CONDENSED TANNINS IN SAINFOIN .2. OCCURRENCE AND CHANGES DURING LEAF DEVELOPMENT

A histological study examined condensed tannin (CT) formation in plant tissue samples taken from the meristematic area of very young sainfoi n (Onobrychis viciifolia Scop.) seedlings and from leaflets sampled at various stages of development in mature plants growing in the greenho use. Light and electron microscopy revealed no CT in the seedling meri stem and leaf primordia, but CTs were seen very early in leaf developm ent forming first in the vacuoles of discrete cells of the abaxial sub epidermal layer when the leaflets were recognizable, but still folded. Immature leaflets collected from the growing point of a mature sainfo in plant show similar CT formation with the abaxial cell vacuoles fill ed with CT when the new leaves have reached the 90 degrees-fold stage. As the leaflets unfold and mature, CTs begin to appear in the vacuole s of small, but discrete cells in the adaxial subepidermal layer while the tannin-containing cells in the abaxial subepidermal layer begin t o lose CT. The CT continues to increase in the adaxial layer until typ ical enlarged tannin idioblasts or sacs are observed at full leaflet e xpansion and maturity. By this stage, the vacuoles in the abaxial laye r are almost empty. In senescing leaflet samples collected from the le af rachis attached to the last and second to last node near the base o f the plant, the cells in both subepidermal layers have lost the major ity of the CT that was originally formed. At senescence all tannin-con taining cells appear as empty shells. We speculate that a finite amoun t of CT is formed in the two subepidermal layers of new leaves at diff erent stages of early leaf development, does not increase during the m ature phase, and is catabolized in older leaves and during senescence.