The roots of the allium vegetables are comparatively thick and sparsely branched as compared with those of most crop species. They lack root hairs except when they are grown in moist air rather than in soil or solution culture.
Sections through onion roots show the usual series of layers from an outer epidermis, a multicellular, thick cortex and an endodermis that surrounds the central stele, which contains phloem and xylem vessels with their associated parenchymatous cells (see Fig. 2.13A; de Mason, 1990, Stasovski and Peterson, 1993).
Onions have been one of the most frequently used species in investigations of root anatomy and physiology, probably because they can be conveniently produced from bulbs and, being fairly thick, straight and unbranched they are easy to handle. Hence there is a wealth of detail available on their structure and also ultrastructure, as revealed by electron microscopy (Ma and Peterson, 2001), and how this relates to both nutrient uptake (Cholewa and Peterson, 2004) and water uptake (Barrowclough et al., 2000). As the root develops, various cell layers change and mature (see Fig. 2.14 ).
Of particular interest is the development of the exodermis immediately below the outermost epidermal layer of cells (see Fig. 2.13). The exodermis consists of long and short cells (see Fig. 2.13C) and, somewhat later than the endodermis (see Fig. 2.14), these cells develop a water-impermeable, suberized Casparian strip in their radial walls. The long cells and some of the short cells
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 = 70m. (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
the exodermis. These cells, along with the remaining unsuberized exodermal short cells, which expose a living plasmalemma surface representing just 0.47% of the root surface-area (Kamula et al., 1994), can remain alive for up to 200 days (see Fig. 2.13E).
In addition to controlling the pathway of nutrient flow into the root cortex, there are other roles that the exodermis may play in the functioning of roots.
The short cells may provide entry points for arbuscular mycorrhizae (AM) (see Fig. 2.14. Diagram of an onion root (not to scale) showing positions of early metaxylem (central ladder-hatched structure), Casparian bands (dashed lines), Casparian bands plus suberin lamellae (solid lines) in the endodermis (at edge of central stele) and exodermis (near root surface). Segment A, immature endodermis and exodermis; segment B, endodermis with Casparian bands, immature
exodermis; segment C, developing exodermis with Casparian bands and sometimes also suberin lamellae; segment D, endodermis with Casparian bands, exodermis with Casparian bands and suberin lamellae; segment E, endodermis and exodermis with both Casparian bands and suberin lamellae. The water conductivities of Zones 1,2 and 3 were measured in experiments (from Barrowclough et al., 2000. Courtesy of Journal of Experimental Botany).
Chapter 5). The AM hyphae increase the nutrient-absorbing surface of the root and may prolong its absorbing life. Also, the suberized exodermis, by preventing the death of cortical cells in drought, may provide a protected environment for AM as well as giving protection from pathogens and pests (Peterson, 1992).
Apart from very young seedlings, the bulk of the root system is made up of the adventitious roots which originate in the primary thickening meristem near the top of the stem (see Figs 2.5 and 2.10). These emerge from all sides of the stem and tend to grow near horizontally for some distance before turning downwards.
Laborious investigations of root development in onion, leek and garlic were made by Weaver and Brunner (1927). All three crops have similar root systems, and Fig. 2.15 shows the leek root system at various stages of development. The roots vary between 2.0 and 0.5 mm in thickness and are fairly sparsely branched, with one to two lateral branches per cm of primary
Fig. 2.15. Stages in the development of the root system of leek plants grown in rows 1 m apart with 10 cm between plants within the rows. Seeds were sown in mid-March in a well-structured loam at Norman, Oklahoma, USA: (a) roots of 2-month-old plant; (b) half of the roots of a 3-2-month-old plant; (c) 25% of the roots of a 4.5-month-old plant. The scale lines show a 30 cm (1 foot) square grid (from Figs 11, 12 and 13 of Weaver and Brunner, 1927).
root. Lateral branches rarely re-branch; as a result, the root length per unit volume of soil, LV, under allium crops is low compared with that of other crop species. There are only small differences in root morphology between different onion cultivars, although older Dutch cvs had a slightly higher LV than modern cvs (de Melo, 2003).
However, a comparison of the roots of bunching onion, A. fistulosum, with common onion showed that the former produced 1.29 cm of root per mg of shoot dry weight as against 0.52 cm/mg for onion. A. fistulosum also spread its roots further from the plant and deeper into the soil. The difference in LVis a result of more fine lateral branches on the main roots of A. fistulosum (see Fig. 2.16).
The development pattern of root systems is always variable and can be influenced by both soil compaction and the distribution of nutrients in the soil.
In one study the root length under an onion crop increased as the crop grew as described by the equation:
logeL = 3.4 + 1.5 logeW 0.035T (Eqn 2.1)
where L is the total root length per unit of soil surface (km/m2), W is the crop shoot dry weight (t/ha) and T is days from sowing (Greenwood et al., 1982). Ninety per cent of the root length was found in the top 18 cm of soil throughout the season, unlike other crops that rooted more deeply as the
Fig. 2.16. The distribution of root length density (mean length of root per unit volume of soil (cm/cm3) in different root diameter categories in A. fistulosum and in Eastern European and old and modern Dutch onion cultivars (from de Melo, 2003.
Courtesy of Wageningen University, The Netherlands).
season progressed and their root weight increased. It may be that the rather sparse root system is a reflection of these crops having evolved from wild species in which mycorrhizal (VAM) enhancement of the root-absorbing surface was usual (see Chapter 5).