rates of leaf senescence [29,30]. Additional support is provided by studies with transgenic plants that have
increased internal sugar levels [11,26,31–35]. These plants have chlorotic or yellow leaves with reduced
rates of photosynthesis.
II. INCREASED SOURCE STRENGTH: ELEVATED CO 2 AND THE
“TEMPORAL SHIFT” MODELDespite the attractiveness of the “feedback inhibition” hypothesis to explain the patterns of change that
occur in photosynthetic rates during leaf development, very few studies have directly investigated the
impact of carbohydrates on leaf developmental programming. We have previously examined various
photosynthetic parameters during tobacco leaf development under conditions of increased source
strength (carbohydrate production) [28]. In these experiments, individual leaves were examined under
ambient CO 2 levels (approximately 350 L/L) or enriched CO 2 concentrations (950 L /L). Leaf 10
(counting up from the base) was chosen for analysis because of its large final size. The elevated CO 2
regime was initiated at the time of visible leaf emergence, and measurements were made at various time
points until the leaf abscised. As illustrated in Figure 1, ambient CO 2 –grown leaves exhibited increas-
ing CO 2 exchange rates (CERs) up to day 12 (coincident with leaf expansion), a transient maximum (at
full expansion), then a steady decline from day 14 onward. The high CO 2 –grown leaves, on the other
hand, attained a similar photosynthetic maximum, but they reached this maximum significantly earlier.
The patterns of senescent decline in photosynthetic rates were comparable in both sets of leaves inas-
much as the duration of the senescence phase appeared to be unchanged. The major difference was that
118 RODERMEL ET AL.Figure 1 Photosynthetic rates (CERs) during development of tobacco leaves grown under ambient or
elevated CO 2 conditions (top panel) or compared with developmentally similar leaves from Rubisco antisense
plants (bottom panel). The wild-type and antisense plants were maintained under identical growth conditions.
For the increased source strength studies, wild-type plants were moved into high CO 2 when leaf 10 reached 1
cm in length. In all cases, day “1” status was assigned when the leaf reached 3 5 cm in length. Each point
represents the average ( SD) of multiple measurements on leaves from at least four different plants. (Adapted
from Refs. 27 and 28.)