Carbonic anhydrase activity and CO2-transfer resistance in Zn-deficient rice leaves

It has been reported that carbonic anhydrase (CA) activity in plant leaves is decreased by Zn deficiency. We examined the effects of Zn deficiency on the activity of CA and on photosynthesis by leaves in rice plants (Oryza sativa L.). Zn deficiency increased the transfer resistance from the stomatal cavity to the site of CO2 fixation 2.3-fold and, consequently, the value of the transfer resistance relative to the total resistance in the CO2-assimilation process increased from 10% to 21%. This change led to a reduced CO2 concentration at the site of CO2 fixation, resulting in an increased gradient of CO2 between the stomatal cavity and this site. The present findings support the hypothesis that CA functions to facilitate the supply of CO2 from the stomatal cavity to the site of CO2 fixation. We also showed that the level of mRNA for CA decreased to 13% of the control level during Zn deficiency. This decrease resembled the decrease in CA activity, suggesting the possible involvement of the CA mRNA level in the regulation of CA activity.