The Modified Stomata of the Floral Nectary of Vicia faba L.

Abstract

A comparison of developmental features and physiological responses between the modified stomata (MS) of the floral nectary and the stomata of 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 projections 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 those of their foliar counterparts. Average pore widths of the nectary MS remained constant throughout the day and night, contrary to those of leaves; maximal foliar apertures were similar to average pore widths of MS of nectaries. Experiments with plasmolytica demonstrated a higher osmotic potential in nectary MS, and that sister GCs of the nectary only occasionally closed their pores by movements, even when they were plasmolysed, whereas GCs of leaves were found to be plasmolysed only after their pores had shut. Abscisic acid (ABA) was always detected naturally in floral nectar. The results of experiments involving K+‐localization and the secretion of large quantities of ABA in nectar suggested that GCs of the MS lack operational systems involving K+‐influx and ABA‐receptor sites. All information gained is consistent with the earlier conclusion that the MS do not have a regulatory role in nectar secretion by flowers of V. faba.

A Permeable Cuticle, Not Open Stomata, Is the Primary Source of Water Loss From Expanding Leaves

“…Once that cuticle tears and the outer cuticular ledge is formed, Q. rubra stomata are capable of sustaining maximum water loss rates through the pore. These cuticle coverings in young stomata have been observed multiple times in A. thaliana (Serna and Fenoll, 1997;Nadeau and Sack, 2002;Hunt et al, 2017), in Hydrocotyle bonariensis (Koch and Barthlott, 2009), the stomata on the flowers of Vicia faba (Davis and Gunning, 1993), and now Q. rubra. Given that we observed these in both Q. rubra and A. thaliana, and stomatal development and developmental genes are highly conserved across land plants, this cuticular covering of young stomata may be a feature common to all vascular plants (Chater et al, 2017).…”

“…Cuticles also appear to cease developing in chemical composition once leaves cease expanding (Hauke and Schreiber, 1998). Furthermore, very young stomata are covered in a cuticle (Davis and Gunning, 1993;Nadeau and Sack, 2002;Hunt et al, 2017). Breaking of this cuticle covering layer in leaf development to form the outer cuticular ledge may be responsible for reported increases in leaf gas exchange as leaves expand (Constable and Rawson, 1980).…”

Comparative Nectary Morphology across Cleomaceae (Brassicales)

“…Nectarostomata are often described as continuously open and unable to control nectar secretion [ 4 ]. Yet, nectarostomatal aperture regulation has only been thoroughly studied in Vicia faba (Fabaceae) [ 2 , 25 , 26 ]. In this taxon, nectarostomata development is asynchronous with most opening a few days prior to anthesis and rarely closing once mature [ 25 ].…”

Anatomical analyses of floral and extrafloral secreting structures indicate the presence of nectaries and colleters in Stanhopea grandiflora Lindl.

“…Nectariferous stomata are considered to be ''modified'' with respect to leaf stomata because they are not able to finely regulate their aperture (Davis and Gunning 1993). The nectar exits through modified stomata that remain permanently open or through specialized trichomes (Wist and Davis 2006;Vassilyev 2010).…”