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

2.1. Temperature
Effect of Preharvest Factors 3
2. GROWING ENVIRONMENT
Temperature is a major factor controlling quality in vegetables and it is very difficult
to modify especially for resource poor smallholder farmers. The extreme
temperature range of plants is between killing frost at 0 °C and death by heat and
desiccation at 40 °C with most plants permanently immobilized at 10oC and most
plants ceasing to photosynthesize efficiently above 30 °C (Nonnecke, 1989).
Seasonal temperature variation can change as little as 3 °C all year round in the
tropics and as much as 45 °C in the polar zone. Temperature decrease with increases
in altitude, a drop of about 6 °C occurs with each 1000 m in elevation. Cardinal
temperatures, minimum and maximum (where plant growth ceases) and optimum
(where growth occurs most rapidly) vary with different families, genera and species.
For every 10 °C rise in temperature, growth doubles at the temperature range of 5
to 35 °C depending on the vegetable (Van’t Hoff’s Law).
Most vegetables are sensitive either to high or low temperatures. Tropical vegetables
are mainly sensitive to low temperatures. Most vegetables are either injured
at temperatures at or slightly below freezing (freezing injury). Tropical and subtropical
plants may also be killed or damaged by the cold at temperatures below
10 °C but above freezing (chilling injury). These are discussed in Sections 3.1
and 3.2 respectively. Susceptibility to cold damage varies with different species
and there may be differences among varieties in the same species. Susceptibility
to cold damage also varies with stage of development, with most plants more
sensitive to cold temperature shortly before flowering through a few weeks after
anthesis. Chilling and frost damaged produce is not accepted by the consumers at
the markets.
High temperature in many semi-arid and arid regions may be a limiting factor
in the production of vegetables. Leaf temperatures can reach 8 °C above air temperature
under high insulation and high humidity. Heat destruction of the protoplasm
results in the death of the cells and this occurs at 45 to 50 °C. High temperatures
are detrimental to the growth and development of many vegetables. High temperatures
are implicated in flower fall in many tropical and subtropical vegetables
for example tomatoes, peppers and paprika. The Cruciferae leaf vegetables such
as kale (Brassica oleraceae) and mustard rape (Brassica juncea) have a bitter taste
after exposure to high temperatures.
Temperature does not only affect growth but also development and in some
cases it interacts with photoperiodism to affect growth and development. Some bean
cultivars behave as day neutral plants at low temperatures and as short day plants
at higher temperatures. Thermoperiodism (diurnal temperature range) also affects
development, growth and quality of plants. Generally a large diurnal range is
favourable for net photosynthesis. High night temperatures are not beneficial as
respiration rates increase using up photosynthates produced during the day. In
tomatoes, adult plant growth is most rapid and flowering more profuse with
10–12 °C difference between night and day temperatures. Some vegetables need
to be vernalised (exposed to low temperatures to induce or accelerate flowering)