THE INFLUENCE OF LIGHT ON STOMATAL DENSITY IN THE TOMATO

SUMMARY Stomatal densities (stomatal number per unit leaf area) were determined on tomato plants grown at ‘high’ (100 W m−2) and ‘low’ (20 W m−2) light in a growth cabinet. The upper epidermis of a fully expanded leaf in low light had fewer than one stoma mm−2, whilst that in high light had thirty stomata mm−2. On the lower epidermis there were about eighty and 100 stomata mm−2 in low and high light respectively. Similar differences were observed on plants growing in natural light. Stomatal initiation is most active early in the development of the leaf and effects of light on initiation were greatest at this stage, being observable within 3 days of a change in light level. Since light influenced the rate and duration of all stages of leaf expansion, the final stomatal densities achieved under natural conditions varied with the light history of the leaf. Stomatal density reached a peak before the leaf had expanded to 10% of its final area. Stomatal initiation and stomatal density both decreased from then until leaf expansion ceased. Fully‐expanded leaves of plants grown continuously in the same environment had similar stomatal densities irrespective of their position on the plant. A quantitative relationship is described between stomatal density and leaf area during the growth of a leaf.