Abstract:
An inherently short vase life is a problematic characteristic of cut flowers and
foliage for otherwise attractive native Australian Acacia spp. Reasons underlying the
poor postharvest water uptake of cut acacia stems have been elusive. A. holosericea
was used to investigate possible bacteria-induced and wound-induced xylem
occlusion. The effects of bacterial- and wound-induced xylem blockage on water
uptake were investigated by light and scanning and transmission electron microscopy.
Observations were made on cut stems that stood into either deionised water (DIW;
control) or 0.5 mM Cu2+ solution and on stems pulsed with 2.2 mM Cu2+ solution and
then stood into DIW. The stem-end region of cut A. holosericea that stood into DIW or
Cu2+ solution became covered with bacterial growth after 3 days. Regardless of the
bacterial biofilm, the Cu2+ treated stems had improved water relations and vase life.
Therefore, the biofilm had little or no effect on cut A. holosericea longevity. Further
observations revealed presence of a vessel-occluding substance (gel) originating from
axial parenchyma cells in direct physical contact with xylem vessels. The gel exuded
into vessel lumens through pit membranes, evidently as a wound-response. Xylem
occlusion by gels in A. holosericea may be especially problematic due to an abundance
of secretory contact cells relative to xylem elements. Nonetheless, active wound
response processes may be the key determinant of short postharvest longevity for this
and possibly other cut Acacia spp. Cu2+ treatments, however, disrupted the secretory
function of axial parenchyma cells thereby preventing vessel occlusion by the gels.
