Agriculture Reference
In-Depth Information
summer months, with night temperatures generally to be kept lower than day tem-
peratures in order to reduce respiration and heating costs. Minimum air tempera-
tures are the lowest occurring plant-specific temperatures, which plants can toler-
ate in the short-term without permanent damage. For technical reasons frequently
experienced in cold regions when it is snowing, the minimum temperature has to
be regulated, so that snow can be defrosted by the greenhouse cover. The stage at
which the grower commences cooling under increasing temperatures is of great
importance, to avoid extreme heat stress situations. It is necessary to distinguish be-
tween optimal physiological and economical temperatures. Physiologically optimal
air temperatures are temperatures at which, under the given irradiation, provide for
maximum plant development per time-unit. By contrast, economically optimal air
temperatures, take other features into account such as culture duration, yield, size,
quality, and energy costs. Economically optimal air temperatures are frequently
lower than optimal physiological temperatures with the differences usually a little
higher for vegetables than for ornamental plants (Jansen et al. 1989 ).
New ways to increase energy efficiency and reduce costs of production include
limiting the cultivation period to periods of adequate solar radiation, and lowering
the economically optimal temperatures for heating and lowering the target tempera-
ture, to reduce energy consumption. For example, Elings et al. ( 2005 ) calculated
that lowering day and night temperature set points for tomato by 1 °C lead to a re-
duction of 8 % per year energy consumption. However, lowering temperatures can
adversely affect the leaf area development and light interception, resulting in lower
production, extending the development time of the plant and adversely affecting
product quality. This could be a problem especially for date cultures, e.g. poinsettia
(  Euphorbia pulcherrima ), as well as early protected vegetables crops. Therefore,
one of the main decisions that growers have to make is either to have a longer
growth period, with lower energy requirements associated with often lower yields,
or a shorter cultivation time with higher heating expenses and more often better
returns by advancing the crop. Both cases incur additional costs that obviously will
be higher for longer than shorter growth periods.
Day and Night Temperatures
Generally, a constant temperature is less favorable than a fluctuating one between
day and night, when associated with high temperatures during the day and lower
ones at night. Of importance here is the decrease in respiration at lower tempera-
tures (night) and the increased photosynthesis at elevated temperatures (day).
The DIF-concept, or the difference between the day and night temperatures, was
introduced in the horticultural literature in the 1990's. The DIF can be positive (day
temperature = DT, is higher than night temperature = NT) or negative (DT is lower
than NT). Langton and Cockshull ( 1997 ) reported that absolute day and night tem-
peratures explained internode length rather than DIF, when an equal photoperiod
of 12 h (day/night) was applied to chrysanthemum. Carvalho et al. ( 2002 ) went
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