Study Notes on Rhizology | Arid Zone

The term rhizology was suggested by Vysockij in 1905 and since then it has been used by several workers. The Rhizology is the study of the root systems of plants in their natural environments.

This branch occupies a significant position in ecological studies due to special relationship between the root and the soil.

Terrestrial plants live in two distinct environments – the atmosphere and soil. These make a major division within the plant-shoot and root.

Roots in any soil, under any climate, are continuously subject to the stresses of the soil environment, since temperature or water supplies rarely remain within the narrow limits, optimal for growth.

Arid conditions merely provide exceptional problems since the stresses are more extreme or of longer duration, so that many root functions, particularly those dependent upon concomitant root growth, are confined to comparatively short periods by water availability and season.

Roots function in:

(a) Absorption and translocation of water and inorganic nutrients;

(b) Synthesis and transport of organic compounds, including growth regulators;

(c) Storage of metabolites, and sometimes water; and

(d) Anchoring the plant in the soil.

Types of Root:

The broad division of root systems into ‘tap root’ (as in most dicotyledons and gymnosperms) and ‘fibrous’ (as in most monocotyledons) needs little elaboration. The tap root and its laterals are usually supplemented by adventitious roots which develop from the stem base. The laterals, together with adventitious roots, are sometimes referred to as feeder roots during their early growth.

Classification of Root Systems:

Environmental factors can exert an over-riding influence on root extension, branching, orientation and longevity, resulting in wide differences in root form between individuals within a species.

One way of regarding the ‘strategy’ of root distribution in arid land soils, recognising that water is the major factor controlling root growth, is to consider the length of time water remains available at different depths and the types of root system and life cycle best suited to intercept this efficiently. On the basis of depth of water penetration and rooting zone, four categories of root systems can be distinguished.

i. Shallow rooting ephemerals:

The much of the water from light rains penetrating the soil to a depth of 10-30 cm may be lost by evaporation, but its duration of several weeks may be sufficient for roots of fast-growing annuals and herbaceous perennials to complete their life cycle. The root systems of these plants are often shallow and poorly branched.

ii. Shallow-rooting perennials:

The roots of cacti such as Opuntia fulgida are often located not more than a few centimetres beneath the soil surface even in deep soil, and therefore benefit from the lightest rain.

iii. Deep-rooting perennials with superficial laterals:

In perennial shrubs there is well developed tap root system or vertically orientated laterals to absorb water from deeper stores of water.

iv. Deep-rooting perennials depending on a water table phreatophytes:

Plants which develop roots reaching to the water table are rendered independent of water supplies from rainfall. Their laterals often branch copiously in the capillary fringe above the water table.

It is cleat, however, that the soil environment must be favourable for extended periods to allow roots to penetrate to considerable depths, and for this reason phreatophytes are usually restricted to dune areas, alluvial soils and habitats receiving water from run-off.

Remarkable rooting depths have been recorded for this category of plants. Roots of Prosopis sp. have been found at a depth of 53 m and Zizyphus sp. to depths of 60 m.

Root Patterns in Some Arid Zone Plants:

i. Tribulus terrestris: Tribulus terrestris possesses straight tap root system which is whitish to light yellow in colour. On sand dunes, the root is 120 cm deep. The surface root system is well developed which is made up of thin, wiry lateral roots which are horizontally and obliquely to vertically placed. The laterals ramify in all directions of various levels. The length, number of laterals and ramifications vary from habitat to habitat.

ii. Fagonia cretica has a straight tap root nearly 50-90 cm deep. The surface root system is almost absent. Primary laterals are short and horizontally or obliquely placed. The fungal hyphae are also associated with root hairs.

iii. Calotropis procera has a tap root system. The main root may be 2 m or more in length. The tap root is curved or straight which is generally forked towards the end. The surface root system is composed of only thin and obliquely placed roots. The laterals are distributed all along the whole root length. The forked end branch is mostly vertically placed.