Wednesday, 28 September 2011

Back to grass roots

The roots of grass plants are hidden and concealed - they are dull and brown - they are ordinary and mundane.

But what grass roots do, how they function and the way they interact with other living creatures in the soil is anything but ordinary.

A brief tour of some of the highlights of the rhizosphere – the zone of intense biological life in and closely around roots in the soil – reveals some of the extraordinary things that grass roots ordinarily do to maintain the domesticated and wild grass swards that sustain most terrestrial animal and human life on the planet.

Grass plants have a dense, multi-branched, multitudinous network of fibrous roots that infiltrate and extend their fine filamentous fingers into every moist and nutrient-rich nook and cranny in the topmost layers of the soil. The grass root network has no predetermined, fixed structure or plan but roots branch adventitiously, elongating and extending dynamically (also dying and shrinking back rapidly in times of scarcity and stress) to gain a purchase in the soil and seek out and absorb moisture and nutrients. Roots get thinner as they branch; ending in a fine root tip sheathed with fine root hairs and tipped with a sensory elongation zone and a root cap that sensitively perceives and reflexively responds to changes in the physical and chemical state of its immediate environment.

Form and structure of the root tip
The leaves of a grass plant enjoy the luxury of easily accessible and mostly uniformly available sunlight and the atmospheric gases they absorb, but roots have to delve deeply into and unceasingly explore a highly heterogeneous environment, in which vital resources are patchily and unpredictably distributed. Water, and especially vital mineral nutrients the plant needs to function, grow, flower and reproduce, are hard to come by, often tightly bound to soil particles or in intractable organic forms. Myriad other tiny and microbial forms of life swarm through the rhizosphere (take a quick peek) also seeking and competing for the very resources the roots need to acquire.

Water and nutrients selectively filtered and absorbed by the root hairs are transported upwards to the leaves where they are synthesised into a range of simple and complex molecular products using solar energy and carbon harvested by the green leafy canopy. Roots also produce plant-stimulating hormones. In turn, the leaves supply the roots with the protein and energetic compounds they need to fuel their crucial exploratory, interactive and absorptive activities.

Roots are not just passive absorbers but actively work, through their plenteous and varied secretions, to modify the zone of soil and life activities in their immediate vicinity. Water and soil particles are bound and held to the root by mucilaginous ooze while root exudates serve to communicate with friends and foe lurking in the soil. Among the diverse array of chemicals exuded by the root are compounds that recognise and poison roots of foreign plants, pathogenic microbes or fierce root-devouring denizens such as nematode worms. 

Root exudates and root hairs create a rhizosheath of soil around the roots
 Intriguingly and perhaps most importantly, plants give away up to almost a fifth of their hard-earned energy by actively exuding sugars and other soluble molecules into the rhizosphere to feed hordes of insatiable soil bacteria. Energised by these root exudates, soil microbes set upon and transform complex organic material in the soil and release, for the roots to absorb, simple minerals and nitrogen-rich substances vital for plant life. Grass plants give up even more sweet secretions when grazed but are rewarded by a larger bounty of compounds they need for regrowth.

Many grasses and other plant species in natural vegetation connect to and interact closely with other living networks in the soil. Filaments of particular fungi (mycorrhizae) inosculate intimately with the root system, penetrating into the outer root cells to obtain food directly from the plant. The benefits to the grass of this symbiotic association are many (disease and pest protection, for instance) but most importantly, these ‘fungal-roots’ serve to vastly extend the reach of the roots. They fetch and transport water and many essential macro- and micronutrients – some of which do not easily move through the soil – back to the root. Different grasses have their own distinct mycorrhizal communities and these need to be purposely reintroduced (along with ample organic matter) into degraded soil of barren lands to restore the vital interactions and connections between plants and a living soil.

grass roots
nourish, nurture, hold
Earth’s green

The dyanamic interactive rhizosphere ecosystem

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