Agriculture Reference
In-Depth Information
6.8
Development
temperature is considered, such responses
being independent of the thermal regime
(day/night temperature), within certain
limits. This capacity of integrating the
thermal fluctuations in time (responding
to the average thermal values) is not lim-
ited to periods of 24 h (being possibly
higher), but is observed only in developed
crops with closed canopies, which com-
pletely cover the ground (Challa
et al
.,
1995). This can optimize short- and long-
term responses: for instance, using the
daytime temperature to optimize photo-
synthesis and managing the night temper-
ature to obtain the desired average thermal
values (Challa
et al
., 1995).
6.8.1
Introduction
Development is a qualitative notion of the
stage of the plant. It is the ordered change
towards a higher or more complex stage of
the plant (Challa
et al
., 1995). The time
intervals between different developmen-
tal stages constitute the development
phases. The development of a crop fol-
lows a basic pattern according to its
genetic make-up, which may be modified,
although not changed, by the environment
(Challa
et al
., 1995).
Flower differentiation is a development
phase and takes place when a series of con-
ditions are met. Some of these conditions
are internal, for instance, that the plant
reaches a certain maturity (corresponding to
an age or a certain number of leaves) that
allows the meristems to differentiate from
vegetative to flowering. Other conditions
are external, for instance, the existence of
certain conditions of photoperiod or tem-
perature. One of the most used development
indicators, at the whole plant level, is the
number of leaves.
When the photoperiod and nutrition
conditions are favourable, the rate of
development depends primarily on tem-
perature, its response being linear from
the lower thermal threshold (known as
vegetative zero, characteristic of each spe-
cies, below which there is no growth) to
the top limit (characteristic of each spe-
cies) from which the development rate
decreases. The vegetative zero of the
majority of the horticultural species ranges
from 0 to 6°C.
The thermal integral received from a
certain moment determines, in many cases,
the beginning of a certain developmental
stage. In a similar way, the photo-thermal
integral (thermal integral corrected accord-
ing to the received radiation) determines,
when reaching a certain value, the begin-
ning of a developmental stage, for instance
flowering (Berninger, 1989).
When considering how a crop responds
to temperature, in terms of growth, devel-
opment and production, the daily average
6.8.2
Development stages in
greenhouse crops
The most important development stages in
greenhouse crops are: (i) the germination
and sprouting (of bulbs and corms); (ii) flow-
ering; and (iii) the formation of reserve
organs.
The essential factors for good germina-
tion are humidity and temperature, although
some seeds need a pre-treatment that inter-
rupts their latency. After germination the
seedlings need light to expand their first
leaves.
Many species don't need flower induc-
tion, as they flower when they reach adult
stage: such is the case with the majority of
vegetables. Other species need exogenous
signals to flower under natural environmen-
tal conditions, such as photoperiodic signals,
alone or associated with temperature or radi-
ation. The direct control of temperature on
flower induction occurs, for instance, in
Chinese cabbage whose flowers are induced
by low temperatures, commercially depreci-
ating the product (Hernández, 1996).
In the first stages of development, many
plants do not flower, even if they receive the
right stimuli. However, with greenhouse
vegetables such as tomato and pepper, trans-
planting takes place at an advanced devel-
opmental stage with flowers clearly
developed, to maximize the utilization of
expensive production inputs.
