EVALUATION OF A MODEL WHICH PREDICTS WHOLE PLANT RESPIRATION FROM PHOTOSYNTHESIS AND BIOMASS.

摘要

Over time, whole plant growth is nearly the balance between photosynthesis and respiration. This research consisted of devising and testing a model which predicts plant respiration rates from knowledge of the photosynthesis rate and initial weight of the biomass, on an instantaneous basis. Parameter determination, establishment of initial conditions, calculations of photosynthesis rate, and model testing were carried out using only carbon exchange rate (CER) data and the final dry weight in carbon equivalents.;Previous research has shown that a plant grown in a constant light and temperature regime will reach a state where no net change in POOL occurs over a light-dark cycle. In this state, a plant's daily respiration was found to equal (1-YG) of the daily photosynthesis plus a fraction of the plant biomass in carbon equivalents. Simulation studies indicated this model can replicate these findings. The model is dynamic and can predict plant growth and respiration even if POOL varies due to changes in photosynthesis rate. Additionally, observed effects of POOL size on photosynthesis were replicated.;To evaluate the model and the effect of temperature on its parameters, grain sorghum (Sorghum bicolor (L.) Moench) plants were grown at 25, 30, and 35(DEGREES)C for about 3 weeks, reaching the 9 leaf stage. Plant response for each growth temperature was tested at 25, 30, and 35(DEGREES)C resulting in a 3 growth temperature by 3 test temperature factorial design. The plants were placed in an assimilation chamber and CER data collected. The plants were initially subjected to about 48 hours of darkness allowing determination of E, B, and initial levels of SDM and POOL. YG was assumed to have a value of .75 from previous work. MAXGRO was never achieved and thus, not required.;The model was tested by turning on the lights continuously except for brief, intermittent, periods of darkness. Photosynthesis was calculated by adding the CER measurement to a value of respiration predicted from the POOL and SDM levels. In turn, new values of POOL and SDM were computed from photosynthesis, predicted growth, and maintenance rates. The test of the model was the comparison of measured and predicted respiration rates during the dark periods.;The model considers the plant to consist of two quantities, the photosynthate pool (POOL) and the structural dry matter (SDM). These are acted upon by three processes: photosynthesis, SDM synthesis, and SDM maintenance. Photosynthesis fills the POOL. A fixed fraction of the POOL, the yield of growth (YG), is converted to SDM. The remainder provides energy for SDM synthesis and is respired as growth respiration. SDM synthesis proceeds at a rate proportional to POOL size until a maximum growth rate (MAXGRO), proportional to SDM is reached. When SDM synthesis is less than MAXGRO, POOL usage rate is determined by the POOL efflux coefficient (E), a fraction of POOL per hour. Energy costs for maintaining SDM are borne by SDM, giving rise to maintenance respiration. This is described by the maintenance coefficient (B), a fraction of SDM per hour.;It was concluded that: (1) The model can predict whole plant respiration with an accuracy of (+OR-) 25% using as an input CER data with an accuracy of (+OR-) 10%. (2) No temperature effect on E or B was observed. (3) Since MAXGRO was never achieved, photosynthesis, rather than SDM synthesis limited biomass production. (4) The model and its limits are well-defined, such that its use can be restricted to those situations where it is sufficiently accurate.