THE MODIFIED STOMATA OF THE FLORAL NECTARY OF VICIA-FABA L. .3. PHYSIOLOGICAL-ASPECTS, INCLUDING COMPARISONS WITH FOLIAR STOMATA

A comparison of developmental features and physiological responses bet ween the modified stomata (MS) of the floral nectary and the stomata o f leaves of Vicia faba L. has revealed several significant differences . In mature tissues, the frequency per unit area of MS is three times that of foliar stomata, and when only the distal quarter of nectary pr ojections is considered, it is twelve fold higher. The walls of guard cells (GCs) of MS near the pore were four to five times thinner than t hose of their foliar counterparts. Average pore widths of the nectary MS remained constant throughout the day and night, contrary to those o f leaves; maximal foliar apertures were similar to average pore widths of MS of nectaries. Experiments with plasmolytica demonstrated a high er osmotic potential in nectary MS, and that sister GCs of the nectary only occasionally closed their pores by movements, even when they wer e plasmolysed, whereas GCs of leaves were found to be plasmolysed only after their pores had shut. Abscisic acid (ABA) was always detected n aturally in floral nectar. The results of experiments involving K+-loc alization and the secretion of large quantities of ABA in nectar sugge sted that GCs of the MS lack operational systems involving K+-influx a nd ABA-receptor sites. All information gained is consistent with the e arlier conclusion that the MS do not have a regulatory role in nectar secretion by flowers of V. faba.