Production Practices and Quality Assessment of Food Crops. Vol. 1

6 R. M. Madakadze and J. Kwaramba
800–1000 m above sea level is beneficial to many crops especially onions and
leaf crops in the Cruciferae family in the tropics. Where there is a steep sloping
or uneven land soil conservation measures are needed to save the soil and water
from erosion. If the land is steep, erosion could be a problem and so ridging or
terracing may be necessary. High elevations are linked to frost therefore freezing
and chilling injury of tropical and sub-tropical crops are more common in such areas.
In hilly areas the rainfall on the west-facing or leeward slopes is much less especially
on coming down to sea level. If streams are coming from the mountain-tops
the leeward side of hilly areas, are suitable for horticultural crops as long as
irrigation is carried out.
2.5. Light
Light is necessary for photosynthesis. Daylight is also responsible for certain
morphological induction’s, and the quality of light can significantly affect growth
patterns. Plants grown in full light develop several thicknesses of palisade tissue
compared to those grown in low light indicating improved photosynthetic activity
(Nonnecke, 1989). Leaves of salad vegetables such as celery and lettuce are generally
considered to be of higher quality and more tender if they are grown under
partially overcast skies. Photosynthesis is practically stopped at 4.31 lux, the
compensation point (at which photosynthesis equals respiration). Plants generally
respond to 350 to 780 nm wavelength light. Different wavelengths can affect some
physiological processes in plant such as stimulation of bulbing in onions at 1000
to 720 nm light and suppression of bulbing in onions and red pigmentation formation
in tomato at 690 to 650 nm light.
The flowering response of plants to relative length of day or night is called
photoperiodism. Plants that develop and reproduce normally only when the photoperiod
is less than a critical maximum are called short day plants and those that
flower only when the photoperiod is greater than the critical minimum are long
day plants (Yamaguchi, 1983). For short day plants the duration of the dark period
is the critical condition, rather than the duration of the light period. Phytochromes
which absorb red and far red lights, is the pigment necessary for the response in
photoperiodims. In day neutral plants flowering is not affected by photoperiods.
A few specific examples of vegetables fitting into the three categories of photoperiods
are listed below:
a. Long-day vegetables e.g. spinach, radish and Chinese cabbage;
b. Short-day vegetables e.g. soyabean, sweet potato, whinged bean and amaranth;
c. Day-neutral vegetables e.g. tomatoes, squashes, beans, peppers, eggplant and
most cucurbits.
Exotic vegetables in the tropics are the ones mostly affected by day length as they
generally originate mostly from temperate regions. These regions experience day
lengths longer than 12–13 hours per day. Cultivar selection is therefore necessary
in such crops so that the farmer does not produce poor quality produce due to unsuitable
day lengths. This information is also useful for timing production of specific
vegetables.