Agriculture Reference
In-Depth Information
the energy stored in them. It is analogous to the need
for oxygen when a fire burns, where the energy stored
in the fuel (trapped from the sun by plants millions of
years ago) is released as heat.
In the absence of oxygen, inefficient anaerobic
respiration takes place in the cytoplasm of cells
(see Figure 9.7). Incomplete breakdown of the
carbohydrates produces alcohol (ethanol) as a
waste product, with much energy still trapped in the
molecule. If a plant or plant organ such as a root is
supplied with low oxygen concentrations, such as in
a waterlogged or compacted soil, or an overwatered
pot plant, the consequent alcohol production within
the cells may prove toxic enough to cause root death.
Furthermore, the small amount of energy released is
insufficient for growth, repair or reproduction and only
enables the plant to 'tick over' until aerobic respiration
can be restored.
Figure 9.8 Lysichiton americanus (skunk cabbage) is
a plant adapted to waterlogged soils and has many air
fi lled spaces in its root and stem tissue (aerenchyma)
which interconnect and enable oxygen to reach its
roots
glucose
alcohol (ethanol) + carbon dioxide + energy
(ATP and heat)
Figure 9.7 Equation for anaerobic respiration
of respiration therefore shows a similar pattern
of increasing rate with increasing temperature up
to an optimum, beyond which the rate decreases.
Plants adapted to high temperatures have a higher
optimum than those from temperate or cooler
regions.
In some horticultural situations, a high rate of
respiration is desirable - for example, in propagation
of cuttings or seed germination where new cell
growth needs plenty of energy, heat may be given to
speed up respiration (see Chapter 11). Alternatively,
where low respiration rates are required, such as in
seed storage (see 'Seed Storage and Respiration'
on the companion website) or to delay ripening and
senescence (see p. 74) in stored produce such as
fruit and vegetables or cut flowers, temperatures
are reduced, sometimes with control of the gaseous
atmosphere as well. Growers, distributors, retailers
and consumers have developed a 'cool chain' to keep
produce at consistently low temperatures, between
0°C and 10°C along the supply chain, reducing waste
and enabling longer shelf life through reducing
respiration rates. As well as reducing respiration,
cold storage can have other benefits. Cuttings stored
at low temperature root more readily later, while
strawberry runners kept in cold stores over winter
maintain their quality and are also stimulated to flower
the following year.
Sometimes, anaerobic conditions can be advantageous.
For the plant it enables it to survive periodic inundations
which would otherwise be fatal. For gardeners, the
viability of stored seeds can be greatly increased if they
are stored in a 'modified atmosphere' that is within
sealed, airtight packets (see Figure 5.3). The oxygen is
removed as the seeds respire and carbon dioxide levels
rise which combine to reduce the respiration rate and
inhibit germination, enabling longer storage.
In fruit and vegetable storage, inhibition of
respiration is desirable to prevent produce going
beyond the ripening stage and into senescence (see
p. 74) with accompanying loss of quality. In addition,
once harvested, photosynthesis ceases but respiration
continues so carbohydrates will continue to be broken
down and the dry weight of plants or produce will
be reduced. Packaging may provide a 'modified'
atmosphere similar to that of the seed packet with
low oxygen and high carbon dioxide levels within. For
some fruits such as apples, 'controlled atmosphere'
storage in sophisticated large-scale airtight stores
enables fine control of oxygen and carbon dioxide
levels, which, along with temperature control, can
extend the storage times well into the following year.
9
Temperature
As in photosynthesis, many of the reactions
involved in respiration involve enzymes. The rate
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