Agriculture Reference
In-Depth Information
processes have occurred. Each of these aspects will now be considered in more
detail.
The quantity of light absorbed (i) depends on the quantity of light radiating
on the field, the percentage absorbed by the leaves and on the duration of
growth of the yield-bearing components. Clearly then, a high-yielding crop
must produce a leaf canopy with a high percentage absorption of incident light
during a season of high solar irradiation. Furthermore, the longer the duration
over which such a leaf canopy is transferring photosynthate to harvestable
material, the higher the yield. Much of this chapter is concerned with explain-
ing how the season of, and magnitude of, leaf canopy development is controlled
by physiological responses to environmental conditions. For the bulb-forming
species the situation is complex, because bulb development ultimately leads to
a cessation of foliage leaf production and this prevents the renewal and
maintenance of the crop leaf canopy. There is no such finite limit to the crop
leaf canopy for non-bulbing species like leek and Japanese bunching onion.
Nevertheless, the growth of allium crops can also be curtailed by the onset of
unfavourable weather conditions or by flower stalk development (bolting).
The proportion of the total incident light intercepted by leaves depends on
the area of leaf surface per unit area of ground, the Leaf Area Index (LAI).
Figure 4.1 shows the relationship between LAI and percentage incident solar
radiation intercepted by the leaf canopy (I%) for onions in summer in central
England. This is well summarized by the equation:
e
-0.377.LAI
)
I% = 85.4(1
(Eqn 4.1)
Fig. 4.1.
The relationship between percentage light (photosynthetically active
radiation, PAR) interception by leaf canopies of onion cv. 'Robusta' and Leaf Area
Index (LAI) determined non-destructively from leaf surface areas calculated as leaf
length x maximum width x 2.2 (from Mondal, 1985).
