Productivity assimilation rate

Jorgensen, N. O. G. 1987. Free amino acids in lakes Concentrations and assimilation rates in relation to phytoplankton and bacterial production. Limnology and Oceanography 32 97-111. 

While the triazine resistant biotypes of virtually all weed species studied differ substantially from their susceptible counterparts in photosynthetic efficiency at the level of the light reactions, data describing whole plant performance is actually quite variable. For example, in one report, relative differences in C02 assimilation rate between susceptible and resistant biotypes varied among six species examined 136). In other reports, resistant biotypes were actually more productive and competitive than susceptible under some conditions (49-51). It is difficult to interpret such data since most research to date has been conducted with weed populations of uncertain genetic backgrounds. 

The recovery of assimilation rate included both an increase in the efficiency of the carboxylation processes and the level of carbon reduction cycling. This was probably due to increasing reductant production as the light reactions recovered, which would both induce carboxylation enzymes and allow greater levels of carbon cycling. 

These data show that morphological and physiological modifications associated with water stress able cvar Clare to maintain higher rates of photosynthesis and plant production, but also that the water economy indexes were superior in this cultivar even under irrigation, showing the efficiency of this indexes and also the existence of intraespecific variability in photosynthesis rates under drought that able cultivar Clare to maintain better CO2 assimilation rates by water stressed leaves. 

Significant differences in net photosynthetic assimilation of carbon dioxide are apparent between C, C, and CAM biomass species. One of the principal reasons for the generally lower yields of C biomass is its higher rate of photorespiration if the photorespiration rate could be reduced, the net yield of biomass would increase. Considerable research is in progress (ca 1992) to achieve this rate reduction by chemical and genetic methods, but as yet, only limited yield improvements have been made. Such an achievement with C biomass would be expected to be very beneficial for foodstuff production and biomass energy appHcations. 

The subsequent fate of the assimilated carbon depends on which biomass constituent the atom enters. Leaves, twigs, and the like enter litterfall, and decompose and recycle the carbon to the atmosphere within a few years, whereas carbon in stemwood has a turnover time counted in decades. In a steady-state ecosystem the net primary production is balanced by the total heterotrophic respiration plus other outputs. Non-respiratory outputs to be considered are fires and transport of organic material to the oceans. Fires mobilize about 5 Pg C/yr (Baes et ai, 1976 Crutzen and Andreae, 1990), most of which is converted to CO2. Since bacterial het-erotrophs are unable to oxidize elemental carbon, the production rate of pyroligneous graphite, a product of incomplete combustion (like forest fires), is an interesting quantity to assess. The inability of the biota to degrade elemental carbon puts carbon into a reservoir that is effectively isolated from the atmosphere and oceans. Seiler and Crutzen (1980) estimate the production rate of graphite to be 1 Pg C/yr. 

The quantification of gross root production, rhizodeposition, microbial assimilation, and the production of organic materials in soil has made increasing progress ever since stable ( C) and radioactive ( C) carbon isotopes have been used (see Chap. 12). Measurements of soil organic matter dynamics without these isotopes are difficult due to the large amount present as compared to the smaller rates of input. 

Several studies have measured DFAA concentrations and turnover (see Chapter 4 and Munster, 1993), but here we concentrate on those that compare DFAA uptake with bacterial production. The fraction of bacterial production supported by DFAA is one index for the relative importance of amino acids, not only in supporting bacterial growth but also in the overall flux of DOM. ( Flux is used here to indicate both production and uptake in a quasi-steady state.) If DOM concentrations are constant, DOM production will equal total uptake rates by microbes there is no evidence of photo-oxidation of amino acids and of the other compounds discussed here (see Chapter 10). Total uptake includes respiration and assimilation into biomass. Here assimilation is defined as the appearance of a radioactive compound in cells (both cellular LMW and HMW pools) respiration is excluded. 

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