Measurements of the orientations of leaves of 5 well-irrigated sunflower plants were made in the field during a 24-h period from 5th--6th March, 1974. The leaves moved from a maximum easterly azimuth 12 min before sunrise (06h07) to a maximum westerly azimuth 14 min before sunset (18h49). Throughout the day the azimuth of the leaves lagged an average of 12 Β°, or 48 min in time, behind the sun's azimuth.
As a result of the heliotropism of the leaves, the maximum angle between the leafnormal and the sun's beam was, in the early morning and late afternoon, about 27 Β° less than for a spherical distribution of leaves.
The average daily interception of direct sunlight by the leaves was 40% greater than for a spherical distribution, and 9.4% greater than the maximum which can be obtained from a fixed set of leaves.
It is inferred from calculations that for sunlit leaves heliotropism may increase average daily photosynthesis by 10--23% compared to static leaf arrangements, other things being equal.
In contrast to plants in the field, plants in a growth cabinet with constant overhead illumination 16 h a day exhibited no heliotropism, the average azimuth angle varying by only 8Β°throughout the day.