Photoinhibition of Intact Attached Leaves of C₄ Plants: Dependence on CO₂ and O₂ Partial Pressures

Abstract

Fully expanded intact attached leaves of Zea mays and other C₄ plants illuminated at 2000 µE m^-2 s^-1 in CO₂-free N₂ containing 10 mbar O₂ for a 3-h treatment period showed substantial inhibition of the capacity for both CO₂-dependent and light-dependent CO₂ assimilation. This photoinhibition was reflected at the chloroplast level as substantial inhibition of photosystem II activity of the mesophyll chloroplasts. Photoinhibition was largely insensitive to the O₂ partial pressure maintained throughout the treatment period. This result suggests that internal CO₂ generation by photorespiration with subsequent carbon cycling, or O₂ uptake in a Mehler-type reaction are largely unable to alleviate photoinhibition. The extent of photoinhibition was, however, markedly affected by the CO₂ partial pressure present throughout the treatment period. This result is consistent with the CO₂-concentrating function of C₄ photosynthesis, which allows substantial rates of CO₂ assimilation even at low mesophyll CO₂ partial pressures. Apparently, a certain level of CO₂ turn- over is beneficial in providing a sink for photochemically generated energy, thereby maintaining the ability of illuminated leaves to transduce and dissipate light energy.