November/December – 1991
Story Title: Mass By Gas
Author: Steven Carruthers
Carbon dioxide (C02) enrichment is one of the most interesting curiosities of modern horticulture, and is yielding valuable primary data as research continues into the effects of increased levels Of C02 in the atmosphere.
Unfortunately, the data collected is not generally published outside scientific journals, and seldom reaches the agricultural industry where the effects of increased levels Of C02 can be better understood.
During the past century, the level Of C02 in the atmosphere has been Dsteadily rising, largely from the combustion of fossil fuels. The atmospheric level Of C02 has climbed from an average of about 0.028% (280 ppm) in 1860 to 0.034% (340 ppm) by 1981, an increase of more than 1%. Current estimates predict that C02 levels will continue to rise, perhaps doubling within the next 70 years (Kellogg 1978).
While C02 in this context might be regarded as a pollutant by those concerned with the climatic implications of an increased “greenhouse effect”, elevated levels Of C02 are likely to be beneficial to agriculture.
The genetic capability of plants to absorb higher levels Of C02 stems back to primordial times when plants adapted to C02 levels three to four times that which exists on earth today. In fact, for many years horticulturists have practised C02 enrichment in controlled environments to increase crop yields.
An important focus of current research is the exchange of carbon dioxide between the biosphere and the atmosphere, and the effects of elevated C02 levels upon plant species of economic value, in order to predict the likely outcome of future crop yields. On current projections, an increase in atmospheric C02 Will induce higher global crop yields. Kanemasu (1980) estimates that there will be a wheat yield increase in the United States of about 59% with C02 levels double that of today.
However, the implications for agricul-ture will depend strongly upon other weather-related factors, such as changes in rainfall patterns and the length of the growing season. Such predictions are also dependent upon:
1.The continued destruction of the great tropical and sub-tropical forests, since they act as the lungs of the planet for converting C02 into oxygen;
2.How much C02 is absorbed by the world’s oceans, since they play a strong interactive role in both the global carbon cycle and the climate system; and
3.The continued burning of fossil fuels given likely alternative technologies of the future.
Tucker (1981) suggests that a doubling Of C02 could give an increase in photosynthesis of somewhere between 30% and 60%, but this may not necessarily be reflected in increased crop yields. He reasons that any increased yields are more likely to be attributed to increased precipitation, which in turn is attributed to global warming as a result of higher C02 levels.
Given a 30% increase in photosynthetic efficiency and a two-degree increase in average temperature, Baker and Lambert (1980) estimate a net increase in crop growth and development of 14% to 38%, depending upon the availability of water. Pimentel (1980) further notes that a decrease in rainfall of between 10% and 30% over the USA, together with a temperature variation of plus or minus two degrees, but with no change in ambient C02, will reduce expected wheat and corn yields by 10%-15%.
At present, these predictions are little more than conjecture owing to our inability to make accurate climate forecasts. However, there are a small number of scientists currently working to understand aspects of the biological consequences of elevated C02 in the future. Scientists can make much more progress in understanding the effects upon existing genotypes and the ensuing phenotypes. For example, yield responses can be studied for different crops, applying existing climate variations in order to estimate the future climate matrix, even if some of the more precise requirements concerning meteorological variables are the subject of conjecture. Much of this research is conducted using hydroponic techniques.
Carbon dioxide fixation and water use efficiency are only now beginning to be understood in detail. When considering C02 fixation, plants are grouped into three main biological classifications – C3, C4 and CAM, abbreviated names for plants that share a predominance of the same chemical bonding sites (receptor sites) for carbon dioxide. Once the bonding process is complete, a series of chemical reactions occur to break down the C02 and water to create carbohydrates.
To help understand the process more simply, plants absorb C02 using C3, C4 and CAM receptors, much like the body absorbs oxygen with haemoglobin, which has a high affinity for oxygen.
The difficulty facing researchers is that not all plants share the same receptor sites. It is known that C3 plants, of which some 95% or more of the biomass is comprised, utilise ambient C02 directly into their photosynthetic carbon reduction cycle. In terms of water utilisation, this is the least efficient category and includes plants such as cotton, rice, wheat and sugar-beets.
C4 plants, such as maize, sorghum, millet and sugarcane, have a higher water-use efficiency because an enzyme-based, C02-concentrating mechanism begins the photosynthetic process and allows higher stomatal resistance and less transpiration for the same C02 fixation.
CAM (Crassulacean Acid Metabolism) plants have the highest wateruse efficiency because an additional enzyme allows a temporal variation in C02 uptake, fixing C02 from the air at night but reducing it to carbohydrates during the day when the stomata are closed. This group comprises succulent xerophytes most of which have no economic value, with one exception – the humble pineapple.
So much for the scientific explanations, but what does this mean in practice to the agriculturist who would like to put this knowledge to good use?
In general terms, it is possible that the selection of crop species most suited to changing conditions may be one of the first elements in any planned contingency program of adaptation to a high C02 atmosphere of the future. However, the choice of future crops will depend as much upon diurnal and seasonal patterns of temperature and rainfall as it will upon C02 levels. The kind of climate scenario being projected for the middle of the 21st century could lead to major shifts away from crops such as wheat, barley, potatoes and sugar-beet, to more water efficient crops like rice, cassava, sweet potato, maize, sorghum, pearl millet and sugarcane.
A side benefit of an enriched C02 atmosphere is increased water use efficiency in many plants. Under normal C02 atmospheric conditions, C02 diffuses into the leaf while water travels up the root system and transpires through the stomata. But in a C02 enriched environment the stomata shrink, with the result that plants transpire less water. In essence, this means plants become more water efficient.
Research by Downton, Bjorkman and Pike (1980) studied the effects of increasing the C02 level using Nerium oleander, a drought-resistant evergreen shrub, and Tidestronia oblongifolia, a summer-active perennial native to the floor of Death Valley, USA. They found that while increased photosynthesis enhanced dry matter production and flower production, water use efficiency was greatly increased. For N. oleander grown at double normal C02 levels, water efficiency also doubled; for T obiongifolia grown at triple C02 levels, water efficiency tripled, although C02 enrichment did not enhance growth significantly. Their research did show that such plants can be grown in areas currently too arid for agriculture. Of greater significance, however, is the increase in dry matter production in areas where plants already grow.
Over the past decade there have been numerous studies on the effect Of C02 enrichment, but growth and yield rates have varied from model to model. For tomatoes, Slack (1986) showed a 30% increase in growth and yield, while Yelle (1987) reported a 36% increase. These results support earlier studies by Wittwer and Homma (1969) who, in tomato seedlings, reported accelerated growth rates, root growth promotion, and earlier flowering and fruiting. Other studies have shown a 31% weight increase in lettuce when exposed to 1600 ppm Of C02, and a 23% increase in fruit weight for cucumbers exposed to 1000 PPM C02.
Chrysanthemums, roses and carnations also respond well to elevated C02 levels. Experiments on roses have shown a 53% increase in flower weight when exposed to 1000 ppm of C02, with specific effects of increased stem lengths, a greater number of petals and a shorter cropping time in winter. Carnation yields have been increased by up to 38% with increases in flower weight, stem thickness, and shorter flowering times.
Results on asparagus transplants and in vitro-cultured clones using C02 enrichment and supplemental lighting in greenhouse experiments are nothing short of spectacular. Desjardins, Gosselin and Lamarre (1990) reported increased root and fern dry weight for transplants of 196% and 336%, using 900 and 1500 PPM Of C02 respectively. For clones they reported increases of 335% and 229% respectively. From their C02 models it can be seen that transplants respond better to higher levels Of C02, while clones respond better to lower concentrations.
A similar phenomenon was reported for tissue-cultured strawberries (Desjardins, 1987), and tomato transplants (Hurd, 1968).
For agriculturists, this research has far-reaching implications since transplants are becoming increasingly popular for the establishment of commercial plants. In the case of asparagus, its advantages over conventional plantings of 1-year-old crowns include the prevention of root diseases, superior stand establishment, and reduced production costs associated with high seed prices and digging-replanting operations.
Desjardins, Gosselin and Lamarre concluded that C02 enrichment and supplementary lighting improved plant quality and reduced the nursery period. They further concluded that supplementary lighting contributed significantly to an improvement in plant survival. However, the effects did not contribute to improved yield components, such as number of shoots per plant or height of the plant. Nonetheless, they were able to demonstrate improved growth of plantlets in the field after two years’ growth.
Carbon dioxide enrichment has resulted in a variety of beneficial effects on many other crops. White and Warrington (1 9 84) reported significant growth increases in geraniums.
Leaf area, shoot dry weight, specific leaf weight and plant height all increased, but enrichment gave growth increases only up to the visible bud stage. Similarly, while sunflowers gain increased stem thickness, enrichment was not beneficial at the flowering stage although it significantly increased the number of buds on each plant.
The physiological evidence strongly indicates that most plants have the potential for increased production rates, but the extent of plant response is also dependent upon factors such as temperature, light and nutrition, as well as the level Of C02 in the atmosphere.
Blackman’s Law states that the rate of any process which is governed by two or more factors is limited by the factor in least supply. The process of photosynthesis is a classic example of this rule. For example, on an overcast day it is pointless raising the temperature more than 5°C above the night temperature in a controlled greenhouse environment because the low light intensity will limit the rate of photosynthesis. Any additional heat will be without benefit. Conversely, if it is a bright sunny day and the temperature is not raised, then lack of heat may become the limiting factor for photosynthesis.
Carbon dioxide enrichment can also be a limiting factor in photosynthesis. Elevated levels Of C02 will not be beneficial at low light intensifies or temperatures. If light intensity or temperature is increased, then higher levels Of C02 can be introduced to stimulate further growth increase. It is worth noting that elevated C02 levels can also be expected to increase fertiliser and water requirements.
The relationship between C02 levels, light intensity and temperature and their interactive effects upon photosynthesis are well illustrated in curves developed by Gaastra (1962) for cucumbers. In the lower curve the rate of photosynthesis begins to plateau at a light intensity of about 40,000 lux (3800 footcandles), independent of the temperature being maintained at either 20°C or 30°C. The 300 ppm level Of C02 became a limiting factor at this point.
When the C02 level was elevated to 1300 ppm at 20°C, the rate of photosynthesis increased. At this point the temperature became the limiting factor. With a temperature increase to 30°C, at the same 1300 ppM Of C02, another increase in photosynthesis was recorded.
This does not mean that the rate of photosynthesis will continue to increase with temperature rises. Generally, increases in temperature will affect other photosynthetic processes. Apart from faster growth, relatively high temperatures can cause a reduction in plant quality and yield, such as longer or thinner stems and smaller flowers.
While there is some variation in the results of different studies on the effects of increased C02 levels, the general conclusion must be that the effects are clearly beneficial.
There are several types of horticultural C02 enrichment systems available today. The most sophisticated units rely on very expensive infra-red technology to accurately measure C02 levels, feeding back this information to an electronically controlled C02 injection unit or generator in order to maintain a specified level Of C02 at all times. However, there is currently under development an automatic system which not only maintains a constant level Of C02 in the growing environment, but also adopts data such as the Gaastra curves to maintain the optimum photosynthetic response during the growth cycle of plants. Such a system will point the way to a future of maximal growth for chosen plant