Report on the Processes Occurring in Weeds Killed by Foamstream
Dr. David Hanke
Department of Plant Sciences
University of Cambridge
PHYSIOLOGICAL PROCESSES, timescale of seconds to minutes
1. involving proteins.
At temperatures above 60oC, proteins are rapidly de-natured, i.e. the molecules acquire kinetic energy sufficient to disrupt irreversibly all the myriad weak interactions that hold invidual molecules in shape in a precisely folded water-soluble conformation, and these large molecules come out of solution with almost instantaneous loss of all enzymic activity. At these temperatures there is no browning and leaves die green. At temperatures between 40 and 60oC, proteins de-nature irreversibly but more slowly, with some enzymes, e.g. peroxidases and esterases, more heat-resistant than others. Because increased kinetic energy of their substrate molecules lowers the activation energy needed for enzymic conversion, there is usually a transient burst of increased total activity of these enzymes that triggers browning.
2. involving lipids
A. in membranes
Above 35 to 40oC (the value depends on the climate of origin of the weed, increasing in the order: temperate vs mediterranean vs semi-desert), the weak forces that stabilise the lipid bilayer core of all biological membranes are matched and exceeded by kinetic energy. As a result membranes become leaky, and at higher temperatures lose their integrity. This leads to a number of consequences.
At the outer, plasma membrane (PM) the liquid contents of cells leak out, displacing gas from intercellular spaces. In leaves, a pervasive intercellular gas phase makes up a large proportion of the internal volume and incident light is scattered and reflected from a myriad cell surfaces. With the intercellular spaces filled with liquid, moe of the incident light is transmitted rather than reflected and in consequence the leaf is darker in colour and more translucent. Increased permeability of the plasma membrane also leads to loss of rigidity of all non-woody tissues – the leaves & shoot tips – which rely on turgor for rigidity. Turgor is the internal pressure resulting from the tendency of water to enter the cell down its concentration gradient, against the tight restraint of the rigid cell wall. When solutes are no longer held in by the PM, that pressure collapses, leaves hang limply and shoot tips flop over.
At the inner tonoplast membrane (TP) enclosing the vacuole, the consequencs are more chemical than physical. The vacuole is much the biggest compartment within full-size plant cells and filled with a huge variety of toxic chemical weapons against pests and pathogens, stored inside it for safety including, in almost all perennials, corrosive phenols inactivated and kept inside the vacuole by covalent linkage to form esters and glycosides. In heat damaged plant cells, these conjugated phenols leak across the TP into the cytoplasm where still-active hydrolases cleave off free phenols –
these are toxic due to their reactivity against amino groups of proteins & nucleic acids. Now the free phenols leak across the PM into the cell wall where peroxidases convert them to phenol radicals, some of the most reactive chemicals known, which attack indiscriminately, friend and foe, in a suicidal orgy of destruction visible as browning. This conjugated phenol/ hydrolase/ peroxidase sequence normally plays out at ordinary temperatures as a consequence of breakage of PM and TP in physical damage by herbivores and disease organisms – this is the browning you see on the surface of a cut potato or apple fruit. Damaged cells initiate the reaction which then propagates to neighbouring intact cells, which are sacrificed to generate the chemical firewall. At ordinary temperatures, the chain reaction, damage by phenolics generating yet more phenolics, is damped down by corrective responses of unstressed cells even further from the wound, restricting the browning to the vicinity of the wound. The browning containment reaction includes deploying anti-oxidant chemicals such as ascorbate and tocopherol. For killing perennial weeds, the objective should be to foster the spread of the phenol radical-generating chain reaction and lower the ability of undamaged cells to contain it. One strategy would be to prolong their exposure to heat stress and anoxia; in essence to keep the parts of the plant not exposed directly to membrane-destabilising temperatures at stressful supra-optimal, but not otherwise damaging, temperatures and low oxygen availability under a gas-tight, insulating foam blanket. The extent of spread of damage by phenolics can be assessed by the associated browning.
B. on the surface of the plant.
The entire surface except the absorptive region of root tips is protected from water loss by a cuticle consisting of a thin layer of cutin, a polyester, whose molecular interstices are filled with solid hydrophobic wax exuding out to form a continuous outer coat. At higher than normal temperatures, the wax melts, the layer breaks up into liquid droplets, and this allows unrestricted water loss so that living tissues shrivel and dry to papery crispness.
Amphipathic detergents are included in herbicide sprays to coat surface waxes and render plant surfaces wettable at ordinary ambient temperatures. The same detergents can be used to generate foam at ordinary temperatures. However, detergent foams are molecular bilayers and so unstable at higher temperatures. In any case, for killing weeds by heat, detergents are not necessary because the waxes have gone. In fact hot detergents are micellar and likely to lubricate plant surfaces, interfering with foam adhesion.. Finally, the toxicity of detergents and their residues in the environment is a clear disadvantage.
DEVELOPMENTAL PROCESSES, timescale of days
All plants retain relatively stress-resistant buds in a dormant state as a means of surviving lethal heat, and the loss of leaves and growing shoots to heat kill will trigger the onset of growth in a subset of the total buds. In perennials, a proportion of the buds always remain dormant ensuring the plant survives serial catastrophes.
Buds contain specialised chemicals for stress-resistance, and their cells do not have large vacuoles or phenolics. The most difficult weeds to control are those with stress-resistant buds below ground where the insulating properties of soil moderate the above ground temperature. The only effective strategy is to treat repeatedly at optimum time intervals: long enough for buds to emerge from the ground, short enough to pre-empt any contribution by the new growth to stored reserves or buds.