William Armstrong

Root adaptation to soil waterlogging

Abstract The structural diversity of roots is emphasised and adaptation to soil waterlogging is reviewed. Results pertinent to the mechanism of aerenchyma development are presented, and a modelling approach is used to explore the implications for aeration and rhizosphere oxidation of the structural and physiological adaptations of adventitious roots and laterals.

A convective through-flow of gases in Phragmites australis (Cav.) Trin. ex Steud.

Abstract The first direct evidence of a convective through-flow of gases in a grass is reported for Phragmites australis (Cav.) Trin. ex Steud. The flow appears to be initiated in living leafs sheaths and in the living nodal stomatal regions of the culm. The convected gases are transmitted via air spaces in the culm and underground rhizome, and are vented via old broken culms. The convection is particularly rapid in bright light and at low atmospheric humidities. Consequently, it should cause direct enhancement of rhizome aeration, and by increasing the oxygen regime at the root-shoot junction, cause a greater diffusion of oxygen intto the roots. Static pressure differentials up to 800 Pa and flow velocities of up to 800 mm min −1 per leafy shoot (flow rate, 960 × 10 −6 m 3 h −1 ) have been recorded in the field. The mechanism appears complex, but is probably based chiefly on humidity-induced Knudsen diffusion into the plant. In the light, humidity, temperature, stomatal and possibly photosynthetic factors are involved; at night, the convection is very much slower. The humidity-induced convection has been mimicked using membranes “sealing” humidified chambers.

Pathways of aeration and the mechanisms and beneficial effects of humidity- and Venturi-induced convections in Phragmites australis (Cav.) Trin. ex Steud.

The pathways of diffusive, humidity- and Venturi-induced convective ventilations in Phragmites australis (Cav.) Trin. ex Steud., and the mechanisms of the convections are described. Experimental evidence, indicating those factors, e.g. low relative humidity (RH), light, large leaf-sheath area, which increase humidity-induced convection, and those which increase Venturi convections, e.g. high wind speed, are reviewed. The superior aerating effects of convective as opposed to diffusive ventilation within the plant and their influence on the rhizosphere are demonstrated experimentally and by mathematical modelling.

Rice and Phragmites: effects of organic acids on growth, root permeability, and radial oxygen loss to the rhizosphere.

Young Phragmites plants were grown in two cocktails of monocarboxylic acids (C(1)-C(5)) at pH 6, where the concentration of each acid was innocuous and the total undissociated (potentially toxic) concentrations were 0.35 mmol/L and 0.42 mmol/L. Rice plants were subjected to 1.5 mmol/L acetic acid at pH 4.5 (undissociated concentration = 1.05 mmol/L). In Phragmites, each cocktail curtailed root growth especially and induced premature shoot senescence. In both species, after 3-5 d of treatment, radial oxygen loss (ROL) from apical regions of adventitious roots, and from Phragmites laterals, was reduced to very low values and associated with cell wall lignification and suberization in the surface cell layers. At later stages of treatment, rice responded to acetic acid in similar ways to Phragmites, with the development of intercellular and callus type occlusions in the gas space system, vascular blockages, and the failure of laterals to emerge. The results are relevant to the supply of oxygen from Phragmites roots to sediments for the phytopurification of waste waters, to the efflux of methane and carbon dioxide from wetlands, and to rice cultivation.

A history of pressurised gas-flow studies in plants

Abstract The history of discoveries in the field of pressurised ventilation in plants, and the physical phenomena causing them, is traced from the middle of the nineteenth century to the present day, and identifies two major periods of activity. Descriptions are given of observations, experiments and controversy during the early years (1840–1920) which revealed that pressurisation and flows were widespread among flating-leaved plants, and which broadly identified the major causal mechanisms as thermal transpiration and humidity-induced diffusion. The more recent period is one in which the flows (identified also in emergent macrophytes) have become better understood, and recognised to be of major importance in wetland plant survival and competition, and in the emissions of greenhouse gases from wetlands.

Pressurised ventilation in emergent macrophytes: the mechanism and mathematical modelling of humidity-induced convection

Abstract Humidity-induced diffusion (HID) and convection (HIC: pressure flow) are described for a simple physical model and are mathematically modelled. The physical model comprises a cylindrical chamber, partially filled with water, and capped by a micro-porous partition (membrane); an outflow pipe with a tap vents gases from the header space to the outside. The mathematical model comprises a series of equations which embrace the effects of header space depth and boundary layer thickness, the diffusive- and any Poiseuille-flow resistances of the partition, and the venting path resistance. For membrane pore diameters within (i.e. ≤ 0.1 μm) or outside the Knudsen regime, close correlations are found between experimental values of static and dynamic pressure and convective flows, and those obtained from the mathematical model. The findings suggest that for plants, compromises are necessary between factors such as small pore widths, which can help generate high dynamic pressures, and the need for wider pore widths to support greater flows. It is shown that the fastest flows are generated at pore diameters of ca. 0.2 μm, and it is suggested that the high rates of flow found in Phragmites are achieved because of an optimum leaf sheath stomatal pore width coupled to very low venting resistance through the plant. The benefits of a sustained humidifying source close to the base of the pores is also highlighted, and attention is drawn to thermally enhanced HIC and the continuance of HIC when temperatures within the plant might be lower than outside. The results have important implications for understanding convective gas flow generation in plants and its potential for enhancing ‘greenhouse gas’ emissions from wetlands.

Plants and flooding stress

Floods early this year in Queensland, Australia, received a great deal of attention in the media because they affected a land area the size of Germany and France combined. However, on a world scale this is not exceptional as in some years the land area exposed to flooding is > 17 million km, equal to twice the size of the USA. These dramatic floods occur in all continents of our planet and result in annual damage costs of >

Senescence, and phytotoxin, insect, fungal and mechanical damage: factors reducing convective gas-flows in Phragmites australis

80 billion (http://floodobservatory. colorado.edu/). Many wild plant species and nearly all crops are intolerant to these floods and thus excessive water will affect the natural patterns of plant distribution and biodiversity (Silvertown et al., 1999) and have a devastating impact on crop growth and survival and thus on food production (Normile, 2008). Flooding is a compound stress composed of interacting changes inside plant cells induced by the flood water surrounding the plant. The concentrations of oxygen (O2), CO2, reactive oxygen species (ROS) and ethylene change upon flooding and can occur in various combinations, as determined by the flooding regime.

Cauliflower shoot-culture : Effects of different types of ventilation on growth and physiology

Premature senescence, insect bore holes and callus development in the gas-transport pathways were each shown to impair convective (pressurised) gas-flows in Phragmites australis (Cav.) Trin. ex Steudel. Premature senescence resulting from aphid and fungal attack, and possibly from the uptake of soil-borne phytotoxins, was found on average to have reduced the gas-flow potential of living culms by at least 70%. This was attributed chiefly to a reduction in humidity-induced convection resulting from a loss of healthy leaf-sheath area. Insect bore holes, found in living culms, reduced potential humidity-induced convective flows by varying amounts, by causing the culms to be leaky. It was shown that the degree of leakiness of culms should be dependent upon the position of the bore holes along the culm. Irrespective of their position and the wind speed, bore holes in old dead culms reduced Venturi-induced convection to almost zero. However, such holes could also increase convective flows by reducing resistance to venting from the underground parts, basal holes conferring lower resistance than more apical ones. In addition, it was concluded that callus development within gas pathways of the plant is stimulated by insect damage, by mechanical damage/flooding and by uptake of phytotoxins such as acetic acid and sulphide, and that extensive callus production might completely impede all pressurised gas flows. It is suggested that each of the above factors has the potential to contribute to Phragmites die-back by adversely affecting rhizome, root and rhizosphere aeration.

Evaluation of a closed system, diffusive and humidity-induced convective throughflow ventilation on the growth and physiology of cauliflower in vitro

Abstract Hypocotyl cuttings of in vitro grown cauliflower ( Brassica oleracea var. botrytis L.) were cultured in 60 cm 3 glass vessels containing MS medium supplemented with BAP (1.0 mg l −1 ) and NAA (0.5 mg l −1 ) which led to callus induction and new shoot proliferation. The effects of various methods of ventilations (sealed, diffusive and humidity-induced convective throughflow ventilation) on growth and physiology of the regenerated shoots and callus were investigated and described. The humidity-induced convective throughflow ventilation (HICT-ventilation) system proved to be the most effective to procure best growth and photosynthesis of the regenerated shoots. On the contrary the sealed vessels showed very poor growth with little leaf and shoot number, fresh weight and callus volume. The diffusive treatment is being intermediate between the sealed and the HICT-ventilation treatment. Throughout the 30 days experiment, the ethylene, CO 2 and oxygen concentrations in the culture atmosphere were also monitored. High ethylene accumulation was observed in sealed (1.75 μl l −1 ) and diffusive system (0.55 μl l −1 ), which could not accumulate in the culture vessels grown under HICT-ventilation system. Thus the inhibition of growth observed in the sealed and diffusive system is thought to be associated with the accumulated ethylene. Very high CO 2 concentration (5%) was observed in sealed system despite the continuous illumination; maximum CO 2 output originated from the callus. In the HICT-ventilation system it was below atmospheric. Oxygen concentrations in the sealed and the diffusely vented vessels fell (7.1 and 15.1%, respectively) as CO 2 level rose, which remained fairly constant at levels a little below atmospheric in HICT-ventilation.