spiration rate of small radish separated in leaves and tubers. Postharvest
storage conditions, weight and dimensions of the single small radishes
were nearly constant. Transpiration of the tubers and the leaves showed
a pronounced seasonal effect. In comparison to early summer and au-
tumn, the transpiration rate of both product parts in summer was lower
and attributed to an increased cutin and suberin synthesis. The rising
cutin production was induced by the low relative air humidity (nearly
30%) during the growth period. In a similar way, increasing soil tem-
peratures associated with lower soil humidity might lead to a stronger
suberin insertion in the external tissue layer of the tubers. In compari-
son with the tubers, the leaves showed a reduced transpiration rate, which
might be explained by a better adaptation to high water potential dif-
ferences.
EFFECT OF POSTHARVEST STORAGE CONDITIONS
ON TEXTURAL PROPERTIES
The altering gas exchange (i.e., changes in respiration, transpiration
and ethylene production) was found to be closely correlated with changes
in the textural properties of the cell wall during development and ripen-
ing (Kader, 1987; Dick and Labavitch, 1989; Huyskens, 1991; Mend-
linger et al., 1992; Sonego et al., 1995). Softening is dependent on the
physiological stage of the fruit or vegetable and results from a transfor-
mation of insoluble pectic substances to water-soluble pectin leading to
losses of quality (Huyskens-Keil et al., 1998).
The strong relationship between the change of total pectic substances
and respiration rate during postharvest storage and changes in water con-
tent, related to the fresh matter at harvest time, are presented in Figure
5.6 for lettuce. The data were estimated by a regression analysis, where
respiration rate was used as an independent variable among other vari-
ables. Respiration and the decomposition of total pectic substances de-
pended on the water status of the product. A decrease of pectic substances
occurred at a low water content of lettuce (65%) (Schreiner et al., 1995).
Reduced decomposition of pectic substances with rising respiration may
be due to the enhanced energy demand for detaching monosaccharides
linked to pectic substances and for utilizing them as additional respira-
tion substrates. At a high water status of lettuce (87%, 85%), there was
no decomposition of pectic substances. The negative values of pectin
degradation at higher water status indicate preferred respiration of stor-
age carbohydrates.
Effect of Preharvest Storage Conditions on Textural Properties 91