|
|
|
|
WIMOVAC Respiration ModuleThe oxidation of carbohydrate to CO2 and H20 in living cells is generally termed respiration. In plant vegetation there are two main types of respiration. The first is called dark respiration (Rd) and includes various pathways of substrate oxidation such as glycolysis, the oxidative pentose phosphate pathway and the tricarboxylic acid (TCA or Krebs) cycle. These conserve some of the free energy in carbohydrate in the high energy bonds of ATP, reduced pyridine nucleotide (NADH) and FADH2. The term dark respiration also covers the further oxidation of NADH and FADH2 by transfer of electrons through the various electron transfer complexes of the mitochondrial electron transport pathway. The second type of respiration in plants is called photorespiration (reviewed by Ogren, 1984 [Ogren, 1984 #558]). This is the pathway of CO2 production via the photorespiratory carbon oxidation (PCO) cycle. The same enzyme (Rubisco) that catalysis the carboxylation of RuBP as the first step of the PCR cycle can also catalyse the oxygenation of RuBP to phosphoglycollate as the first step of the PCO cycle. Details of respiratory metabolism in plants are reviewed in Amthor (1991) [Amthor, 1991 #1751]. A number of approaches to modelling respiration have been taken. Amongst the first, considered plant respiration to be a fixed rate per unit plant weight. However this approach was rejected by McCree (1970) [McCree, 1970 #1822] who showed that respiration could be more closely related to photosynthesis. McCree (1970) showed that respiration could be related to a linear function, in which canopy respiration equals a fixed proportion of gross photosynthesis in addition to a rate determined by the dry weight of the plant. A formal expression of McCree (1970) formulation is given by Equation 129,
where a and b are experimentally determined coefficients (day-1), Agross is the gross photosynthetic rate (g m-2 day-1) and w is the vegetation dry matter (g m-2 ). Typical values of a and b reported in the literature suggest that respiration accounts for 25% of gross photosynthesis plus 1.5% of existing plant dry weight per day. The respiration associated with photosynthesis (a.Agross) is often termed growth respiration and that involved with turnover and repair of plant material (b.w ) maintenance respiration. In response to difficulties in our present understanding of respiration and in the measurement of key respiratory parameters wimovac contains a number of interchangeable respiration processes. These are controlled by a software switch in the respiration section of the model parameter database and consist of the following options: i). The model assumes that no respiration takes place and growth is therefore a hypothetical potential growth. ii) Dark respiration (Rd) is applied at the leaf level according to the Farquhar & von Caemmerer (1980) and Collatz et al. (1992) models but no plant structure maintenance respiration is considered. iii). The McCree (1970) equation is combined with the canopy photosynthesis and growth modules to give total plant respiration. iv). A modified form of the McCree (1970) equation in which maintenance respiration is considered to be a simple constant.
v). A modified form of the basic McCree (1970) equation suggested by Penning de Vries (1972) and Thornley (1970) in which the fraction of gross photosynthesis associated with growth respiration is assumed constant but the respiratory cost of maintaining plant structures varies according to a largely assumed pattern of metabolic activity.
A temperature response model is applied to the calculated respiration values according to the model parameter database switch which has 3 options: i). There is no temperature response and respiration is assumed to be independent of temperature. ii). An Arrhenius function is applied which has a similar structure to Equation 17 iii). A Q10 function with a user defined coefficient (default of 2) is applied. |
Send mail to humph@essex.ac.uk with questions or comments about this web site.
|
