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

Other responses that can also be caused by different daylenghts beside flowering
in vegetables are listed below:
a. Long days for bulbing in onions;
b. Short days for tuber initiation in Irish potato, yams and Jerusalem artichoce;
c. Short days for root enlargement in cassava and sweet potato.
2.6. Winds
Winds are caused by differences in air pressure. Temperature differences produce
pressure gradients which give rise to air movements. Winds affect the humidity in
the atmosphere. They can remove the humid air adjacent to leaf surfaces, increasing
transpiration rates and decreasing temperature. Winds can replenish the carbon
dioxide supply to the leaves deep within the canopy thereby improving photosynthesis
and vegetable growth. Winds can also injure or break the above ground
portions of the plant thereby affecting quality of the product.
All the above must be considered as they do not only affect growth and development
of vegetable but the quality of the produce. It is therefore necessary to check
before introducing any vegetable into the tropics or any area, if its optimal growth
temperature and photoperiodic requirements are compatible with local climatic
conditions. Even for the quality of local varieties, it is important not to ignore the
environmental conditions as they also affect quality of the produce. Where local conditions
are marginal, artificial means can be used to promote the growth of the
crop for better quality. These can be windbreaks, shades and green houses to
protect the crops from wind and high solar radiation respectively.
3. MAJOR PRE-HARVEST DISORDERS IN VEGETABLES
3.1. Chilling injury
Effect of Preharvest Factors 7
Chilling injury is the permanent or irreversible physiological damage to mostly warm
season plant tissues, cells or organs, which results from the exposure of chilling
sensitive plants to temperatures below some critical threshold for that species or
tissue. A chilling temperature is any temperature (above freezing) below the critical
threshold temperature that causes injury. The critical temperature for chilling injury
varies with commodity, but it occurs when produce is kept at below 10–12 °C.
All stages of growth and development of the entire plant (except perhaps dry seed)
are susceptible to chilling injury in warm season crops. Factors that determine the
extent of this injury include the temperature, duration of exposure to that low
temperature, whether exposure is continuous or intermittent, physiological age or
condition of the plant, part exposed and the relative responsiveness (sensitivity)
of the crop to chilling. Chilling sensitive species except asparagus and potatoes
are mostly warm season crops of tropical and subtropical origin. Prolonged exposure
to temperature below the chilling threshold in these species results in death. Low
temperature limits the geographical areas and time of year for production of chilling
– sensitive crops.