern Asia).

C. Analyze these data to test the validity of the energy-equivalence model.


The evolution of vascular plants followed the colonization of terrestrial habitats by ancestors of Chlorophyta, green algae, during the Devonian period (which began about 400 million years ago). The three most significant structural innovations in that process are responses to selection through the availability of water resources: 1) the cuticle, a waxy covering of the epidermis that retains water; 2) stomata, openings that penetrate the cuticle through which water and carbon dioxide are transported; and 3) a vascular system, plant tissues through which water moves.

Measurements of gases trapped in ice cores provide atmospheric concentrations of the distant, as well as the recent, past. Life must adapt to changes in the environment. Woodward examined samples from the Cambridge herbarium of several trees (Nature, 327, 1987) to determine the stomatal index (percentage of epidermal cells that contain a stoma). In 1720, when the herbarium samples were collected, the carbon dioxide concentration in Earth’s atmosphere was 225 ppm. In the year of the study, 1987, it was 340 ppm (it is 370 ppm in 2016). The following table presents some of Woodward’s reported results.

Tree Genus CO2 (ppm) Stomatal Index (%)
Acer 225 14.9 ± 0.8
  340 6.7 ± 1.1
Quercus 225 17.4 ± 1.1
  340 9.6 ± 1
Rumex 225 15.5 ± 0.7
  340 11.8 ± 0.9
Table 23.1

Teng and co-workers (PLoS ONE, 2009) followed the dependence of Arabidopsis, a member of the Brassica family of vascular plants, grown under a range of elevated CO2 concentrations for 15 generations. They found elevated stomatal densities for each generation that were not heritable.

Engineer and co-workers (Nature, 513, 2014) discovered a mutant Arabidopsis in which stomatal density increases as CO2 concentration increases. Measurements of a component of the set of mRNA molecules for epidermal patterning factor 2 (EPF2), responsible for stomatal density, are shown for plants grown in low and high CO2 concentrations.

The bar graph is titled Measurements of E P F 2 for Plants Grown in Low and High Carbon Dioxide Concentrations. The x-axis shows wild-type plants on the left and mutant plants on the right. The y-axis shows E P F 2 m R N A level. The E P F 2 m R N A level for each type of plant is shown for low carbon dioxide level 150 p p m and high carbon dioxide level 500 p p m. For wild-type plants, the bar for low carbon dioxide level is slightly over half the height of the bar for high carbon dioxide level. For muta
Figure 23.53

A. Analyze these data in terms of the likelihood that the effect of carbon dioxide concentration on stomatal density involves negative feedback at the level of i) translation, ii) post-transcription, or iii) changes in genotype.

B. Changes in precipitation patterns are expected to accompany an increase in atmospheric carbon dioxide. Predict the effect on a forest where trees that have matured over decades are suddenly under drought stress. Justify your prediction in terms of positive or negative feedback where stomatal density is high and a drought occurs.

C. In a favorable environment, trees continue to accumulate biomass and increase in height until the flow rate of water through the xylem (plant vascular tissue that transports water and minerals from roots to shoots) is no longer sufficient to support the negative water potential at the interface between root and soil. Fluid dynamic models predict that increasing the diameter, d, of the xylem greatly increases the rate of flow of water, leading to greater productivity when water is abundant. Under conditions of drought stress, the water potential is reduced, and an air bubble can disrupt the flow of water in that vessel entirely. A larger stem diameter permits a larger number of small vessels.

Describe a model of the evolution of xylem in trees in terms of selection under conditions of unlimited and limited water resources.

D. Olson and Rosell (New Phytologist, 197, 2013) investigated the question of whether xylem diameter was determined by water availability or by plant height and, consequently, stem diameter. A summary of their data is shown on the left with lines of best fit through data with the corresponding color.

The graph is titled Xylem Diameter in Relation to Stem Diameter. The x-axis is labeled Log stem diameter in centimeters and ranges from negative 0.5 to 2.5 in increments of 0.25. The y-axis is labeled Log xylem diameter in micrometers and ranges from 1.0 to 2.5 in increments of 0.25. Five lines indicate dry savanna 1513 m m, tropical rain forest 3454 m m, tropical dry forest 794 m m, tropical rain forest 2091 m m, and temperate forest 807 m m. Dry savanna 1513 mm has points around a straight line running
Figure 23.54

Analyze these data and summarize the pattern that addresses their question. Note: As x increases, log(x) increases.

23.6 Plant Sensory Systems and Responses


Like the animal intestine, the organ system principally responsible for nutrient and water uptake, the plant root system, is home to a microbiome upon which the host depends. One important role for the root microbiome is innate immunity. Wheat take-all is a disease caused by the fungus Gaeumannomyces graminis that attacks plant roots and blocks root water channels. When a major outbreak occurs in a wheat field, susceptibility remains high in the following year. But after four to six continued crops of wheat in the same field, susceptibility to the disease declines. This resistance can be transferred with the soil. Burning the soil surface or rotation with another crop returns susceptibility to the next wheat crop. The Fusarium (a fungus) wilt disease of strawberries and potato scab caused by Streptomyces scabies (a bacteria) show a similar disease progression and transferability of resistance (Weller, Ann. Rev. Plant Phytopath, 26, 1988).

A. Plants, like animals, have immune defenses that may involve cooperative interactions between organisms. Describe a model of immune response that accounts for these behaviors.

In plants, the first line of defense is the cell wall. Animal cells lack this protective barrier. Adaptive immunity of vertebrates to pathogens uses specific defenses that are transportable within the organism, such as T-cells, and retains information about earlier infections, such as T-cell receptors. Unlike adaptive immunity, the innate responses of plants are much less effective in defending against neurotrophic (colonizing dead tissue) than against biotrophic (infecting living tissue) pathogens. In animal tissue, the response to infection is inflammation, the recruitment of resources to protect the tissue. In plant tissue, the response is apoptosis.

B. Describe contrasting models of defense strategies for plants and animals that express each of these differences in terms of these strategies: cell boundary, immunological memory, and tissue repair.