Agriculture Reference
In-Depth Information
Fig. 4.32.
Relationships between maturity date and the mean percentage of light
(PAR) intercepted by onion crop leaves during bulbing. The lines are best-fitting
linear regressions for cvs 'Express Yellow',
; 'Keepwell',
- both autumn-sown -
and 'Augusta',
▫
; and 'Robusta',
●
- both spring-sown (from Mondal
et al
., 1986a,
courtesy of
Annals of Botany
).
neighbouring plants was not important. Furthermore, both large neighbouring
onion and carrot plants elicited accelerated bulb initiation in the tested onions.
This indicates that: (i) accelerated bulbing is associated with light competition,
and not with factors in the root environment; and (ii) shading by any green
foliage will accelerate bulb initiation. Table 4.6 also shows that the ratio of red to
far-red wavelengths, i.e. 660:730 nm, in light under the denser leaf canopies is
much reduced compared with daylight. Studies in controlled environments
where onions have been grown under constant photoperiods and temperatures,
but in light with a range of red:far-red ratios, show that the lower this ratio the
faster the rate of bulbing (see Fig. 4.29b).
Many plant developmental processes, ranging from seed germination to
stem elongation, are controlled by the red:far-red ratio in incident light. This
ratio is lowered when light passes through leaf canopies, because leaves absorb
red wavelengths more strongly than far-red. Therefore, this ratio is a sensitive
indicator of the presence of leaves of neighbouring plants. The red:far-red ratio
