Photo: Kochia, SW Montana. © 2020 Delena Norris-Tull.
The Potential Impacts of Climate Change on Plants with C4 Photosynthesis
Research summary and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2021.
Ziska and Bunce (1997) conducted research on plant species that possess the C4 type of photosynthesis, to determine the potential impact on photosynthesis and biomass production of increased carbon dioxide levels in the atmosphere.
Previous research on the much more common C3 pathway species (about 95% of all plant species) has suggested that increases in carbon dioxide levels “stimulate net photosynthetic increases in C3 plants by increasing the CO2 concentration gradient from air to the leaf interior and by decreasing the loss of CO2 via photorespiration.” Research has shown that C3 species, when exposed to elevated carbon dioxide levels, show an increase in their photosynthesis and/or biomass production.
Previously, it was thought that increased CO2 levels would not have a similar effect on C4 plants. Because of differences in the photosynthetic pathway, it was assumed that C4 plants should be saturated at current atmospheric CO2 levels.
My interview with George Beck, at Colorado States University, pointed me to research by Lewis Ziska. Dr. Beck pointed out that, “Lewis Ziska, with the NRCS, did research that showed that when carbon dioxide levels increase, as we are seeing occur now with climate change, the C3 and C4 plants respond more rapidly. In other words, the weeds grow faster when CO2 levels increase.” Dr.Beck commented, “Also refer to research by Harold Mooney on yellow star thistle & CO2 levels. The brush species are expanding as CO2 levels are increasing. Alaska has not had concerns about weeds in the past, but are now starting to see problems with knapweed and sweet clover.”
Ziska and Bunce (1997) found that, of six weedy species and four crop species they tested, eight of these C4 species (all six weed species and only two crop species) showed a significant increase in photosynthesis. The “weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO2 concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C4 weeds,” but not in any of the crop species. Their research challenged the previous hypothesis that the cause of increased photosynthesis and biomass may be stomatal closure, which theoretically would result “in an increase in water use efficiency and improved water relations.” They found that “elevated CO2 did not result in an improved water potential for any species.” They also stated that “the exact mechanism (for increased photosynthesis or biomass) is unclear.”
Ziska and Bunce concluded that, in the C4 species they tested, “photosynthesis or biomass can increase by 40-60% with a doubling of CO2 concentration. Such increases are on a par with the observed stimulation of photosynthesis and growth in some C3 species.” They concluded their summary by stating that, “It is generally acknowledged that while the C4 pathway occurs in relatively few plants, C4 species are disproportionally over-represented in lists of major weeds. For example, of the 76 ‘worst’ weeds, 42% are C4 plants.” “It is unfortunate that, to date, no studies of the relative yield response at elevated CO2 of either C3 or C4 crops in competition with C4 weeds have been reported under field conditions.”
In addition, elevated CO2 levels can impact seed “germination, flowering times, pollen output, seed yield, and the onset of senescence” (Ziska & Runion, 2007, page 263).
To date, research on the impact of increased temperatures on plant growth are inconclusive. Some plants increase productivity with increased temperatures and other plants have decreased productivity.
“No studies are available on the interactions among drought, rising CO2, and weed-crop competition” (Ziska & Runion, 2007). But as temperatures increase, we can expect weeds that previously did not grow at high latitudes and altitudes, because their expansion has been limited by low winter temperatures, to move northward and upward. Kudzu, previously only seen in the southeastern U.S., has already expanded its range as far north as Chicago and Massachusetts.
Ziska and Runion (page 278) conclude with, “It is remarkable, given the importance of weeds, insects, and diseases to crop production and food security, that so few experimental data are available assessing the impact of rising atmospheric CO2 or rapid climatic change on their biology.”
Treharne (1989) found that weedy species often have a greater genetic diversity and therefore greater physiological plasticity when compared with crops.
Hager, Ryan, Kovacs, & Newman (2016) challenged the results of this earlier research. In closed chamber experiments, they compared the growth responses of eight C3 and seven C4 grasses, both invasive and native species, to varying increases in CO2 levels. They concluded that, “Differences in trait means between invasive and noninvasive species tended to be similar across CO2 levels for many of the measured responses. However, noninvasive C3 grasses were more responsive than invasive C3 grasses in increasing tiller number and root biomass with elevated CO2, whereas noninvasive C4 grasses were more responsive than invasive C4 grasses in increasing shoot and root biomass with elevated CO2. For C3 grasses, these differences could be disadvantageous for noninvasive species under light competition, whereas for C4 grasses, noninvasive species may become better competitors with invasive species under increasing CO2.”
Given the propensity of weedy species for differential responses to climate change and elevated CO2 levels, when compared to crop species, the future of weed management remains unpredicted and unpredictable.
I pose a question that deserves careful consideration. Will some of today’s “weeds” become tomorrow’s agricultural products?
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The Potential Impacts of Climate Change on Plants with C4 Photosynthesis
Research summary and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2021.
Ziska and Bunce (1997) conducted research on plant species that possess the C4 type of photosynthesis, to determine the potential impact on photosynthesis and biomass production of increased carbon dioxide levels in the atmosphere.
Previous research on the much more common C3 pathway species (about 95% of all plant species) has suggested that increases in carbon dioxide levels “stimulate net photosynthetic increases in C3 plants by increasing the CO2 concentration gradient from air to the leaf interior and by decreasing the loss of CO2 via photorespiration.” Research has shown that C3 species, when exposed to elevated carbon dioxide levels, show an increase in their photosynthesis and/or biomass production.
Previously, it was thought that increased CO2 levels would not have a similar effect on C4 plants. Because of differences in the photosynthetic pathway, it was assumed that C4 plants should be saturated at current atmospheric CO2 levels.
My interview with George Beck, at Colorado States University, pointed me to research by Lewis Ziska. Dr. Beck pointed out that, “Lewis Ziska, with the NRCS, did research that showed that when carbon dioxide levels increase, as we are seeing occur now with climate change, the C3 and C4 plants respond more rapidly. In other words, the weeds grow faster when CO2 levels increase.” Dr.Beck commented, “Also refer to research by Harold Mooney on yellow star thistle & CO2 levels. The brush species are expanding as CO2 levels are increasing. Alaska has not had concerns about weeds in the past, but are now starting to see problems with knapweed and sweet clover.”
Ziska and Bunce (1997) found that, of six weedy species and four crop species they tested, eight of these C4 species (all six weed species and only two crop species) showed a significant increase in photosynthesis. The “weed species (+19%) showed approximately twice the degree of photosynthetic stimulation as that of crop species (+10%) at the higher CO2 concentration. Elevated carbon dioxide also resulted in significant increases in whole plant biomass for four C4 weeds,” but not in any of the crop species. Their research challenged the previous hypothesis that the cause of increased photosynthesis and biomass may be stomatal closure, which theoretically would result “in an increase in water use efficiency and improved water relations.” They found that “elevated CO2 did not result in an improved water potential for any species.” They also stated that “the exact mechanism (for increased photosynthesis or biomass) is unclear.”
Ziska and Bunce concluded that, in the C4 species they tested, “photosynthesis or biomass can increase by 40-60% with a doubling of CO2 concentration. Such increases are on a par with the observed stimulation of photosynthesis and growth in some C3 species.” They concluded their summary by stating that, “It is generally acknowledged that while the C4 pathway occurs in relatively few plants, C4 species are disproportionally over-represented in lists of major weeds. For example, of the 76 ‘worst’ weeds, 42% are C4 plants.” “It is unfortunate that, to date, no studies of the relative yield response at elevated CO2 of either C3 or C4 crops in competition with C4 weeds have been reported under field conditions.”
In addition, elevated CO2 levels can impact seed “germination, flowering times, pollen output, seed yield, and the onset of senescence” (Ziska & Runion, 2007, page 263).
To date, research on the impact of increased temperatures on plant growth are inconclusive. Some plants increase productivity with increased temperatures and other plants have decreased productivity.
“No studies are available on the interactions among drought, rising CO2, and weed-crop competition” (Ziska & Runion, 2007). But as temperatures increase, we can expect weeds that previously did not grow at high latitudes and altitudes, because their expansion has been limited by low winter temperatures, to move northward and upward. Kudzu, previously only seen in the southeastern U.S., has already expanded its range as far north as Chicago and Massachusetts.
Ziska and Runion (page 278) conclude with, “It is remarkable, given the importance of weeds, insects, and diseases to crop production and food security, that so few experimental data are available assessing the impact of rising atmospheric CO2 or rapid climatic change on their biology.”
Treharne (1989) found that weedy species often have a greater genetic diversity and therefore greater physiological plasticity when compared with crops.
Hager, Ryan, Kovacs, & Newman (2016) challenged the results of this earlier research. In closed chamber experiments, they compared the growth responses of eight C3 and seven C4 grasses, both invasive and native species, to varying increases in CO2 levels. They concluded that, “Differences in trait means between invasive and noninvasive species tended to be similar across CO2 levels for many of the measured responses. However, noninvasive C3 grasses were more responsive than invasive C3 grasses in increasing tiller number and root biomass with elevated CO2, whereas noninvasive C4 grasses were more responsive than invasive C4 grasses in increasing shoot and root biomass with elevated CO2. For C3 grasses, these differences could be disadvantageous for noninvasive species under light competition, whereas for C4 grasses, noninvasive species may become better competitors with invasive species under increasing CO2.”
Given the propensity of weedy species for differential responses to climate change and elevated CO2 levels, when compared to crop species, the future of weed management remains unpredicted and unpredictable.
I pose a question that deserves careful consideration. Will some of today’s “weeds” become tomorrow’s agricultural products?
Previous Sections: Next Sections: