GETTING TO THE ROOT OF IT ALL
Root nodules of a bean plant.
reproductive growth, and such findings may be speciesspecific, as it appears that the roots of some plant species
can sense the roots of neighboring plants and respond
accordingly. Further studies might help us determine
how plants grown side-by-side may influence the growth
of each other in hydroponic systems.
Root Temperature
The temperature of a root zone strongly affects shoot
growth. In fact, root zone temperature plays more of a role
in growth and development than the temperature of the
air surrounding the plant because the root tissue sends
numerous, non-hydraulic messages to the shoot, which
influences the way the shoot responds to its environment.
So, with many plant functions under the control of what
goes on down in the roots, root zone temperature becomes
an extremely important factor to monitor. Research has
shown that even less than 30 minutes of root zone heat
buildup can have a negative impact on many crops, which
cannot be countered by having a low daily temperature
average. Just a few minutes a day of root zone temperatures
of more than 86°F will slow the growth of heat-sensitive
crops such as lettuce and parsley.
Chilling the root zone and nutrient solution is one solution
to this unique aspect of plant physiology. Nutrient solution
chilling allows the tops of heat-sensitive crops like lettuce
“ROOT ZONE temperature plays
more of a role in growth and
development than the temperature
of the air surrounding the plant.”
76
Maximum Yield USA | November 2015
to withstand higher-than-optimal temperatures. This method
of root-zone chilling assists plants in a number of ways. Cool
nutrient solutions hold more dissolved oxygen for root uptake,
which means oxygen starvation is less likely to occur. Cooling
the roots well below ambient air temperatures also allows for
higher assimilation rates by reducing both photoinhibition
and stomatal closure, which typically occur once the plant
becomes heat-stressed. The positive effects of nutrient chilling
seem to be largely the result of changes in the production of
plant growth hormones (abscisic acid and cytokinins) in the
root tissue, which control a wide range of plant responses.
Salinity and Phytotoxicity
While salinity and phytotoxicities can be serious issues in
both soil and soilless production, hydro growers can largely
avoid these issues, allowing crop growth in areas where soil
salinity and toxicity would otherwise prevent cultivation.
When salinity, or EC, is too high, root cells lose moisture and
often die. There are wide differences in tolerance to high EC
and salinity among plant types, even those that have similar
growth requirements. Tomatoes, for example, can tolerate
high salinity through changes in root physiology, which
prevents salt damage, whereas salinity-sensitive crops like
lettuce and strawberries are easily damaged when EC becomes
higher than optimal.
Toxicities occur when root cells are damaged or destroyed
by compounds such as high levels of certain trace elements
or, more commonly in hydroponics, plasticizers leaching from
unsuitable materials in contact with the nutrient solution or
roots. When root cells are damaged, plant pathogens such as
pythium often invade the site of the damage, leaving growers
wondering what the initial cause of the root damage was.