in [13]), which can result in lower rates of photosynthesis under
CO2 limited conditions. This down-regulation was evident in high
CO2-grown wildtype plants where the CO2-saturated rate of photosynthesis
was slightly lower in plants exposed for 7 days (Fig. 2B)
and 50% lower in plants exposed for 2 weeks (data not shown)
compared to plants cultured under ambient CO2. In contrast, CO2-
saturated photosynthesis was the same or somewhat higher in
high CO2-grown TLUp1-9 plants than in ambient CO2 plants. CO2-
enriched TLUp1-9 plants accumulated about 40% more leaf starch
than WT plants grown under the same conditions. Hence, TLUp1-9
plants may have avoided the down-regulatory response in photosynthesis
observed with prolonged exposure to elevated levels of
CO2 due to their increased capacity for starch synthesis which, in
turn, prevented feedback on photosynthesis.
The buildup of starch in response to prolonged exposure to
elevated atmospheric CO2 has been proposed to reduce photosynthesis
due to damage to chloroplasts [21] or increased diffusive
resistance to CO2 [22,23]. Our results, however, demonstrate that
excess accumulation of leaf starch alone does not inhibit photosynthesis
as evident by the properties exhibited by CO2-enriched
TLUp1-9 plants. This plant line maintained normal or slightly elevated
rates of CO2 assimilation rates although accumulated more
than 2-fold greater levels of leaf starch at the end of the day than
plants grown under ambient conditions. Hence, the reduction in
photosynthesis in plants grown under prolonged elevated CO2
conditions is more likely due to the limitation in the conversion
of triose-P into carbohydrates resulting in photosynthetic feedback
where the levels of Rubisco or photosynthetic apparatus are
reduced [13].