Maximum Yield Australia/New Zealand March/April 2020 | Page 50

The mechanism for this damage is not completely clear,
but appears to at least be partially a result of the hyper
accumulation of starch and soluble sugars produced in the
leaf under the prolonged lighting conditions that can cause
a range of symptoms, the most common being leaf chlorosis
(yellowing) and necrosis (death of leaf cells). Continuous
light may also accelerate leaf aging and photo-oxidative
damage due to excessive light levels and there may be other
factors involved with foliar damage.
Studies have found that running a lower temperature for
several hours in each 24-hour period under continuous
light benefited a number of species — tomatoes grown
with 28/16°C alternating temperatures and cucumbers with
a short-term exposure to 12°C decreased the severity of
injury symptoms caused by continuous light. Leaf injury
under continuous light appears to be not just caused by the
long light exposure itself but by an interaction between
light duration, light intensity, and light quality as well as
temperatures provided by thermoperiods.
Continuous Light and Assimilate Unloading
One of the limiting factors for the use of continuous light
is the process of assimilate unloading — the transport
of photosynthesis products out of the leaves where
photosynthesis has been occurring to the sites where it
is required for growth (such sinks for assimilate include
Capsicum is a fruiting crop that could benefit from continuous light
rapidly developing cells in fruit, buds, flowers, and new root
if leaf injury could be prevented.
growth). Under normal light/
dark conditions, photoassimulate
produced during the light period
can be exported out of the leaf
during both the light and the dark
period when no photosynthesis
of whether continuous light may be beneficial to certain crops is
is occurring, thus preventing a
the fact temperature interacts with light on several different levels.”
build-up of sugars in the leaf
cells. If the assimilate produced
via photosynthesis is not
exported out of the leaf cells fast enough, there is a build-up
of these sugars that creates a negative feedback and slows
or even stops further photosynthesis.
So, even if sufficient light is provided, the build-up
of assimilate in the leaf and subsequent slowing of
photosynthesis means that no increases in yield and
productivity will occur even under continuous light.
In fact, a reduction in growth can happen. This is seen
as the major drawback in the use of continuous light for
increasing the rate of food production. The other issue is
this overproduction of assimilate that builds up in the leaf
cells and can’t be shipped out of the leaves fast enough.
This may be what is responsible for the damage and leaf
injury some species experience under continuous light.
To further confuse this process of whether continuous light
may be beneficial to certain crops is the fact temperature
interacts with light on several different levels.
First, temperature determines the rate of photosynthesis,
so more assimilate is produced under continuous light when
optimal warmth is provided than when it is cooler than
optimal, so an excess build-up of starch is more likely to
occur in foliage under these warm conditions.
Second, temperature plays a major role in the rate of
transport of assimilate out of the leaves (the source) to the
sinks (fruits, flowers, buds). The temperature differences
that typically occur under a normal day/night regime
are warmer during the day and cooler at night. Under
continuous light, temperature often remains the same right
Eggplant is one of the crops that are prone to leaf injury under continuous
throughout the 24-hour period in order to allow maximum
light at constant temperatures.
rates of photosynthesis to occur.
“TO FURTHER CONFUSE THIS PROCESS
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