The positive effect of [CO2] on crop growth may be counteracted by the effect of a concomitant temperature rise. Such effects cannot be traced back to one or a few critical physiological and morphological characteristics because the linkages between plant productivity and physiological characteristics are concealed by feedback control mechanisms by interactions among varying environmental conditions, and changes in crop characteristics during its phenological development. Simulation models are probably the only tools for interpreting and predicting the impact of an environmental change on plant production or evaluating the best strategy for further experimental research. Several crop growth models have been formulated for potato that differ in approach and detail (Fishman et al., 1984; Ng and Loomis, 1984; MacKerron and Waister, 1985).
A simulation model of potato growth was used to highlight the importance of CO2/temperature interactions on potato productivity and their implications for varietal choice (Schapendonk et al., 1995). The model was previously described (Spitters and Schapendonk, 1990). Briefly, that model calculates the light profile within the canopy on the basis of a leaf area index and the extinction coefficients for both the flux of direct solar radiation and diffuse sky light. The rates of photosynthesis at various heights within the canopy are calculated using the von Caemmerer and Farquhar (1981) model. Discrimination was made between shaded leaf area receiving diffuse radiation only, and sunlit leaf area receiving both diffuse and direct radiation (Spitters et al., 1986). Daily gross canopy assimilation rates are obtained from instantaneous photosynthetic rates of individual leaves and the light profile in the canopy, integrated over the canopy and day (Goudriaan, 1986). A death rate of leaves due to senescence depends on the maturity class (Spitters and Schapendonk, 1990).
Simulations were made with this model to calculate total dry matter production and tuber yield of different potato cultivars, growing on a sandy loam under Dutch weather conditions. Averaged over the years 1988-1991, a simulated increase of the [CO2] from 350 |mmol mol-1 to 700 |mmol mol-1 increased the tuber dry matter production by 22% for late cultivars and 29% for early cultivars. The effects were smaller for late cultivars, irrespective of the occurrence of a drought period. Elevated temperature reduced the positive effects of elevated [CO2], because the stimulation of leaf area expansion in the juvenile stage was offset later in the season by earlier foliage senescence. Temperature increases had only a net positive effect on production in situations with optimal water supply and high irradiance during the juvenile stage, combined with a severe late drought. In those situations, the larger foliage prevented a rapid decrease in light interception during senescence. Recently, a simulation study was performed within an EU-funded project (CLIVARA) to determine the effect of climatic change on potato yields at several sites and on regional scales. A detailed potato model (NPOTATO) was used to analyse the effects of climatic change, climatic variability and increased [CO2] on potato production throughout Europe (Wolf, 1999a). In addition, a simplified potato model (POTATOS) was used to analyse the effects of climatic change on potato at a regional scale (Great Britain: Downing et al., 1999; Wolf, 1999b). The model simulations for both studies were made for baseline climatic and future climatic scenarios (corresponding with the period 2035-2064) obtained using the HADCM2 (Hadley Centre unified model climate change experiment) greenhouse gas experiment (HCGG, greenhouse gas only integration) and the HADCM2 greenhouse gas with sulphate aerosol experiment (HCGS, sulphate and greenhouse gas integration), both with an increase in climatic variability (HCGGv, greenhouse gas only integration, including changes in climatic variability; and HCGSv, sulphate and greenhouse gas integration including changes in climatic variability) and without. For the baseline climate, the [CO2] was set at 353 | mol mol-1 and for the four scenarios at 515 |mmol mol-1.
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