|T1||The definition of construction costs (CC) of plants|
|T2||How to estimate CC?|
|T3||Are there alternative methods?|
|T4||Why is it attractive to measure CC with the carbon content?|
|T5||What to take care of?|
|Construction costs are defined as the amount of glucose required to build 1 gram of biomass from "scratch", that is from minerals and photosynthates. They can be calculated for an organ, or for 1 gram of total plant. Penning de Vries (1974) has pioneered this approach for plants.|
|There are circa 250,000 different compounds known that can be found in plants.
Theoretically, one needs to know the biosynthetic pathways, and the amount of ATP and NAD(P)H
required (or produced) to drive these reactions for each of these 250,000 compounds, as well as their
concentration. This is of course not feasible.
If constituents are grouped into larger units, and average construction costs are
calculated for these groups, life becomes much easier. The eight groups I discern and
their estimated costs (g glucose required to produce 1 gram of compound) are:
|Compound||CC in g/g|
|Protein (with NO3)||
|Total Structural Carbohydrates (TSC)||
|Total Non-structural Carbohydrates (TNC)||
So, with these numbers it only requires determination of the concentration of these groups of compounds! How to do that? Preferably by using rather old methods, which are designed to measure a whole group of compounds at the same time. However, before doing that you have to separate a plant (organ) chemically into several fractions. A relatively simple method, is the following:
|This scheme requires to extraction steps, one Bligh & Dyer extraction yielding a soluble
fraction S2 and a residue R2, and a digestion of starch, yielding the fractions S3 and R3.
Most determinations are relatively simple and standard. More information can be found in, e.g.,
Poorter & Villar (1997).
The determination of lignin is explained here.
|If you add the estimated concentrations of these eight groups, you should get values close to 100%. However, especially with biomass that contains a lot of secondary compounds, even simple determinations may get fooled. Depending on organ and species, you may come within 5% of total recovery. However, in some cases you may miss out on 10 - 20%. You would not like to miss out on CC just because your determinations missed out on a part. One solution is to assume that the missing part has similar chemical composition as the part that is known. An alternative is to assume that this missing gap was (hemi)-cellulose, as this is very easily overlooked.|
|The advantage of the above method is that you know exactly which compound(s) is/are responsible for higher CC in one species or treatment than another. However, the determinations are still a lot of work, so short-cut methods have been sought. One of these alternative methods is to determine the caloric heat of a sample of plant that is combusted (Williams et al. 1987). In a second method, the elemental composition (CHNOS) is determined (McDermitt & Loomis 1981). Vertregt & Penning de Vries (1987) realized that it could even be simpler, and just derived the CC from the carbon and mineral content. The reason was that they found a nice correlation between the [C] of various compounds and their construction costs. However, there is one complication: the form of N which is taken up by the plants. If plants take up ammonium by the roots, then this form of anorganic N can be converted immediately into amino acids. However, if plants take up N as nitrate, they first have to reduce nitrate to ammonium, and this is an expensive step in terms of reducing power. The energy can come from NADH formed while breaking down sugars. Alternatively, it may come for free if plants are reducing nitrate in the chloroplasts when they have excess lights. On top of these uncertainties, for plants in nature it is sometimes not known how much N they take up in the form if nitrate and ammonium. Hence, people often calculate the CC using nitrate as a basis, and call this the maximum CC.|
Still, it is quite attractive to determine CC from C, if you correct for the nitrate-reduction step.
The concentration of C and N can easily be determined with an elemental analyser.
The total amount of minerals needs to be known and can be estimated by ashing a sample in a muffle furnace.
However, the amount of ash is not equal to the amount of minerals, as NO3 and organic acids disappear
during the ashing, but leave an oxide behind that reacts with CO2 upon cooling to form a carbonate.
Both nitrate and organic acids are easily determined. And, if you know the nitrate content,
you can deduct the nitrate-N from the total N, and this gives you an estimate of the organic N.
And as this is most protein, you can estimate the additional costs that were required to reduce the nitrate
that was necessary to produce that amount of protein. The equation then becomes (Poorter 1994):
|where C, Min and Norg are the concentration of C, Minerals and organic N (all in mg/g), respectively. The advantage of using this shortcut method over determining heat of combustion is that you know, with not too much work, the concentrations of minerals, organic acids, protein, as well as the [C] of your material, and so you have already fairly good insight in a number of parameters related to the C-economy of the plant.|