Handbook of Plant and Crop Physiology

were sampled just prior to flowering. The leaves were fully expanded (and thus senescing) with the ex-

ception of the youngest ones at the top of the plant. These analyses showed that photosynthetic rates

(CERs) are depressed in the antisense leaves but that the overall patterns of change are similar in the mu-

tant and wild-type plants: after attaining a maximum in young fully expanded leaves, photosynthetic ca-

pacities decline progressively in the older leaves. Alterations in chlorophyll content and in intercellular

CO 2 concentrations (Ci) did not closely parallel the changes in CER in either the wild type or mutant, sug-

gesting that light harvesting and stomatal conductance do not strongly limit photosynthesis during leaf

development in these plants. By contrast, the patterns of change in CER correlated well with changes in

Rubisco initial and total activities as well as with changes in Rubisco content (Figure 4A). “Initial” Ru-

bisco activities provide an estimate of the amount of activated enzyme in the leaf sample at the time of

harvest, and “total” activities provide a measure of the amount of Rubisco that is capable of being acti-

vated in the sample.

The correlation between initial activities and Rubisco contents suggests that Rubisco activity is pri-

marily a function of holoenzyme concentration in leaves from the antisense and wild-type plants, regard-

less of leaf nodal position. Consistent with this notion, the activation state of the enzyme (the ratio of ini-

tial to total activities) was similar in all of the leaves from both sets of plants. Collectively, these data

suggest that Rubisco is a primary determinant regulating photosynthetic rates during leaf development,

regardless of holoenzyme concentration. This is consistent with flux-control measurements on first fully

expanded leaves of the antisense plants showing that Rubisco activity can explain ~70% of the control on

photosynthetic rates under moderate to high light intensities [47,48].

Jiang and Rodermel [13] also examined the mechanism of Rubisco accumulation in the antisense

plants: is it similar to that in plants growing in tissue culture on sucrose-containing medium? For these

analyses, Rubisco subunit mRNA levels were measured by RNA gel blot analysis as a function of leaf

nodal position (Figure 4B and C). As mentioned earlier, Rubisco concentrations in both sets of plants

are highest in the youngest fully expanded leaves at the top of the plant and decrease progressively to

the oldest leaves at the bottom of the plant (Figure 4A); LS and SS proteins are not present in excess

in either set of plants. The RNA gel blot analyses showed that, in the wild type, the alterations in Ru-

bisco abundance are due primarily to coordinate changes in rbcS and rbcL transcript accumulation. In

the antisense plants, however, Rubisco concentrations appear to be controlled by the abundance of

rbcS, but not rbcL, mRNAs; the levels and patterns of change in rbcL mRNA were normal in the mu-

tants even though they accumulated less LS protein. This suggests that LS accumulation is regulated

posttranscriptionally during antisense leaf development, mirroring the situation in the tissue

SOURCE STRENGTH AND LEAF DEVELOPMENT 121

Figure 3 Growth of Rubisco antisense and wild-type (WT) tobacco. Plant height (cm) was plotted as a
function of days after planting. The samples included WT and antisense plants with either 40% (Mutant 1) or
20% (Mutant 2) of WT Rubisco amounts. Arrows signify the beginning of flowering. (Adapted from Ref. 13.)