The Guard Cell Chloroplast: Properties and Function

Abstract

Chloroplasts are a predominant feature of guard cells, with the orchid genus Paphiopedilum constituting the only documented exception to that invariance (1). Hypotheses on roles of the guard cell chloroplast (GC Ch) in stomatal movements have recurred in the history of stomatal physiology: Von Mohl (2) in 1856, Shaw and Maclachlan (3) in 1954, and Hsiao (4) in 1976 are a few examples. Despite these trends, a conclusive elucidation of the function of GC Ch in stomatal movements remains elusive and contemporary stomatal physiologists have often considered the GC Ch unimportant or dispensable for stomatal function. This paper reviews current information on the GC Ch, including unpublished findings from our laboratory that point to a capacity for photosynthetic carbon fixation in guard cells. We also reevaluate the function of GC Ch on stomatal movements and on the coupling between stomatal conductance and mesophyll photosynthesis.