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increases in developing seeds, until it
induces seed dormancy when it has
reached optimal concentration. If leaves
experience water stress, ABA amounts
increase rapidly, causing the stomata to
close. In this way, ABA protects plants
that experience water stress (drought)
conditions. ABA also prepares plants to be
ready for winter by slowing plant growth
and causing buds to become dormant.
Production of the natural plant hormone
ethylene has long been linked to herbicidal
effects of hormone-, or auxin-type,
herbicides such as 2,4-D and MCPA. It is
ethylene that causes the typical ‘epinastic’
(malformation of leaves and stems) growth
responses in plants that are susceptible Even with drastic intervention, the yellow nutsedge residues (alive or dead) left in the soil, can exert
to the auxin-type herbicides. Recently it crop-damaging allelopathic effects on a crop such as these tomatoes.
has been revealed that an increase in ABA
production is actually the primary plant Soya beans have one distinct literature revealed at least one study
response to those herbicides, and ethylene advantage over nutsedge weeds; it conducted by researchers at Purdue
production the secondary response. Ethylene can rapidly develop a dense canopy University on soya bean allelopathy.
at phytotoxic concentrations causes early that makes it highly competitive for Field and greenhouse experiments
senescence of plants due to breakdown of the one growth factor that yellow showed significant retardation of wheat
plant cell constituents and tissue collapse, nutsedge needs a lot of – sunlight. germination and seedling emergence
and plant death can follow within days. Research showed that crops maintaining when wheat seed was sown into soya
Ethylene, which is a gaseous molecule, dense canopies for an appreciable part bean residues shallowly incorporated
finds commercial application in the form of the growth season, which created a into soil after harvest. This would
of synthetic ethylene used for ripening of regime of low light intensity during a suggest that soya bean litter exudes
unripe fruit under controlled conditions. long yellow nutsedge growth period, allelochemicals; if the growth of wheat
Under natural conditions, ethylene can suppressed tuber formation more strongly could be inhibited, why can the same
be released in the form of a gas into the than crops that shadow the weed for process not be followed with weeds?
environment. It is therefore plausible a relatively short period of time. The allelopathic characteristic has
that ABA, ethylene, and possibly other A crop such as maize, planted in 75cm already been proven for several crop
phytotoxic allelochemicals can be exuded rows, did not provide enough shade to species, and is exploited in practice for
from yellow nutsedge tubers, and perhaps prevent yellow nutsedge from producing weed suppression (e.g. use of intercropping,
also from other underground plant parts, tubers. Less light reaching the plant means cover crops and organic mulches), and even
to have phytotoxic (allelopathic) effects lower photosynthesis, which results in lowered in the quest to develop novel herbicides
on crops that are infested by the weed. production of energy-rich carbohydrates from biochemical leads or precursors,
Although the discussion here is limited used in plant growth and reproduction. i.e. allelochemicals, present in plants.
to yellow nutsedge, various studies have Tubers, as storage organs, are particularly Hopefully more research will be
reported similar effects (allelopathy and strong ‘sinks’ for carbohydrates produced forthcoming, as it makes sense and improves
competition) on crops for its close relative, by photosynthesis in leaves, the ‘source’. economics if the crop can assist with
purple nutsedge (Cyperus rotundus). In Glyphosate-tolerant crops such as controlling weeds by biochemical means,
terms of notoriety the latter shares the weed soya beans have made weed control thereby relieving pressure on the use of
status of yellow nutsedge. Globally and easier, especially in zero-tillage systems, synthetic chemicals such as herbicides.
locally both species are severely harmful and fortunately, glyphosate-resistant
weeds. Locally, yellow nutsedge apparently yellow and purple nutsedge have Charlie Reinhardt is professor
prefers dry land conditions, whereas not been reported anywhere in the of agronomy in the Agricultural
purple nutsedge tends to thrive under world, yet. The best practice herbicide Sciences Programme at North-West
irrigation. Both are formidable opponents programme for nutsedge control would University, Potchefstroom campus,
of the farmer seeking to destroy them. include an array of herbicide modes of and research leader in the South
action that can be applied pre-plant, African Herbicide Resistance Initiative
Soya bean fights back pre-emergence and post-emergence. (SAHRI) at the University of Pretoria.
Can soya bean defend itself against Do soya beans possess allelochemicals Contact him on 083 442 3427 or
yellow nutsedge and other weeds? that can suppress weeds? A survey of dr.charlie.reinhardt@gmail.com.

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