Symptoms of light insufficiency vary with the type of foliage.
Thin-leaved, rapidly growing plants quickly deplete the small
reserve of chlorophyll stored in their tissue. When light con-
ditions are such that more cannot be manufactured, the leaves
of such plants turn lighter in color. On the other hand, thick-
leaved, slow-growing plants whose leaf surface does not increase
so rapidly can make their stores of chlorophyll last longer.
Thus, their foliage will not show the symptoms so quickly. The
color designating good growth is a medium shade between the
darkest and lightest hues of green. Trained observers can rec-
ognize deviations from this color within a few days after light
becomes poor. But to do this you must know your plants intimately.
Inadequate light may also be reflected in an increase ot
foliage at the expense of fruit. This has been particularly no-
ticeable in several hydroponicums where tomatoes were grown
in winter. The solution was heated and consequently the root
temperature was high. The plants grew very fast. Leaves and
flowers were large but fruit was sparse and often deformed.
The last effect was due to lack of light for pollination. Artifi-
cial illumination offered no relief. Additions of carbon di-
oxide to the air did give encouraging results but data on this
point are still too incomplete to allow conclusions to be drawn.
There is a theory that setting of fruit is affected by the ratio
of carbon to nitrogen in the plant's composition. Carbon is
necessary for the production of starch, sugar, and cellulose. It
is taken from the carbon dioxide in the air and fixed in the
plants through the process of photosynthesis. Thus, to build
up large amounts of the food products mentioned, direct sun-
light is needed. When light becomes poor, less carbon is fixed
by the plants and its ratio to nitrogen, whose intake by the
plants is less affected by light conditions, becomes narrower. It
has been found that, when the light supply is low, better growth
can be obtained by lowering the nitrogen intake of the plants.
Lowering the temperature of the solution to slow up the growth
is also advantageous.
When intense sunlight falls upon a surface of high reflecting
power, temperature great enough to injure the root crowns and
tops of the plants may arise. Injury from this source is not
detectable at the time. It is reflected later in such factors as
hastened maturity and poorer quality of the product. Bulbs,
tubers, and perennials may not show injury until the following
season. Your best protection against such an occurrence is to
take temperature readings at the surface of the seedbed on hot
days. From this you can gain an idea of the reflecting power
of the litter and change it if necessary.
Inadequate light cannot cause injury by reflection. Instead
it lowers the air temperature until it is so close to that of the
solution that the root-top temperature gradient is poor. The
solution temperature is fairly constant so long as the liquid is
covered. Consequently, air temperature should not be^allowed
to drop below certain levels. This does not mean, of course,
that air temperature determines the rate of growth. As we
know, root temperature is an important contributing factor in
this respect. It is thus possible for a given root temperature
to be ideal in summer when light is good and too high in winter
when light is poor.