3.1.1  Introduction

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Figure 3.1 Root systems of young (a) wheat and (b) lupin plants. Wheat, a monocotyledon, has a dual root system. Seminal roots emerge from the seed and nodal roots (thicker roots on the outside of the picture) emerge from the crown, a group of closely packed nodes from which tillers emerge. Lupin, a dicotyledon, has a tap root from which lateral roots emerge and which thickens with time as continued cambial activity leads to secondary growth

Roots keep shoots anchored and supported. A great diversity of overall architecture among root systems is fashioned as much by soil conditions as by genotype; hard subsoils, for example, restrict roots to surface soil layers. Root systems of monocotyledons and dicotyledons are genotypically distinct (Figure 3.1). Dicotyledons frequently develop tap roots from a single radicle that emerges from a seed. This tap root plus primary lateral roots emerging from it form a framework on which higher-order lateral roots are formed. Such a framework strengthens due to secondary thickening as a plant matures, leading to massive roots that are often seen radiating from the base of a tree trunk (Figure 3.2a). Monocotyledons such as grasses do not have a facility for secondary thickening and develop a fibrous root system comprised of one to several seminal roots, which emerge from the seed, plus nodal or adventitious roots, which emerge from lower stem nodes. Monocotyledonous stems are typically anchored by these nodal roots, which are much stronger and more numerous than seminal roots.


Figure 3.2 (a) Dimorphic root system of a six-year-old Banksia prionotes tree growing in Yanchep, Western Australia, on a deep sand with dominant winter rainfall. The lower trunk (T) is connected through a swollen junction (J) to the root system. A system of lateral roots (L) emerge horizontally from the junction, some bearing smaller sinker roots (arrows). Other laterals give rise to ephemeral cluster roots (CR) as described in Case study 3.1. The remainder of the root system comprises a dominant sinker root (S) which gives rise to smaller sinkers (S2). (b) Sinker roots penetrate up to 2.6 m into the sand and extract water through a low-resistance (high hydraulic conductivity) xylem pathway. Xylem in lateral roots is significantly narrower, raising axial resistance to water flow by at least one order of magnitude (Based on Jeschke and Pate 1995; reproduced with permission of Journal of Experimental Biology)

Roots do much more than anchor a plant. In addition to their obvious role in taking up water and nutrients (Section 3.6), they are also a source of hormones such as gibberellins, abscisic acid and cytokinins, which modify shoot physiology (Chapter 9). Concentrations of some hormones respond to soil conditions, allowing roots to act as sensors of soil conditions which might affect overall plant performance. Roots also act as storage organs; examples from the Australian flora are found in the Proteaceae (Clematis pubescens, Stirlingia latifolia), Portulaceae (Calandrinia spp.), Juncaginaceae (Triglochin procera) and even the bladderwort, Utricularia menziesii. Also very importantly for native vegetation, roots fashion soil profiles, creating niches (biopores) which can be colonised afresh each season and which enable roots to traverse otherwise inhospitable subsoils to gain access to water at depth. Complex physical and biological interactions between roots and soil occur in the rhizosphere (Section 3.3), where bacterial activity and root exudates stabilise biopores and modify soil chemistry.