Carbon Dioxide Damage in the Flora and Climate Change
Saturday, December 24, 2011 6:31 PM
Rise or Fall of Flora's Realm
The Threat of the Real Greenhouse Effect!
By Johan van der Galiën (M.Sc. in Chemistry)
With thanks to Dr.Ir. P.J.A.L. de Lint for his very useful corrections and discussions.
2. "Carbon dioxide damage" to plants
3. Ethylene as a factor causing leaf damage
4. Hypothesis about leaf damage waiting to be verified
Notes and references
This paper is a literature survey and a stimulus for more research on the topic of leaf damage observed during carbon dioxide enrichment of greenhouse plants. The theories in this paper have consequences for the biodiversity and agriculture on our planet. Because some plants might show severe leaf damage above carbon dioxide concentrations of 700 ppm! A concentration to be exceeded in the atmosphere by our 'hydrocarbon society', which is using fossil fuels for energy production, in eighty years from now. This might seriously derail the flora and agriculture.
Beside the fact that elevated carbon dioxide might cause for instance chlorosis and necrosis, the exponents of leaf damage, in some plants. It certainly affects the opening or closure of the stomata, starch content of the leaves and photosynthesis rate of all plants. Because of this some plants will show growth acceleration and some growth retardation. The most favoured ones will overgrow the rest, thus affecting the biodiversity in the flora through the mechanisms of eutrophication and selective parasitic injuries. The last mechanism will occur because of the higher sugar and starch content of the leaves.
The agriculture might be affected because many of the carbon dioxide damage sensitive plants are nutritious crops, essential for the food production, especially in the third world. Since the industrialised world can afford to genetically engineer these plants and bring them to even higher yields. But will the poor third world be able to buy the expensive seeds?
The complex role of ethylene is discussed. A contaminant of the used carbon dioxide sources in experiments whereby leaf damage is observed. It is a plant hormone, which can also cause the main leaf damage symptoms chlorosis and necrosis. So questions remain: Is the factor causing leaf damage ethylene, carbon dioxide or perhaps both? Experiments to answer these questions are described in this paper. Hypothetically it is possible that ethylene is an exogenous and/or endogenous factor for leaf damage, whereby the biochemical production of it in plants is triggered by elevated carbon dioxide.
There is much attention for the effects on the climate that an elevation of the atmosferic carbon dioxide concentration might cause, but perhaps there is still another threat also in the air. Namely, direct damage to some plant species caused by too high carbon dioxide fertilization. This could seriously derail the flora, because plant species will react differently to this change in environment. Some plants will experience growth enhancement while others will experience growth retardation. In other words, the favoured plant species will overgrow the disfavoured ones in their biotope, this will cause many species to become extinct in a short time-span.1 So the argument that the flora and the biodiversity will rise with an elevated atmospheric carbon dioxide level is absurd.19
Certainly one can point out many other dangers for the biodiversity: Poachers, draining of chemical sewage, cutting down of the rainforests, the hole in the ozone layer, climate change etc. But these are all more or less local problems. The affection by carbon dioxide overfertilization to be expected for the last flora is the only threat with world wide consequences, simply because: 'The air is everywhere!'
2. "Carbon dioxide damage" to plants:
The train of thought in the introduction is a logical conclusion from the work of Professor Dr. J. Goudriaan, working in the Theoretical Production Ecology department of the Agricultural University in Wageningen (The Netherlands). According to Goudriaan it is to be expected that with an unchanged energy policy the atmospheric carbon dioxide meter will point towards 1200 ppm in a hundred years from now. (The atmospheric carbon dioxide concentration in the year 2002 is about 360 ppm.)2 Dr. Ir. P.J.A.L. de Lint, former head of the Plant Physiology department of the Research Station for Floriculture and Glasshouse Vegetables in Naaldwijk (the Netherlands), was the first to realise that with these high concentrations a large number of plant species in the wild and in cultivation will show signs of injury.3 This in analogy of findings during experiments with carbon dioxide fertilization in greenhouses, with some plant species leaf injuries are then found like:4-6,15,22,24,26,27
· The formation of to small leaves (Small Leave Syndrome)
· The curling and deformation of the leaves
· The formation of glasslike spots on the leaves, mainly along the edges, caused by internal destruction of the leaf structure.
· The decolouring of the leaves (chlorosis), it starts from the edges and gradually spreads to the inside, or it begins at the end (Leaf Tip Chlorosis)
· The appearance of leaf parch
· The gradually dying of the plant, this begins with the leaves (necrosis)
· The appearance of gradual defoliation
· The appearance of a brownish fringe on the leaves.
· The leaves become brittle.
At first instance leaf damage is observed with greenhouse plants who were enriched with carbon dioxide in flue gases, from natural gas burning in a greenhouse heater.7,8 A very practical solution, which can turn out to be profitable for the market gardener by raising the crop yield and lowering the energy and water costs. However, flue gases contain besides carbon dioxide and remaining air ingredients also traces of other components: NOx, methane, ethylene, sulphur oxides etc.8 A remark is due here; namely that natural gas contains only very small amounts of sulphur and it forms very clean flue gases with good tuned burners. Some publications about leaf damage describe the effects of elevated carbon dioxide levels caused by dosing flue gases. From these figures one cannot say with 100% certainty that carbon dioxide is causing the observed leaf damage, there still can be another factor in the flue gases responsible for this. However it has been proven for nine or ten plants that they also show leaf damage with "pure" carbon dioxide from a cylinder (between brackets is given an indication of the point at which leaf damage starts to occur, the real point can be lower, to determine this more research is necessary):
· Cucumber (750 ppm, "pure")9,10
· Tomato (550-800 ppm, "pure")10,15
· Paprika (900-1000 ppm, "pure")10
· Eggplant (700 ppm, "pure")15 (350 ppm, from the air and under continuos illumination)30
· Potato (700 ppm, "pure")22
· Basil (1000 ppm, unknown)23,25
· Barley (1000 ppm, "pure")16
· Euphorbia pulcherrima (750 ppm, "pure")11
· Gerbera (750-800 ppm, "pure")11,12,28
· Cotton (675 ppm, "pure" no detectable ethylene)24
For cucumber, tomato, paprika, Euphorbia pulcherrima and Gerbera there has been found no difference between dosing with flue gases and dosing with pure carbon dioxide.10 So I will also make no difference between flue gases and pure carbon dioxide in the course of the discussion in this paragraph!
From experiments in commercial nurseries it has been proven that one third of the ca. 50 greenhouse vegetables show signs of leaf damage above 700 ppm carbon dioxide. The by estimation 1000 ornamental plants, which are grown in greenhouses all over the world, seem to be less sensitive. Nevertheless, involved governmental institutions advise to limit the carbon dioxide enrichment at 1000 ppm.10,11
It is not exactly known how carbon dioxide damage arises. It has been shown by some plant physiologist that with carbon dioxide enrichment the photosynthesis rate is much higher, as a matter of fact it is so high that some plants can not handle the normal processing of the large amounts of formed sugars anymore. Out of necessity these plants make starch from them, with is stored as granules in the leaves and which is meant as an energy and carbon reserve stock. The chloroplasts however get locked and deformed by the large number of starch granules. Finally this leads to retardation of photosynthesis. So, the net effect of to high carbon dioxide enrichment with these plant is not growth enhancement but growth retardation!4,22,23,25,26,27,29
The elevated starch content in these “slow” plants can explain the curling and deformation of the leaves but not all the other observed leaf injuries. Because there have gas enrichment experiments been conducted under circumstances in which no starch can be formed. (This can be achieved with a higher temperature in combination with enough “nitrogen” and other minerals. In this case all formed sugars can directly be used for the growth of the plant.) Still even under these circumstances leaf injuries are observed!10
Hollander and Krug demonstrated with high carbon dioxide concentrations (10.000 ppm continuously) for cucumber plants that it causes, obviously in a direct action, the opening of the stomata. Other controllers, as the osmotic potential of the nutrient solution and transpiration load caused by the saturation deficit of the air, are overlaid. The last ones determine however the degree of water stress, which may cause wilting, drying up as well as a reduction of growth rate up to temporary stop or even reducing of fresh weight.20,21
There have also been found two mutants of Arabidopsis thaliana to become chlorotic when exposed to 20.000 ppm carbon dioxide. These mutants are somehow hypersensitive towards carbon dioxide since the wild type is not affected by this concentration. These mutants were created by mutagenesis with ethyl methane sulfonate.31 It is interesting to know what the precise mechanism is which causes chlorosis in these mutants and at which loci the mutations reside on the plants genome. To say definite things about these things more research is needed. The researchers of this publication do more or less rule out the fact that the chlorosis is caused by the effect carbon dioxide has on the stomata without argumentation. But they do refer to the fact that carbon dioxide concentrations of 20.000 ppm open the stomata in some species. So some questions remain: At what minimum level does carbon dioxide generally open the stomata? At what minimum level does carbon dioxide generally close the stomata? Are these levels surpassed by the civilisation on our planet through burning fossil fuel and emitting carbon dioxide in the atmosphere? Can the effect of carbon dioxide on the stomata explain the observed leaf damage?
A suggestion could be that damage to plants only occurs if the elevation of the carbon dioxide concentration is acute, like with gas enrichment in greenhouses (carbon dioxide shock). It maybe so that a gradual elevation of the atmospheric carbon dioxide concentration, like the doubling from 360 ppm to 720 ppm in the coming 80 years caused by antropogenic emissions, has no effect and the whole flora can adapt without problems. According to this philosophy there is no decline of the global biodiversity to be expected by carbon dioxide damage. But according to De Lint the sensitive plants are genetically adapted to the present-day level of atmospheric carbon dioxide (360 ppm). That there is a link between genetics and carbon dioxide damage is clear from the fact that within the genus Gerbera the variety “Marleen” already is sensitive to carbon dioxide concentrations of > 800 ppm, while the Gerbera varieties “Veronica” and “Gosta” are still insensitive for values up to 3400 ppm.10,28 This difference in carbon dioxide sensitivity can only be determined by the genes! That’s why an acute doubling or a gradual doubling in 80 years does not matter for the sensitive plants, both are for the evolution time scale an instantaneous change (carbon dioxide shock). The present-day variety of species in the flora can probably vindicate it self only in case of an extreme slow doubling, smeared over a period of for instance several hundred-thousand years, to give all plants species enough time to adapt them self genetically without problems, according to the very slow 'trail and error' process with is called evolution.
Carbon dioxide damage must certainly be researched and be discussed as a possible threat to the global biodiversity. Because even with the present-day elevation rate of the atmospheric carbon dioxide above 700 ppm one can expect that certain vegetation species suffer leaf injuries in the end and become extinct. However this elevation also has another effect: eutrophication! Plants react differently to a high carbon dioxide level. A coarse classification based on present-day knowledge and insights in the field of carbon dioxide effects on the flora is as follows:
1) Species that will grow faster, even above 6000 ppm.
2) Species which will grow faster, but less efficient, and which will be overgrown in high concentrations.
3) Species which normally react to more carbon dioxide in a positive way, but for which above certain limits carbon dioxide (> 700 ppm) causes “toxic” leaf parch and other necrotic symptoms. These species do not even survive in monoculture, and also not in the solitary situations with these to high carbon dioxide values.
Because of this differences in response the sensitive species will encounter increasingly growth retardation and overgrowing by less sensitive species. So the knife cuts both ways: photosynthesis retardation versus eutrophication! Plants who even thrive better at a high carbon dioxide concentration will altogether play the first fiddle in the flora, because of this some species will become extinct. After reaching the critical limit (700 ppm) leaf damage also starts to appear by many species, by which still more species become extinct. A vicious circle, which can only be broken by a swift introduction of alternative energy sources without (net!) carbon dioxide emissions, like biogas, solar energy, geothermal sources and wind energy.
Flora and fauna are strongly connected to each other. So the threat does not stop at the realm of plants. Imagine that the bamboo in the forests of central China belongs to the endangered species; than will also the panda beer disappear who only can live from the morrow in the stalks of the shoots from this particular plant. The World Wildlife Fund would than have no animal anymore in its escutcheon.
A number of nutritious crops belong to the sensitive species! So the world food supply certainly comes in danger. With state of the art agricultural techniques is the estimation that 40 milliard people in the world can be fed.13 So there is a large potential buffer for an increase of population qua nutrition. All developing countries have enough time to achieve a level of prosperity and welfare like in the West, without being perished in a war for food reserves. With a higher level of welfare (= healthcare + elderly securities) will the child mortality disappear. A married couple will not have the surviving instinct to give birth to 10 children, of which only 6 scarcely stay alive. But they will get only 2 children, which both survive! A situation which is comparable with in the West where more people die than there are children born.14 A humane solution to the overpopulation on earth lies within our reach, only if our agricultural resources would not suffer from carbon dioxide damage!
3. Ethylene as a factor causing leaf damage:
You can find at least one publication on the Internet about carbon dioxide damage.16 And this brings me to a point of relativism of the above text because there is still a matter to be elucidated. Is the observed leaf damage caused by carbon dioxide or by another factor in the used gas fertiliser?
This factor could well be ethylene, a potential plant hormone which can also cause all of the observed leaf damage for which carbon dioxide is the suspect in the former paragraph.17 During the several industrial chemical production methods of carbon dioxide, which is sold in cylinders and used for instance in the laboratory greenhouse experiments of leaf damage, there is always a fraction of ethylene present.18 To make things more complicated the action of carbon dioxide and ethylene maybe intertwined, with carbon dioxide stimulating, exogenous and/or endogenous, ethylene production and with ethylene acting as a pheromone.
In the Internet article barley suffers from chlorosis at elevated atmospheric carbon dioxide. In a personal email to me the author, Richard Sicher, gives his version of how much ethylene was present during his experiments. He writes about a greenhouse system with 1000 ppm carbon dioxide, that is about three times the normal atmospheric concentration. The carbon dioxide used had a total hydrocarbon concentration (THC) of < 5 ppm. In the greenhouse system there can then be at maximum 5 ppb ethylene present, if the THC is pure ethylene! This comes close to the level of 10-100 ppb whereby ethylene becomes active as an hormone for most plants, but there maybe plants which are hypersensitive towards ethylene, causing leaf damage according to Abeles!17
But to make matters worse atmospheric air also contains some ethylene. So my version of how much ethylene maybe present in Richard Sicher's experiments is as follows: Air can contain 5-500 ppb ethylene mainly from industrial processes, automobile exhaust etceteras. 5 ppb is for the air of rural areas and 500 ppb is for the air of a big city. These figures are from the year 1973. The values for the year 2002 can be expected to be much higher since there is much more industry and traffic nowadays!17 So if you have a glasshouse with barley and one uses Richard Sicher's carbon dioxide then the cumulative (in air + in carbon dioxide) ethylene concentration can be about 10-505 ppb. This overlaps the range of Abeles abundantly!
With these figures in mind one cannot say what causes the observed barley chlorosis: Carbon dioxide, ethylene or perhaps both? And this case represents all experiments were leaf damage is blamed to carbon dioxide.
4. Hypothesis about leaf damage waiting to be verified:
The first: Carbon dioxide is a factor, besides ethylene, that causes the observed leaf damage. The way to verify this is by making sure that the air of gas fertilisation experiments in a greenhouse system are practically free of ethylene (< 1 ppb). This can be achieved by using a large column with the filter system described by James I. L. Morison and Roger M. Gifford,18 keeping the greenhouse as small as possible to assure that the column does not grow out of proportions. The column needs to be large because all the ethylene in the air, carbon dioxide source and which the plants under scrutiny them self produce must continuously be oxidised to ethylene glycol by the potassium permanganate soaked vermiculite in the column. The ethylene concentration should be continuously monitored and be kept under 1 ppb! All the plants which have shown "carbon dioxide leaf damage" should be tested in such a practically ethylene-free environment under elevated carbon dioxide concentrations ranging from 360 ppm to 1.000.000 ppm.
The second: Ethylene is a factor, besides perhaps carbon dioxide, that causes the observed leaf damage. The way to verify this is by keeping the carbon dioxide level to 360 ppm and gradually increasing the ethylene concentration, from a cylinder with an analytical pure ethylene (> 99,9%), until mild or severe leaf damage is observed. The ethylene and carbon dioxide concentrations should continuously be monitored and kept constant during one run of experiments. All the plants which have shown "carbon dioxide leaf damage" should be tested and the results should be compared to the traditionally elevated carbon dioxide results earlier described in the literature (that is without a practically ethylene-free environment as I mentioned above).
The third: Elevated carbon dioxide stimulates the leaf damage sensitive plants somehow to produce ethylene in the air at such levels that this exogenous ethylene causes the observed leaf damage. This exogenous ethylene might be a vicious circle because ethylene could act as a pheromone in these experiments. The presence of ethylene, from one plant specimen, in the air might causes other plant specimens to also produce exogenous ethylene. So you might have two effects mixed: Carbon dioxide stimulating exogenous ethylene production and the pheromone effect of ethylene also stimulating exogenous ethylene both causing leaf damage. The way to verify this is by assuring that the air of the greenhouse is practically ethylene-free (specification < 1 ppb) at the start of each experiment, by first circulating it through the column of James I. L. Morison and Roger M. Gifford until it is under the specification.18 Also the used carbon dioxide should be treated by such a column until the flow into the greenhouse meets the specification. The exogenous ethylene and carbon dioxide concentrations should continuously be monitored and the carbon dioxide should be kept constant during one run of experiments. The experiments should be performed under elevated carbon dioxide concentrations ranging from 360 ppm to 1.000.000 ppm, up to the point that the plants show severe leaf damage. All the plants that do not show "carbon dioxide leaf damage" anymore with the verification of the first hypothesis should be tested and the results should be compared to the traditionally elevated carbon dioxide results earlier described in the literature (that is without exogenous ethylene only environment as I mentioned above).
The fourth: Elevated carbon dioxide causes endogenous ethylene to accumulate in plants, which is the cause of leaf damage. The experiments are essentially the same as with the first hypothesis and can be done simultaneously. Only now one should also analyse the plant material it self somehow for traces of endogenous ethylene. This endogenous ethylene concentration should be correlated to the observed leaf damage.
The fifth: The ethylene present in the air of a gas fertiliser experiment causes an overproduction of endogenous ethylene, which results in leaf damage. The experiments are essentially the same as with the second hypothesis and can be done simultaneously. Only now one should also analyse the plant material it self somehow for traces of endogenous ethylene. The endogenous ethylene concentration should be correlated to the ethylene concentration in the atmosphere of the experimental greenhouse and the observed leaf damage.
It is clear from the above that "carbon dioxide leaf damage" and the ethylene hypothesis needs thoroughly verifying. Because the consequences if the carbon dioxide/leaf damage hypothesis is true are so devastating for the earth and the descendants of its present inhabitants. And from the facts presented in this paper it certainly looks like that carbon dioxide and ethylene are both factors that can cause leaf damage! Ethylene because of its general hormone action17 and carbon dioxide because it affects the opening/closure of the stomata. Opening leads to necrosis as Hollander and Krug demonstrated for cucumber plants.20,21 This is an phenomenon entirely attributed to carbon dioxide because ethylene generally has no effect on the opening of the stomata.17 Closing of the stomata is observed together with a brownish fringe on the leaves of potato.
It could well be that the elevated carbon dioxide concentration in the atmosphere will cause opening of the stomata in some plant species and closing in others, both leading to leaf damage and resulting in loss of biodiversity.
And there still remains the fact of starch granule accumulation in the leaves, which deform the chloroplasts, blocking photosynthesis leading to leaf damage. Also this effect is entirely attributed to carbon dioxide and not ethylene.17
Thus elevated carbon dioxide has at least three negative effects on the biodiversity of the flora. So there is enough reason to be worried about elevated levels of the atmosphere in the future.
It should need no arguments to say: "I want to preserve the last biodiversity on our planet." In my opinion leaf damage to the flora caused by anthropogenic carbon dioxide is the greatest threat to the fulfilment of this goal. Therefore all the mentioned hypotheses should be carefully considered verifying by the scientific community!
Even if the carbon dioxide threat is proven not to be true than there still remains the fact of eutrophication in the flora by elevated levels. This effect will be amplified by selective parasitic injuries.1 This combination will still be a danger to biodiversity! But that is stuff for another article and also for more scientific research!
I hope that I made clear in this article that there is no rise but only fall to be expected for flora's realm, since we continue with the carbon dioxide emissions of our fossil fuel society. Albeit there are enough alternative renewable energy resources available like solar power, wind energy, geothermal energy, biomass etc.
Notes and references:
1) Strictly speaking one can distinguish three possible negative effects of carbon dioxide over-enrichment on the flora, and by which plant species can become extinct:
· Eutrophication = qua growth most advantaged species overgrow the rest.
· Leaf injury or carbon dioxide damage = an in the end lethal plant disease, from which the symptoms are mainly seen on the leaves. Some species are more sensitive then others.
· Parasitic injuries = the higher sugar and starch content in the leaves give rise to a infectious disease caused by bacteria and/or fungus growth with certain plant species, possibly attended with injuries by insects.
2) Van Strien W., ‘Prijs voor model koolstofkringloop’ Bionieuws 17/23 OKT 1992
3) De Lint P.J.A.L.; (a) ‘Geeft meer CO2 buiten ook schade, zoals in kassen?’ LT Journaal 18, 25 november 1993; (b) ‘Stijging CO2 alleen nog te verhelpen door gebruik nieuwe energiebronnen’ Bionieuws 16/9 OKT 1993
4) Van Berkel N., ‘Injurious effects of high CO2 concentrations on cucumber, tomato, chrysanthenum and gerbera’ Acta Horticulturae 162, 101-112 (1984)
5) Van Berkel N., ‘Three physiological disorders in glasshouse cultivation’ Acta Horticulturae 119, 77-89 (1981)
6) Holländer B., Krug H., ‘Wirkungen hoher CO2-Konzentrationen auf Gemusearten 1. Symptome, Schadbereiche und Artenreaktionen’ Gartenbauwwissenschaft 56(5), 193-205 (1991)
7) Van Berkel N., Heij G., ‘Damage to tomato plants by CO2 enrichment’ Annual Report Naaldwijk 48-51 (1969)
8) Van Berkel N. ‘Naar een lagere CO2-concentratie’ G+F 43-45, 28 maart 1979
9) (a) Van Berkel N., Van Uffelen J.A.M. ‘CO2 nutrition of spring cucumbers in the Netherlands’ Acta Horticulturae 51 213-224 (1975)
(b) Heij G., Van Uffelen J.A.M., ‘Effects of CO2 concentration on growth of glasshouse cucumbers’ Acta Horticulturae 162 29-36 (1984)
10) De Lint P.J.A.L., personal communications. See also the annual volumes 1980-1996 of Groente en Fruit and Tuinderij.
11) Papenhagen A., ‘Bessere Ertrage durch CO2 – aber nicht uberall’ Gb+Gw 49 1244-49 (1983)
12) Van Berkel N. ‘Effects of CO2 enrichment on dry matter production and distribution in glasshouse crops; Leaf scorch in Gerbera caused by CO2’ (a) Annual Report Naaldwijk 36-37 (1980); (b) Ibid. 34-35 (1982); (c) Ibid. 30 (1984)
13) Rabbinge R. ‘Wereldvoedselvoorziening kan veel efficienter’ Chemisch Magazine 10 427-430 (1995)
14) Derived from a thesis which I want to call Jimmy Carter’s paradox and it goes something like this: The overpopulation on earth can only be impeded by fighting child mortality.
15) Proefschrift LU Wageningen Dr. Elly M. Nederhoff ‘Effects of CO2 concentration: on photosynthesis, transpiration and production of greenhouse fruit vegetable crops’ (1994)
16) Sicher jr R.C. 'Factors affecting chlorosis of barley primary leaves during growth in elevated carbon dioxide' (1998) webpage.
17) Abeles F.B. 'Ethylene in plant biology' Academic Press (1973)
18) Morison J.I.L., Gifford R.M. 'Ethylene Contamination of CO2 Cylinders: Effects of plant growth in CO2 enrichment studies' Plant Physiology 75 275-277 (1984)
19) Böttcher C.J.F. 'Who is afraid of carbon dioxide' Shell World february 23-25 (1995)
20) Hollander B., Krug H. 'Wirkungen hoher CO2-Konzentrationen auf Gemüsearten: II Wachstum, CO2-Gaswechsel und Stomatawiderstand' Gartenbauwissenschaft 57 32-43 (1992)
21) Hollander B., Krug H. 'Wirkungen hoher CO2-Konzentrationen auf Gemüsearten: III Schäden durch hohe CO2-Konzentrationen bei Wasserstreß am Beispiel von Gurkenjungpflanzen' Gartenbauwissenschaft 57 178-182 (1992)
22) Goudriaan J., de Ruiter H.E. 'Plant growth in response to CO2 enrichment, at two levels of nitrogen and phosporus supply: I Dry matter, leaf area and development' Neth. J. Agric. Sc. 157-169 (1983)
23) Wallick K., Zinnen T.M. 'Basil Chlorosis: A Physicological Disorder in CO2-Enriched Atmospheres' Plant Disease 74 171-173 (1990)
24) Delucia E., Sasek T.W., Strain B.R. 'Photosynthetic inhibition after long-term exposure to elevated levels of atmospheric carbon dioxide' Photosynthesis Research 7 175-184 (1985)
25) Holbrook G.P., Hansen J., Wallick K., Zinnen T. 'Starch accumulation during hydroponic growth of spinach and basil plants under carbondioxide enrichment' Environmental and Experimental Botany 33(2) 313-321 (1993)
26) Madsen E. 'Effect of CO2 concentration on Growth and Fruit Production of Tomato Plants' Acta Agriculturæ Scandinavia 24 242-246 (1974)
27) Tripp K.E., Peet M.M., Willits D.H., Pharr D.M. 'CO2-enhanched Foliar Deformation of Tomato: Relationship to Foliar Starch Concnentration' J. Amer. Soc. Hort. Sci. 116(5) 876-880 (1991)
28) Van Berkel N. 'CO2 doseren bij Gerbera: Meer dan 800 ppm is schadelijk' Vakblad voor de Bloemisterij 25 47-50 (1983)
29) Nafziger E.D., Koller H.R. 'Influence of Leaf Starch Concentration on CO2 Assimilation in Soybean' Plant Physiol. 57 560-563 (1976)
30) Murage E.N., Watashiro N., Masuda M. 'Leaf chlorosis and carbon metabolism of eggplant in response to continuous light and carbon dioxide' Scientia Horticulturae 67 27-37 (1996)
31) Artus N.N. 'Two mutants of Arabidopsis thaliana that become chlorotic in atmospheres enriched with CO2' Plant. Cell and Environment 13 575-580 (1990)
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