These effects of increased yield and improved compositional
quality in hydroponic crops from microbial interactions
are likely to have occurred via complex processes, some
of which are still not fully understood. Some species of
beneficial rhizospheric bacteria are known to improve plant
performance under stressful environments, improving yields
either directly or indirectly. Some studies have shown that
growth-promoting rhizobacteria may provide a direct boost
to plant growth by providing fixed nitrogen, phytohormones,
and iron that has been sequestered by bacterial siderophores
and soluble phosphate. Other species may protect the plant
from potentially highly damaging pathogens that would
otherwise limit plant growth, quality, and yields. However,
it’s likely the majority of root-associated bacteria that play a
beneficial role in hydroponic plant growth do so by producing
the plant hormone auxin as indole-3-acetic acid (IAA).
Rhizobium bacteria develop in nodules on peanut plant root
systems and fix atmospheric nitrogen for plant use.
When plant roots sense an attack by pathogenic microbes,
they release certain exudates called phytoalexins (defence
proteins) and other unknown compounds, engaging in a
process of underground chemical warfare. For example,
the roots of sweet basil plants have been shown to release
rosmarinic acid, an antimicrobial compound, in response
to attacks by Phytophthora cinnamomi disease, which is
possibly just one of many such defence responses we are
yet to identify and understand.
“ RELATIONSHIPS
with beneficial microbe populations in
the rhizosphere occur in hydroponics
just as they do out in the field.”
MICROBES AND HYDROPONIC SYSTEMS
While a new hydroponic system may start off with very
little in the way of microbial life, as soon as moisture
and an organic carbon source (plants) are present,
inoculation naturally begins. Microflora develop rapidly
after planting a crop in a hydroponic system and consume
plant exudates, compounds in the nutrient solution, and
dead plant materials. Some of the microspecies may be
pathogenic, however, these are generally outnumbered and
outcompeted by populations of non-pathogenic organisms.
In most hydroponic systems, the species of beneficial
resident microflora most commonly found are Bacillus spp.
Gliocladium spp, Trichoderma spp, and Pseudomonas spp.
Hydroponic studies into the affects of various species of
beneficial bacteria have found several positive results.
Bacillus amyloliquefaciens has been shown to increase
vitamin C content and water-use efficiency in tomatoes,
while Bacillus licheniformis increased fruit diameter and
weight of tomatoes and peppers, while promoting higher
yields. A strain of Pseudomonas sp. has been found to
promote growth of hydroponic tomato crops and increase
the uptake of calcium, which in turn reduced blossom
end rot of tomato fruit. In hydroponic strawberries,
inoculation with plant growth promoting rhizobacteria
(Azospirillum brasilense) resulted in a higher sweetness
index and a greater concentration of flavonoids
and flavonols in the fruit as well as increasing the
concentration of micronutrients (iron). In hydroponic
tomato studies evaluating a number of different plant
growth promoting rhizobacteria, it was found that
Bacillus spp. (strain 66/3) was effective in increasing
tomato yield significantly — this increase in marketable
yield was 37 and 18 per cent compared to untreated
control plants in fall and spring crops respectively.
48
Maximum Yield
An association with a wide range of beneficial micro flora
has been proven to help maintain healthy root systems.