Agriculture Reference
In-Depth Information
Using this equation we can calculate the LAI needed to achieve a high light
interception. For example, 60% interception requires an LAI of 3.2.
The edible alliums have narrow, upright leaves, termed 'erectophile'
foliage, resulting in a relatively low proportion of incident light interception per
unit of leaf area compared with crops with broader, more horizontal leaves,
particularly when the sun is high in the sky (i.e. at a large solar angle).
Interception of photosynthetically active radiation (PAR) by crop leaf canopies
frequently conforms to the equation:
ln(T/I) = -k.LAI or, equivalently, T/I = e
-k.LAI
(Eqn 4.2)
T = PAR transmitted through the leaves, I = PAR incident on the crop and
k is a constant, termed the extinction coefficient.
Several authors have measured k values for onions, yielding values
ranging from 0.25 (Daymond
et al
., 1997), 0.34 for the data in Fig. 4.1 for LAIs
up to 4 and, assuming the ratio of extinction coefficients of PAR to that for total
solar radiation is 1.21 to 0.47 (Tei
et al
., 1996). In the latter study the values of
k for lettuce and red-beet were 0.66 and 0.68, respectively. The values for
onion are in the range typical of crops with leaves at a steep angle to the
horizontal (Squire, 1990).
A small fraction of the intercepted PAR is reflected or transmitted, and not
absorbed. Tei
et al
. (1996) found that the PAR absorbed by an onion crop
averaged about 93% of the PAR intercepted after allowing for PAR reflected and
PAR reflected from the ground and then absorbed. The erectophile leaf habit
results in a high proportion of the leaf surface being directly sunlit compared
with crops having more horizontal leaves. This promotes efficient utilization of
the incident PAR when the crop is large with high LAI, but spaced seedlings
capture only a small fraction of the incident PAR and are easily suppressed by
light competition from overgrowing weeds with more horizontal leaves. As the
onion leaf canopy grows there is a tendency for the larger leaves to fold under
their own weight, resulting in the leaves becoming more horizontal in the later
stages of growth (Tei
et al
., 1996).
The efficiency with which absorbed light is converted to primary photo-
synthetic products (ii) can be affected by the temperature and water status of
the leaves. Clearly, if temperatures are above or below the optimum for photo-
synthesis, efficiency will be reduced. Similarly, if leaves are water-stressed to the
extent that stomata are closed and the diffusive resistance to CO
2
entry is
increased, then this too will reduce photosynthetic efficiency. Therefore, for
photosynthesis and growth the crop must have adequate supplies of water and
mineral nutrients, and temperatures must be suitable.
The conversion coefficient between the weight of sucrose produced by
photosynthesis and the weight of dry matter stored in the structural and storage
tissues of the plant (iii) depends primarily on biochemical composition. A lower
weight of lipid, protein or lignin is produced per unit of sucrose utilized in bio-
synthesis than is the case for structural or storage carbohydrates (Penning de
