C₄ plant biology

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Higher temperatures are not necessarily ideal for yield, even if the temperatures are below a plants' optimal temperature. At elevated temperatures, plants grow faster which tends to, one, reduce the amount of the time for photosynthesis and growth, resulting in smaller plants, and two, reduce the time for grain fill, reducing yield, particularly if nighttime temperatures are high Hattfield et al.

High temperatures can also reduce pollen viability, be lethal to pollen.

Photorespiration (article) | Photosynthesis | Khan Academy

The multiple effects of high temperatures on plant physiological process and soil moisture likely explain why research has found that grain development and yield are often reduced when temperatures are elevated Hattfield et al. Many factors that are projected to change with climate change could influence plant growth. These include carbon dioxide concentration, temperature, precipitation and soil moisture, and ozone concentrations in the lower atmosphere. Check Your Understanding How will multiple climate change factors that are projected to change together such as temperature, carbon dioxide concentration, and soil moisture availability likely to differ influence crop plant growth and yields?

Photorespiration

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Your Account. Learn More. These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs. These include carbon dioxide concentration, temperature, precipitation and soil moisture, and ozone concentrations in the lower atmosphere.

Check Your Understanding How will multiple climate change factors that are projected to change together such as temperature, carbon dioxide concentration, and soil moisture availability likely to differ influence crop plant growth and yields? We encourage the reuse and dissemination of the material on this site for noncommercial purposes as long as attribution to the original material on the InTeGrate site is retained.

Material is offered under a Creative Commons license unless otherwise noted below. Disclaimer: Any opinions, findings, conclusions or recommendations expressed in this website are those of the author s and do not necessarily reflect the views of the National Science Foundation. Your Account. Learn More. These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.

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2.4: C₄ Pathway

Learn more about using, modifying, and sharing InTeGrate teaching materials. Although atmospheric carbon dioxide CO 2 levels are currently rising, the last 30 million years witnessed great declines in CO 2 , which has limited the efficiency of photosynthesis. Rubisco, the critical photosynthetic enzyme that catalyses the fixation of CO 2 into carbohydrate, also reacts with oxygen when CO 2 levels are low and temperatures are high.

When this occurs, plants activate a process known as photorespiration, an energetically expensive set of reactions that—importantly for this story—release one molecule of CO 2. C 4 photosynthesis is a clever solution to the problem of low atmospheric CO 2. It is an internal plant carbon-concentrating mechanism that largely eliminates photorespiration: a 'fuel-injection' system for the photosynthetic engine.

C 4 plants differ from plants with the more typical 'C 3 ' photosynthesis because they restrict Rubisco activity to an inner compartment, typically the bundle sheath, with atmospheric CO 2 being fixed into a 4-carbon acid in the outer mesophyll. This molecule then travels to the bundle sheath, where it is broken down again, bathing Rubisco in CO 2 and limiting the costly process of photorespiration. The evolution of the C 4 pathway requires many changes.

These include the recruitment of multiple enzymes into new biochemical functions, massive shifts in the spatial distribution of proteins and organelles, and a set of anatomical modifications to cell size and structure. A key intermediate step in the evolution of C 4 is the establishment of a rudimentary carbon-concentrating mechanism.

Termed 'C 2 photosynthesis', this mechanism limits certain reactions of the photorespiratory cycle to the bundle sheath cells. A byproduct of these reactions is CO 2 , creating a slightly elevated CO 2 concentration and increasing Rubisco efficiency in these cells.

What Are C4 Plants - Photosynthesis in Plants - Biology Class 12

Though much rarer than C 4 plants, C 2 plants have been discovered in a variety of C 4 -evolving lineages, and are thought to represent a common, if not requisite, intermediate step along the C 4 trajectory Sage et al. One implication of a restricted photorespiratory cycle is the development of a severe nitrogen imbalance between the mesophyll and the bundle sheath cells.

This occurs because every molecule of CO 2 produced in the bundle sheath is accompanied by a molecule of ammonia.


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While this nitrogen imbalance has previously been recognised Monson and Rawsthorne, , it has never been closely studied, and certainly never considered as potentially important to the evolutionary assembly of the C 4 pathway. To investigate this, Mallmann, Heckmann et al. They then modelled the various biochemical pathways that could potentially be induced to balance metabolic fluxes between the mesophyll and bundle sheath cells. This creative combination of models allowed them to evaluate the various metabolic pathways for re-balancing nitrogen in terms of which pathways resulted in the highest biomass yield a proxy for fitness.

Remarkably, when low levels of C 4 enzyme activity are permitted in the model, key elements of the C 4 cycle are favoured as the nitrogen-balancing pathway. What's more, this model predicts that with a C 4 cycle established, increasing the activity of the enzymes results in a linear increase in biomass yield. Allowing for low levels of C 4 enzyme activity is biologically reasonable, as these enzymes are routinely present in C 3 leaves.

Mallmann, Heckmann et al. In other words, once a C 2 cycle is established, the evolution of a fully realized C 4 process is fairly trivial. Once C 4 enzymes are recruited to shuttle nitrogen back to the mesophyll, it is all but inevitable. This can explain in part why C 4 has evolved such a startling number of times, and why many of these origins are highly clustered across the tree of life.

Many C 4 evolutionary clusters likely share an ancestor that had already acquired an elevated likelihood of evolving the pathway Figure 1.

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Many intermediate stages along the evolutionary trajectory from C 3 to C 4 are well known Sage et al. These can be displayed as part of an adaptive fitness landscape, which links biological properties horizontal axis with the fitness they produce right vertical axis; a greater height indicates a greater fitness. The adaptive fitness landscape of the C 4 trajectory was recently modelled as 'Mt.

Fuji-like': a steep linear incline with each step along the trajectory bringing small, incremental increases in fitness Heckmann et al.


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