Production Practices and Quality Assessment of Food Crops. Vol. 1
Production Practices and Quality Assessment of Food Crops. Vol. 1
Production Practices and Quality Assessment of Food Crops. Vol. 1
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2.1. Temperature<br />
Effect <strong>of</strong> Preharvest Factors 3<br />
2. GROWING ENVIRONMENT<br />
Temperature is a major factor controlling quality in vegetables <strong>and</strong> it is very difficult<br />
to modify especially for resource poor smallholder farmers. The extreme<br />
temperature range <strong>of</strong> plants is between killing frost at 0 °C <strong>and</strong> death by heat <strong>and</strong><br />
desiccation at 40 °C with most plants permanently immobilized at 10oC <strong>and</strong> most<br />
plants ceasing to photosynthesize efficiently above 30 °C (Nonnecke, 1989).<br />
Seasonal temperature variation can change as little as 3 °C all year round in the<br />
tropics <strong>and</strong> as much as 45 °C in the polar zone. Temperature decrease with increases<br />
in altitude, a drop <strong>of</strong> about 6 °C occurs with each 1000 m in elevation. Cardinal<br />
temperatures, minimum <strong>and</strong> maximum (where plant growth ceases) <strong>and</strong> optimum<br />
(where growth occurs most rapidly) vary with different families, genera <strong>and</strong> species.<br />
For every 10 °C rise in temperature, growth doubles at the temperature range <strong>of</strong> 5<br />
to 35 °C depending on the vegetable (Van’t H<strong>of</strong>f’s Law).<br />
Most vegetables are sensitive either to high or low temperatures. Tropical vegetables<br />
are mainly sensitive to low temperatures. Most vegetables are either injured<br />
at temperatures at or slightly below freezing (freezing injury). Tropical <strong>and</strong> subtropical<br />
plants may also be killed or damaged by the cold at temperatures below<br />
10 °C but above freezing (chilling injury). These are discussed in Sections 3.1<br />
<strong>and</strong> 3.2 respectively. Susceptibility to cold damage varies with different species<br />
<strong>and</strong> there may be differences among varieties in the same species. Susceptibility<br />
to cold damage also varies with stage <strong>of</strong> development, with most plants more<br />
sensitive to cold temperature shortly before flowering through a few weeks after<br />
anthesis. Chilling <strong>and</strong> frost damaged produce is not accepted by the consumers at<br />
the markets.<br />
High temperature in many semi-arid <strong>and</strong> arid regions may be a limiting factor<br />
in the production <strong>of</strong> vegetables. Leaf temperatures can reach 8 °C above air temperature<br />
under high insulation <strong>and</strong> high humidity. Heat destruction <strong>of</strong> the protoplasm<br />
results in the death <strong>of</strong> the cells <strong>and</strong> this occurs at 45 to 50 °C. High temperatures<br />
are detrimental to the growth <strong>and</strong> development <strong>of</strong> many vegetables. High temperatures<br />
are implicated in flower fall in many tropical <strong>and</strong> subtropical vegetables<br />
for example tomatoes, peppers <strong>and</strong> paprika. The Cruciferae leaf vegetables such<br />
as kale (Brassica oleraceae) <strong>and</strong> mustard rape (Brassica juncea) have a bitter taste<br />
after exposure to high temperatures.<br />
Temperature does not only affect growth but also development <strong>and</strong> in some<br />
cases it interacts with photoperiodism to affect growth <strong>and</strong> development. Some bean<br />
cultivars behave as day neutral plants at low temperatures <strong>and</strong> as short day plants<br />
at higher temperatures. Thermoperiodism (diurnal temperature range) also affects<br />
development, growth <strong>and</strong> quality <strong>of</strong> plants. Generally a large diurnal range is<br />
favourable for net photosynthesis. High night temperatures are not beneficial as<br />
respiration rates increase using up photosynthates produced during the day. In<br />
tomatoes, adult plant growth is most rapid <strong>and</strong> flowering more pr<strong>of</strong>use with<br />
10–12 °C difference between night <strong>and</strong> day temperatures. Some vegetables need<br />
to be vernalised (exposed to low temperatures to induce or accelerate flowering)