Mechanisms of Foliar Penetration of Solutions

From a classical point of view green leaves appear to be organs whose sole purpose is the production of organic materials by photosynthesis. While gases like CO2 and O2, both necessary for and arising from assimilation and respiration, are exchanged by stomata and intercellular spaces, water and mineral salt s are supplied by the roots. However, during the last few decades it has become more and more evident that absorption of inorganic and or­ ganic materials can also take place through the surfaces of leaves. This is rather surprising at first sight, since these surfaces are covered by the cuticle, which has been considered impenetrable. The same is true for excretion. Aside from gases and water vapour, it was thought that other materials, which are no longer useful to the plant or which are surplus, could not leave the assimilating organs but must be stored within the vacuoles or in special tissues. It has been shown, however, that some export of substances takes place from roots and even more from leaves which may sometimes be impor­ tant for the plant itself and perhaps for plant communities. There are numerous reports on foliar spraying, foliar abso rption and foliar leaching, many of which have already been discussed in former reviews in this series (8, 69, 107). Since then, the number of substances applied to leaves has grown enormously; more and more growth regulators, pesticides and nutrients are used for theoretical and practical purposes. The results of such work have been reviewed primarily in relation to applied problems during the last few years (11-13, 17, 36, 44- 46, 108, 109) . While practical aspects were emphasized, the location of the sites of foliar absorption and excretion and the nature of the underlying mecha­ nisms have more recently been investigated. Since substances to be absorbed or excreted by leaves must pass through a cell wall covered by a cuticle, the process of penetration may be somewhat different from that of absorption by root cells, which do not possess an outer layer structurally comparable to that of the leaves. The purpose of this review is to present current views, to reveal gaps in the existing knowledge and to reflect on both. No attempt has been made to refer to the whole body of information available at present. The amount of published work has made it necessary to select only literature directly relevant to the matters considered below. Further literature is cited in reviews referred to above. To understand the mechanisms of foliar penetration it is necessary to find out what the pathways are through which penetrating solutions pass