Advice on Construction Costs

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?
T6 Further reading

     Topic 1: The definition of construction costs (CC) of plants

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.

     Topic 2: How to estimate CC?

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
Soluble phenolics
Protein (with NO3)
Total Structural Carbohydrates (TSC)
Total Non-structural Carbohydrates (TNC)
Organic Acids

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:

Extraction Scheme
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.

     Topic 3: Are there alternatives?

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.

     Topic 4: Why is it attractive to measure CC with the carbon content?

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):
Extraction Scheme
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.

     Topic 5: What to take care of?

  • First, make sure that you only have plant material, not that of pathogens, or soil.

  • Second, to preserve the chemical composition, it is best to freeze-dry your fresh material.

  • Third, it would be good to know what the dominant form of N is that your plants take up.

  • Fourth, these determinations are still laborious, and may take a lot of work if you want to determine this for each individual plant. The solution I follow is to bulk all leaves, stems or roots from 1 treatment and 1 species into two independent samples, and to determine the various compounds in duplo on each of these samples. This will reduce your variability, but also the degrees of freedom, so in the end, the power of your statistical test will be less. But that means you have a conservative approach!

  • A fifth point of attention is that these days, methods are scaled down to work with very small amounts of sample. This requires less amount of reagentia, but at the same time it requires a very good milling and mixing before you take your 3 mg out of a total bulked biomass of maybe 300 gram. That is 0.001% !!

  •      Topic 6: Further reading

  • McDermitt & Loomis (1981) Elemental composition of biomass and its relation to energy content, growth efficiency, and growth yield, Ann. Bot. 48: 275290.
  • Penning de Vries et al. (1974) Products, requirements and efficiency of biosynthesis: a quantitative approach. J. Theor. Biol. 45: 339-377.
  • Poorter (1994) Construction costs and payback time of biomass: A whole plant perspective. In: Roy J & Garnier E (eds). A Whole Plant Perspective of Carbon-Nitrogen Interactions. SPB Academic Publishing, The Hague, pp. 111-127.
  • Poorter & Villar (1997) Chemical composition of plants: causes and consequences of variation in allocation of C to different plant constituents. In: Plant Resource Allocation. Bazzaz F & Grace J (eds). Academic Press, New York, pp. 39-72.
  • Vertregt & Penning de Vries (1987) A rapid method for determining the efficiency of biosynthesis of plant biomass. J. Theor. Biol. 128: 109-119.
  • Williams et al. (1987) Estimation of tissue construction cost from heat of combustion and organic nitrogen content. Plant Cell & Environ. 10: 725-734