The Penetration of Solar Radiation through Leaf Canopies of Different Structure

A comparative study has been made of the penetration of solar radiation through leaf canopies of contrasting structure. For this purpose experiments were undertaken on (a) Gladiolus with nearly vertical leaves, (b) Raphanus sativus with its rosette habit, (c) two varieties of Linum usitatissimum, one an unbranched flax type and the other a multi-stem linseed type, and (d) Agrostemma githago where the apical leaves are sharply inclined and the basal leaves disposed in a horizontal plane. Light gradients were evaluated at different densities and at varying stages of development by means of probes of silicon cells. Using a filter (cut off point 72-5 μ) simultaneous measurements were made of the infra-red and ‘visible’ radiation. The distributions of the leaf and stem area through the canopy were recorded so that light interception per unit surface could be assessed. The shape of the light gradient between the top and bottom of the canopy was sigmoidal for Gladiolus but much less so for the other species. Attenuation of the infra-red radiation with depth was markedly smaller. For valid estimates of the extinction coefficient (K) it is essential to include stem areas; that is on theoretical grounds half the total stem surface. The values of K were low for all species and in particular for Gladiolus. The relationship between light attenuation and leaf area was approximately exponential. For Gladiolus it was established that K can decrease with increasing density but no changes with depth in the canopy were evident. The implications of these observations in terms of canopy structure are discussed.