Agriculture Reference
In-Depth Information
are repeatable and the particular pathogens are allowed
to increase and indeed infect the plants. It also allows
control of the disease when it is time to stop further
infection
Screening breeding lines for abiotic stresses can also
be achieved under greenhouse conditions if the envi-
ronment can be controlled in a repeatable and relevant
manner. Stress screening has been shown to be reliable to
such factors as tolerance to nutrient deficiency, drought,
salinity and heat where it is not always possible or easy
to control the relevant environmental factors involved
under natural conditions in the field.
It should, however, be noted that evaluations
designed to be carried out under greenhouse condi-
tions must first be compared to results that would have
been achieved under natural field condition. There have
been numerous cases where selection has been carried
out under controlled conditions and later found to
bear little, if any, relationship to what subsequently is
experienced under field conditions.
facilities which can be time consuming and expensive if
the number of plants involved is large.
Growth within greenhouses requires artificial irriga-
tion. Irrigation can be by hand, which allows for some
flexibility but does not usually allow for complex irri-
gation management systems. Automatic irrigation is
usually preferable and can be of three forms:
•
Above plant irrigation (or misting) where plants are
sprinkle or mist irrigated from above. This can be rela-
tively inexpensive but can cause problems if plants are
tall. Above plant irrigation can also increase the risk
of plants becoming infected by fungal diseases where
leaf moisture is necessary for infection to take place.
It can also be a problem in generating leaf scorch in
strong sunlight.
•
Below plant irrigation where plants are irrigated by
capillary action by having moist or wet material below
the plant pots. Below plant irrigation avoids the
above-ground plant parts becoming wet although it
can be difficult to establish young plants and main-
tain very large plants with such a system alone and it
is sometimes necessary to hand water as a supplement
to the system.
Environmental control
Drip irrigation where each plant pot is individually
irrigated directly into the soil by a drip line. There
are several different forms of drip irrigation and this
system offers greatest flexibility over all others. This
system is, however, the most expensive to install and is
not always available in all plant breeding greenhouses.
Since the system requires that individual drip lines are
located in each plant pot, there can be some restriction
on the number of plant units that can be grown, so
this need to be carefully considered when setting the
system up.
•
Artificial lighting (fluorescent and/or incandescent) is
nearly always necessary to achieve maximum use of
greenhouse space. Lighting is, however, expensive both
to install and maintain, particularly if different day
lighting regimes are required. When, however, light-
ing is available, it usually allows the greenhouse to be
utilized throughout the whole year.
If plants are to be propagated in the greenhouse
throughout the year it will also be necessary to have
a suitable heating and/or cooling system. A range of
different types of systems is available and these can-
not be adequately covered here. However, it should be
noted that all the types require a relatively high cost to
install and operate. Thus it is usual to expect to have
to justify the costs in terms of likely returns of, for
instance, increased numbers of generations, effectives
of tests etc. Good control of temperature is of course
important if healthy plants are to be propagated. A par-
ticular example in which temperature control is often
needed is in biennial crops where plants require vernal-
ization (chill treatment) before they will flower. Plants
grown under greenhouse conditions can be vernalized
outside the greenhouse (e.g. in a growth chamber or
cold room) but this will involve moving plants between
All methods of irrigation offer the possibility of
applying nutrients along with the water and so they
can be provided 'continuously' thus enabling more
optimized growth of plants over other methods of
nutrient/fertilizer application.
Disease control
Unless disease is to be deliberately encouraged, as in the
case of a screening scheme, it is desirable to avoid as
many diseases and pests as possible in a breeding green-
house. The best results are invariably achieved when the
