Chloroplast Function in Guard Cells of Vicia faba L. : Measurement of the Electrochromic Absorbance Change at 518 nm

Abstract

Stomatal conductance is coupled to leaf photosynthetic rate over a broad range of environmental conditions. We have investigated the extent to which chloroplasts in guard cells may contribute to this coupling through their photosynthetic activity. Guard cells were isolated by sonication of abaxial epidermal peels of Vicia faba. The electrochromic band shift of isolated guard cells was probed in vivo as a means of studying the electric field that is generated across the thylakoid membranes by photosynthetic electron transport and dissipated by photophosphorylation. Both guard cells and mesophyll cells exhibited fast and slow components in the formation of the flash-induced electrochromic change. The spectrum of electrochromic absorbance changes in guard cells was the same as in the leaf mesophyll and was typical of that observed in isolated chloroplasts. This observation indicates that electron transport and photophosphorylation occur in guard cell chloroplasts. Neither the fast nor the slow component of the absorbance change was observed in the presence of the uncoupler carbonylcyanide p-trifluoromethoxy-phenylhydrazone which confirms that the absorbance change was caused by the electric field across the thylakoid membranes. The magnitude of the fast rise was reduced by half in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Therefore, photosystem II is functional and roughly equal in concentration to photosystem I in guard cell chloroplasts. The slow rise was abolished by 2,5-dibromo-3-methyl-6-isopropyl-1,4-benzoquinone indicating the involvement of the cytochrome b(6)/f complex in electron transport between the two photosystems. Relaxation of the absorbance change was irreversibly retarded in cells treated with the energy transfer inhibitor, N,N'-dicyclohexylcarbodiimide. The slowing of the rapid decay kinetics by N,N'-dicyclohexylcarbodiimide confirms that the electrical potential across the thyalkoid membrane is dissipated by photophosphorylation. These results show that guard cell chloroplasts conduct photosynthetic electron transport in a manner similar to that in mesophyll cells and provide the first evidence that photophosphorylation occurs in guard cells in vivo.