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Biochemical warfare:

Soya bean vs yellow nutsedge weed

By Prof Charlie Reinhardt

espite its modest size greatly augments the competition ability but due to the release of phyto (plant)
and relatively sparse of this weed, thus making effective, toxic chemicals called ‘allelochemicals’
above-ground biomass, early-season control essential. from the weed’s litter.
yellow nutsedge (Cyperus Research showed that tuber residues
D esculentus) is rated among Impact on crop yields incorporated in growth medium reduced
the world’s top ten crop-damaging Rhizomes of yellow nutsedge are the growth of soya bean seedlings more
weeds. It is an annual plant (it completes concentrated in the upper 15cm soil than it did maize seedlings. Soya bean
its life cycle in one calendar year), but zone, resulting in 80% and more of the growth was also significantly reduced by
production of tubers on underground tubers occurring in this zone. Tubers the addition of water extracts prepared
stems (rhizomes) effectively extends its form on rhizomes four to six weeks after from tubers to the growth medium. Growth
life with a year, when tubers formed in a seedling emergence. Due to the shallow inhibition of both crops was greatest when
particular year sprout the following year. rhizome and root systems of this weed, the tuber residues in the growth medium
Yellow nutsedge spreads mainly germinating crop seed and seedlings were in direct contact with the crop seeds.
through tubers and not nearly as are in close proximity and even in direct It was postulated that growth-inhibiting
effectively by sexually produced seeds – contact with live or dead parts of the chemicals (allelochemicals) present in the
the seeds are viable, but seedlings lack weed, especially in heavily infested soil. tubers of yellow nutsedge inhibited the
vigour for survival in field situations. Yellow nutsedge’s impact on the yield growth of maize and soya bean in particular.
In South Africa’s summer rainfall of various crops due to competition for
region, it tends to be the first weed growth factors (water, light and nutrient The concept of allelopathy
species to emerge in spring, which elements) is well documented. For Several phenolic compounds that commonly
often necessitates control measures example, cotton yields decreased with occur in plants and are known for having
even before the crop is established. Its an increase in yellow nutsedge densities allelopathic (phytotoxic) effects have been
establishment early in the growing season − average yield loss of 19kg/ha for each identified in tubers of yellow nutsedge.
additional tuber Compounds identified included p-coumaric,
occurring in 1m ferulic, p-hydroxybenzoic, syringic, vanillic,
crop row length, salicylic, protocatechuic and caffeic
and 18kg/ha lost acids, with p-coumaric and ferulic acids
for each additional occurring in the highest concentrations.
nutsedge plant per High concentrations of the natural plant
square meter. hormone, abscisic acid (ABA), were found
Beware, it is not in tubers that were in a state of dormancy.
only competition The highest ABA concentration attained in
from live yellow dormant tubers was 6,81 microgram per
nutsedge plants gram tuber material. ABA concentration
that pose a threat gradually dissipated as spring approached
to crops; plant until, upon sprouting of tubers, ABA
litter of the weed, was no longer present, or present in
which is present in an undetectable low concentration.
or on the surface Despite its name, ABA does not directly
of soil, can also initiate abscission (leaf drop, fruit drop)
inhibit crop growth. in plants, although in the 1960s, when it
This is not due to was named, botanists thought that it did.
Yellow nutsedge plant with tubers on rhizomes. Each tuber can sprout later
in the same season in which it was produced, or remain dormant until the competition for light, The principal effect of ABA is inhibition of
following growing season. nutrients or water, cell growth. ABA concentration gradually

20 June 2020