Agriculture Reference
In-Depth Information
on to clarify the validity of the DIF concept by investigating cut chrysanthemums
(
Chrysanthemum
cv 'Reagan Improved'), grown in growth chambers at 16 com-
binations of 4 day and night temperatures (16, 20, 24 and 28 °C) with a 12 h day
length. The research group found that DIF could predict final internode length only
within a temperature range 18-24 °C where the effects of DT and NT were equal in
magnitude and opposite in sign. Internode appearance rate, as well as stem length
formed during the experiment, showed an optimum response to DT, with the au-
thors concluding that plants do not respond to DIF itself, but rather to the combina-
tion of independent effects of temperature measured during day and night periods.
Similarly a negative DIF strategy is still used, in order to reduce and substitute
growth retardants used in controlling plant height or stem elongation of a number
of different horticultural crops. Moe et al. (
1992
) reported that the most appreciable
inhibitory effects on poinsettias were observed when lowering the growing tem-
perature for 2-4 h before the dawn (cool morning strategy).
Peet and Bartholemew (
1996
) and Abdelmageed and Gruda (
2009
) emphasized
the role of night temperatures on pollen characteristics and reported that total and
percentage normal pollen grains were higher in tomatoes grown under normal night
temperature than at high night temperatures. High day/night temperature differenc-
es or wide fluctuations in temperature can also induce disorders, like the cracking
of tomato fruits (Peet
1992
).
Low and High Growing Temperatures
Plastic greenhouses are also widely used for horticultural production in warm re-
gions where high radiation and mild temperatures make production successful. By
contrast, during the cold season, suboptimal temperatures and low irradiation can
adversely affect growth and yield, and reduce the product quality of crops. Since
warm-season crops are most likely cultivated under protected cultivation, damages
could happen even at temperatures above freezing point. After long low tempera-
ture exposure leaves wilt and yellow and show various metabolic process distur-
bances. Furthermore, low temperature exposure has an influence on external and
internal product quality.
Several authors have reported that low temperatures can cause fruit malforma-
tion and distortion, seedlessness, pericarp cracking, and pigmentation formation in
various fruits and vegetables (Gruda
2005
). Moreover, low night temperatures can
reduce the number of pollen grains per flower and impair the germination ability of
vegetables such as tomatoes and peppers with a tendency to develop parthenocarpic
fruits.
Temperature can also influence the color intensity in most flower and fruits.
Usually, low temperatures in combination with high light intensities hinder the col-
oring of flowers, bracts, and leaf parts, whereas some petunia cultivars show an
increase in color intensity at higher temperatures. Fruits such as tomatoes, peppers
and eggplants require relatively high temperatures for dye synthesis with color and
color intensity interacting with growth factors, such as light (Jansen et al.
1989
). For
