The effect of starch accumulation levels on CO2 assimilation
rates were studied in more detail by comparing the properties of
WT, TL46 and TLUp1-9 grown under ambient CO2 and elevated CO2
(1000 ppm) levels. TLUp1-9 was selected as it exhibited the most
uniformity in growth phenotype among the transgenic lines. In
ambient CO2-grown plants the rates of photosynthesis were lowest
in TL46 and progressively greater in WT followed by the highest
rates in TLUp1-9 particularly at higher CO2 levels (Fig. 2A), which paralleled
starch accumulation patterns (Table 1). In high CO2-grown
plants the differences between the three genotypes were accentuated.
In this case, the rate of CO2 assimilation measured under high
CO2 in TLUp1-9 was 8 mol m−2 s−1 greater than that of WT and
16 mol m−2 s−1 greater than that of TL46 (Fig. 2B). These heightened
differences in CO2 assimilation rates were due to the divergent
responses by these plants under prolonged culture under elevated
CO2 concentrations. Typically when plants are grown under CO2
enrichment they down-regulate their capacity for CO2 assimilation
(e.g. reduction in Rubisco content, see results with WT Arabidopsis