Levels of ribulose bisphosphate carboxylase (Rubisco) and Rubisco
activase were compared in leaves
above the ear in two genetically related populations (Z0 and
Z20) of maize (Zea mays L.). Z20 was
obtained from Z0 after twenty agronomic selection cycles for
grain
yield improvement (c. 90% above
Z0). Plants were cultivated in the highlands of Mexico and leaves
were sampled weekly during the
grain-filling period. Chlorophyll, soluble protein and Rubisco activity
were
measured. Chlorophyll
and soluble protein content slowly decreased during this period, the former
faster than the latter, with
no significant differences between populations. During the first 40 days
after anthesis, Rubisco
activity was significantly greater in the high-yielding population
(Z20), although Western blot analysis
of Rubisco showed similar values for both populations within this period.
However, the same analysis
for Rubisco activase indicated a greater amount of this protein in the
higher-yielding population (Z20)
than the original one (Z0) during the early and middle part
of the
grain-filling period. The addition
of Rubisco activase and an ATP-generating system to Z0 leaf
extracts
resulted in increased Rubisco
activity. It was concluded that during grain-fill in maize, the level of
Rubisco activase has a regulatory effect on Rubisco activity expression.