Agriculture Reference
In-Depth Information
Fig. 2.13.
Sections of onion roots illustrating, in particular, the structure of the
exodermis with long cells that become suberized and water impermeable, and short
cells that remain non-suberized and alive in older roots and even after long periods of
water stress. (a) Transverse section stained with toluidine blue. Scale bar = 250
m.
c, cortical parenchyma; e, epidermis; en, endodermis; ex, exodermis. (b) Cross-section
stained with Sudan red 7b, which shows up the deposition of water-impermeable
suberin in cell walls: e, epidermis; ex, exodermis. The walls of the exodermal cells are
positively stained except for those of a few short cells, which are indicated by asterisks.
Scale bar = 25
m. (c) A paradermal section, partially cleared and stained with trypan
blue. Short cells, such as the one indicated by an asterisk, stain blue. Scale bar =
70
m. (d) Median longtidunal section of an onion root stained with Sudan red 7b;
e, epidermis. The inner tangential wall (arrowhead) of the short cell (asterisk) is
unstained, indicating that it is not suberized and is therefore likely to be permeable to
soil water. Scale bar = 15
m. (e) Paradermal section of root from a plant that had not
been watered for 200 days. Stained with fluorescein and viewed under blue light.
There is a striking accumulation of fluorescein in the cytoplasm and nuclei of the short
cells of the epidermis, which indicates that they were alive, whereas long cells are
unstained and were therefore dead. Scale bar = 100
m. (a and e from Stasovski and
Peterson, 1993. Courtesy of
Canadian Journal of Botany
; b, c and d from Kamula
et al
.,
1994. Courtesy of
Plant, Cell and Environment
).
go on to develop water-repellent suberin lamellae on all their cell walls.
Consequently, water and nutrient ion entry into the roots becomes restricted to
those short cells lacking suberin lamellae. Hence, water and ion entry into the
inner cell layers of the root involves crossing the living plasmalemma
membrane bounding the cytoplasm of these cells and is subject to metabolic
control there (Barrowclough.
et al.
, 2000; Cholewa and Peterson, 2004).
When onion roots are subject to drought the root tips die, as do the epidermal
cells external to the suberized exodermis (Stasovski and Peterson, 1993). The
inner tissues remain protected from water loss by the outer suberized shell of
