Chloroplast movements in the field

An ecophysiological understanding of chloroplast movements in leaves requires measurement of these movements under field conditions. A field-portable instrument was constructed, based on a pulsed measuring beam and lock-in detection that measures chloroplast movements in attached leaves by sensing the resultant changes in leaf transmittance. In the instrument and generally in nature, leaves are illuminated obliquely, in contrast with the perpendicular illumination used in most laboratory experiments on chloroplast movement. Microscopic analysis of cells illuminated obliquely with bright light verified that chloroplasts move out of the light path, and transmittance changes in response to oblique light were robust. Chloroplast movements in Alocasia brisbanensis under natural sunlight express kinetics and light requirements expected from laboratory observations: chloroplasts were in the periclinal position at dawn and dusk, anticlinal position in full sunlight midday, and in an intermediate position at night. Movement in response to bright light was rapid allowing responses to brief sunflecks. Movements in Helianthus tuberosum, Eustrephus latifolius and Cissus hypoglauca were qualitatively similar with differing kinetics and magnitude. In all four species, chloroplasts were in motion most of the time, rarely achieving the extreme anticlinal or periclinal positions.