Agriculture Reference
In-Depth Information
in light is detected by the phytochrome pigment system. The main ecological
significance of the phytochrome system seems to be the detection of the degree
of shading experienced by a plant, or part of a plant, thereby enabling growth
to be modified in an adaptive fashion (Smith, 1982).
In the case of onions, decreased red:far-red ratio in the incident light
accelerates bulbing and crop maturity. Both increases in crop plant density and
weed competition increase LAI and accelerate bulbing. It is not difficult to
imagine that accelerated bulbing under competition from taller, faster-grow-
ing neighbours could have had survival value to the ancestors of onions. The
low, slow-growing, weakly competive onions could bulb and enter a dormant
phase when competition became intense, and could thereby survive until the
following season.
It can be important that spring-sown crops reach an LAI at which this effect
of decreased red:far-red ratio becomes significant. The resulting extra 'push'
towards bulbing ensures that bulbs mature despite the decreases in photoperiod
and temperature of late summer. Factors that decrease LAI - such as disease,
pest or hail damage to foliage, low plant populations due to poor seedbed
conditions, late sowing, damage from herbicides or stress from lack of nutrients
or water during seedling growth - could all contribute to a failure of bulbing to
reach completion, the reversion to leaf blade growth and thick-necked plants
(see Fig. 6.11). This will particularly be the case in cool summers, when the rate
of bulbing is slowed by low temperature (see Eqn 4.21).
Studies in which bulb-inducing photoperiods were interrupted by 3 h of
light of a specific wavelength have produced an 'action spectrum' for onion
bulbing showing that the most effective wavelength is 714 nm, i.e. just beyond
the visible red in the far-red region of the spectrum. The promotive effect of a
period of far-red light can be reversed by following it immediately with red
light, confirming the involvement of phytochrome (Lercari, 1983). The bulb-
promoting effect of a period of far-red light is maximal if it occurs near the
middle of the photoperiod. The importance of the timing of such treatments
relative to the 24 h cycle of light and dark suggests that daily (circadian)
rhythms are involved in the phytochrome response. Circadian rhythms are
implicated in many plant photoperiodic responses (Thomas and Vince-Prue,
1996). The photoperiodic, red:far-red and temperature responses of bulbing
may all be interrelated via some, as yet unknown, common control system that
involves phytochrome.
Plant and leaf age effects
Experiments in which plants of different ages were defoliated to a similar leaf
area and then transferred to bulb-inductive photoperiods showed the older
plants bulbing more rapidly than the younger ones. On the other hand,
removal of either young leaves or old leaves on plants of the same age, and then
