age root (carrot), a combined hypocotyl and root (beetroot), a rhizome
(Jerusalem artichoke) to a large number of folded leaves emanating from
the same central core carrying the apical meristem (winter white cab-
bage) or swollen leaf bases emanating from a dwarf stem (onion bulbs).
With ceasing growth in the autumn, the storage organ enters into an im-
posed or true dormant period with diminished metabolic rate, which
makes them adapted for long-term storage.
INFLUENCES OF THE ENVIRONMENT
Temperature
Temperature is one of the most important factors determining the
keeping quality of horticultural produce due to its influence on physi-
cal, biochemical and inductive processes. Lowering the temperature re-
duces respiration as well as other metabolic processes, and therefore
delays senescence. There is a linear relationship between the logarithm
of oxygen consumption rate and tissue temperature. The effect of a 10°C
increased temperature on respiration, the Q 10 value, averages 2.0 to 2.5
within the 5 to 25°C temperature range. At temperatures25°C, Q 10
generally decreases probably due to a gradual denaturation of enzymes
and limited diffusion rate of oxygen from surrounding air to the mito-
chondria.
With decreasing temperature, the kinetic energy of water molecules
diminishes, resulting in a reduced vaporization rate of liquid water with
a lower transpiration and water loss. Maintaining freshness and quality
during the postharvest chain is thus mainly a question of temperature con-
trol but the optimum temperatures for keeping quality of various prod-
ucts vary considerably. A number of plants of tropical or subtropical
origin develop chilling injury when stored at temperatures below a cer-
tain limit (Saltveit and Morris, 1990). Among vegetables affected by chill-
ing injury, the tomato group consisting of squash, cucumber, water melon,
aubergine, and tomato has a lower limit at 12°C while the potato group
consisting of snap beans, sweet pepper, and potato has a lower limit at
5–8°C. Vegetables originating from the temperate zone are preferentially
stored at a temperature between zero and 2°C. All produce affected by
chilling injury must therefore be kept at temperatures that limit shelf life
due to the high metabolic rate compared with chilling tolerant produce.
Rapid precooling close to harvest and an unbroken cool chain con-
tribute to a prolonged shelf life but the benefit of precooling is depen-