Clippings Galore

Every week, I make a point of visiting the library or searching online for research I can pull into what frequently becomes my "Greening Up" column for the World Climate Report Online. What I am looking for, of course, is further evidence of the impact rising atmospheric concentrations of carbon dioxide will have on plant life. Some of the research lends itself to a single article it is so impressive. At other times, I am able to group research into a theme about a particular family of plants – grasses, for example. I’ve mowed my way through the stacks again and what I have this time are clippings galore.

Tischler et al. grew bagpod, cotton, mesquite, cucumber, and hemp seedlings in glasshouses with atmospheric CO2 concentrations maintained at 365 ppm (which is the ambient concentration) and nearly double that at 700 ppm. The elevated CO2 increased total biomass in the five species by 11 percent to 36 percent within just three days of emergence. By the sixteenth day of growth, doubled CO2 increased total biomass by 56 percent to 82 percent.

Bioscientists at the UK’s University of Nottingham raised potatoes in open-top chambers with atmospheric CO2 concentrations maintained at three different concentrations: 350 ppm, approximately fifty percent greater at 550 ppm, and not quite doubled at 680 ppm. Donnelly et al. went a step further and exposed the plants to varying concentrations of potentially harmful ozone (O3). What they determined from their research is that the elevated CO2 increases photosynthetic rates, decreases stomatal conductance with the effect of making the plants more water-use efficient, and increases tuber yield at crop maturity by 40 percent. If that weren’t sufficiently good news, the team reports that the higher levels of atmospheric CO2 "greatly reduced the damaging impact of O3 on chlorophyll content and visible foliar damage."

Dag and Eisikowitch grew melons in a greenhouse in southern Israel with some plants exposed to ambient levels of atmospheric CO2 while others had the CO2 concentrations set at 1000 ppm in the morning, 400 ppm in the afternoon, and 600 ppm in the dark periods. The researchers collected nectar from the plants during the early flowering stage of their development and found that elevated CO2 effectively doubled the volume of nectar per flower. They discerned no difference in the nectar’s sugar concentration so, in effect, doubling CO2 doubles the amount of sugar produced. Dag and Eisikowitch note, "Improvement in nectar reward can increase the attractiveness of the flowers to the bees, increase pollination activity, and consequently increase the fruit set and the yield." Sweet news.

Li et al. grew spring wheat in central Arizona. They maintained the atmospheric CO2 concentrations at 370 ppm (ambient) and 550 ppm. They made a series of complicated kernel weight measurements throughout the spikelet (the part of the wheat plant that contains the grain) and found an approximate 6 percent increase in kernel weight given the 180 ppm increase in CO2.

Italian and English researchers grew a large number of poplar seedlings in outdoor plots where the atmospheric CO2 concentration was maintained at ambient levels (near 360 ppm) and 550 ppm. Their focus was on the impact of the elevated CO2 on leaf growth of the trees. Ferris et al. found that the mature leaf area increased from 19 percent to 61 percent, depending upon poplar species at their location in Italy. They conclude, "The information on leaf area development suggests that species with larger leaves will develop larger canopies more quickly and may become more productive in a carbon-rich environment".

But what about super-concentrations of carbon dioxide in the atmosphere?

Admittedly, the literature on plant responses to levels of CO2 near or above 1,000 ppm (three times ambient) is rather small in comparison with the literature on the effects of a doubling. But, what about the effect of 10,000 ppm CO2 – or approximately thirty times the current level? We keep hearing that carbon dioxide at some concentration has to be a bad thing. Surely those who argue it’s a pollutant at some concentration could accept 10,000 ppm might meet their definition.

Researchers in Singapore conducted an experiment growing orchids in transparent zipped plastic bags at ambient CO2 levels of 350 ppm and at thirty times that concentration, at 10,000 ppm. They maintained temperature between 22°C (72°F) and 30°C (86°F). The orchids were provided 12-hours of simulated sunshine each day and were well-watered. After three months of the "high life" they were harvested and measured.

Gouk et al. found an array of statistically highly-significant biological responses to the super-elevated CO2. Compared to their Clark Kentish (ambient) brethren, the super CO2-saturated orchids resulted in much higher young leaf dry weight, root dry weight, total dry weight, relative growth rate, root-to-shoot ratio, young leaf area, and overall leaf area (Figure 1). Total chlorophyll rose in the leaves and roots of Super Orchid. The authors noted increased photosynthetic capacity and enhanced growth of the roots would combine to increase the survival rate of orchid plantlets grown under stressful field conditions.

The authors were skeptical, going in, and anticipated that the high levels of CO2 would trigger some mechanism within the plants to slow-up or reverse the relative "goodness" that seems to come from enhanced CO2. The mechanism never materialized. The orchids thrived in a world of 10,000 ppm of atmospheric CO2.

References

Dag, A., and Eisikowitch, D. 2000. The effect of carbon dioxide enrichment on nectar production in melons under greenhouse conditions. Journal of Apicultural Research, 39:88-89.

Donnelly, A., Craigon, J., Black, C.R., Colls, J.J., and Landon, G. 2001. Does elevated CO₂ ameliorate the impact of O3 on chlorophyll content and photosynthesis in potato (Solanum tuberosum)? Physiologia Plantarum, 111:501-511.

Ferris, R., Sabatti, M., Miglietta, F., Mills, R.F., and Taylor, G. 2001. Leaf area is stimulated in Populus by free air CO2 enrichment (POPFACE), through increased cell expansion and production. Plant Cell and Environment, 24:305-316.

Gouk, S.S., He, J., and Hew, C.S. 1999. Changes in photosynthetic capability and carbohydrate production in an epiphytic CAM orchid plantlet exposed to super-elevated CO2. Environmental and Experimental Botany, 41:219-230.

Li, A., Hou, Y., and Trent, A. 2001. Effects of elevated atmospheric CO2 and drought stress on individual grain filling rates and durations of the main stem in spring wheat. Agricultural and Forest Meteorology, 106:289-301.

Tischler, C.R., Polley, H.W., Johnson, H.B. and Pennington, R.E. 2000. Seedling response to elevated CO2 in five epigeal species. International Journal of Plant Science, 161:779-783.

 

Figure 1. Orchid enhancements caused by high levels of CO2.