S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253 251
winter. This suggested that the south plant in E–W row canopy was affected more by its neighbouring plants
than that in N–S row canopy. In addition, the regular fluctuation following the leaf number in values of nor-
malized daily leaf irradiance was obviously caused by the differences in their azimuth directions. For exam-
ple, leaf number 2, 6 and 10 on the south plant have a azimuthal direction to south see Fig. 2, consequently
comparatively higher normalized daily leaf irradiance was observed from these leaves in most cases Fig.
6. Some exceptions about this, e.g., lower values of leaf number 6 than leaf number 5 at 35 and 45
◦
N for E–W row plant, and at 55
◦
N on the winter solstice for both E–W and N–S plants, further suggested that the
amount of direct solar radiation accepted by a certain leaf is affected by many factors and under certain cir-
cumstances, shade-effect from the surrounding plants or leaves was also an important factor affecting values
of normalized daily leaf irradiance.
Comparisons of normalized daily leaf irradiance be- tween the north plants under E–W and N–S row ori-
entations are shown in Fig. 7. On the winter solstice, leaf numbers 2 and 3 of the N–S row plant gave higher
normalized daily leaf irradiance than that of E–W row plant, but the comparison between the rest of the leaves
showed the opposite results in general in every lati- tude region. On the vernal equinox, leaf numbers 2, 3,
6, 7, 8 at 35
◦
N, and 2, 3, 7 at 55
◦
N of the N–S row plant had higher normalized daily leaf irradiance than
that of E–W row plant. And on the summer solstice, higher values of normalized daily leaf irradiance were
shown in most leaves on N–S row plant than that of E–W row plant. Furthermore, compared with the re-
sults for the south plant Fig. 6, the regular fluctua- tion in normalized daily leaf irradiance following the
leaf number i.e., azimuthal direction was not so ob- vious. This was especially true for the cases on the
winter solstice and vernal equinox. Instead, leaves on the upper nodes of the plant i.e., leaf numbers 1, 2,
3 generally gave higher normalized daily leaf irradi- ance than those on lower nodes. Similar results were
obtained on the plants in the central part of the row canopies under both row orientations data not shown.
This suggested that, for those plants which located in the central or northern parts of the canopy, the leaf
position has more important effect on the amount of direct solar radiation reaching it than its azimuthal
direction.
4. Discussion
The results of scale-model experiments under ar- tificial direct light showed that the amount of direct
solar radiation received by the canopy leaves varied with latitudes as well as seasons. During the winter
months, namely, the main seasons of greenhouse pro- duction, the E–W row canopy gave a higher normal-
ized daily canopy irradiance than that of N–S row canopy at 35
◦
N, while the opposite results were found at 45 and 55
◦
N. However, during the spring and sum- mer seasons, N–S row canopy gave a higher normal-
ized daily canopy irradiance than that of E–W row canopy regardless of latitude. In addition, larger differ-
ences in normalized daily canopy irradiance between E–W and N–S row orientations were found in win-
ter andor early spring months than those in summer andor late spring months.
Under E–W row orientation, the lower normalized daily canopy irradiance at 45 and 55
◦
N, and the higher normalized daily canopy irradiance at 35
◦
N in the win- ter seasons, suggested that appropriate row orientation
varied in different latitude regions. Some common characteristics can be recognized
from the seasonal variations of normalized daily canopy irradiance shown in Fig. 3. First, a peak
value could be found during period from the winter solstice to the vernal equinox regardless of latitude
and row orientation: on 21 January at 35
◦
N, on 21 February at 45
◦
N and on 21 March at 55
◦
N. It is noteworthy that the solar altitudes at culmination
on the above-mentioned days were 35.2, 34.3 and 35.0
◦
, respectively. This suggested that, in winter and early spring months, the dates with an altitude
at culmination of about 35
◦
could be considered as the appropriate time period concerning acceptance of
more direct solar radiation by the canopies. Allen 1974 analysed the direct-beam radiation
penetration into wide-row sorghum canopies by a mathematical model 17 August, 40
◦
N and indi- cated that low values of light interception in the early
morning and late afternoon by E–W rows resulted in the lower values of daily total intercepted direct
light than N–S rows. Similar results were obtained in the present study and the differences in normal-
ized canopy irradiance between E–W and N–S row orientations were found at the most time of the day
see the results on the summer solstice in Fig. 4. As
252 S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253
indicated previously, the lower values of normalized canopy irradiance in the early morning on the vernal
equinox and summer solstice were obviously caused by the shading effects of opaque north wall, north
roof and the east-, west-gable walls of the lean-to greenhouse Fig. 4. So this does not affect the com-
parison between the results in the present study and those in Allen 1974.
Moreover, results obtained in the present study agreed with those of Iwakiri and Inayama 1974;
Mutsaers 1980; Kurata et al. 1988 with regard to latitude of 35
◦
N, but the difference between row orientations obtained in the present study was much
smaller than their results in winter and early spring months. For example, normalized daily canopy irradi-
ance of E–W orientation was only about 0.02 higher than that of N–S orientation on 21 December in the
present study Fig. 3, see results under 35
◦
N, but this difference was 0.10 in the results obtained by Mut-
saers 1980. However, with regard to the results dur- ing winter and spring months at 45 and 55
◦
N, results in the present study were different from those got by
Mutsaers 1980. In Mutsaers’s study, higher values of daily direct light absorption by E–W row canopy
than that by N–S row canopy were found from 15 September to 15 April at 45
◦
N and from 15 August to 15 April at 55
◦
N. In contrast to this, higher values of normalized daily canopy irradiance were found in
the present study in N–S row canopy than in E–W row canopy though the magnitude of the difference
between row orientations was much smaller in winter and early spring months than those in summer and
late spring months at both 45 and 55
◦
N. Comparisons between the geometrical structural parameters of the
canopies used in the present study and that by Mut- saers revealed that difference in the ratio of H
r
to W
ir
, i.e., row height H
r
to width of inter-row W
ir
could be the main reason to this discrepancy. The value of
H
r
W
ir
in the present study was 9022.5 = 4 Fig. 1, while that in Mutsaers’s study was 10050 = 2, that
is, the H
r
W
ir
value of the former is twice as much as that of the later. Iwakiri and Inayama 1974 indi-
cated that the amount of direct light received by the plants inside the canopy was decreased rapidly as the
reduction of W
ir
i.e., as the increase of H
r
W
ir
, and in the winter months this effect was bigger in E–W
row canopy than in N–S row canopy. Similarly, the different magnitudes in the differences in normal-
ized daily canopy irradiance between E–W and N–S orientations as regard to that in the winter at 35
◦
N be- tween present study and that by Iwakiri and Inayama
Iwakiri and Inayama, 1974, H
r
W
ir
= 150100 = 1.5
could be intercepted by the same way. The information gained from Figs. 5–7 could help
understand some mechanism of the differences of nor- malized daily canopy irradiance Fig. 3 between E–W
and N–S row orientations. On the summer solstice, higher values of normalized daily leaf irradiance in
most leaves Figs. 6 and 7 and normalized plant irra- diance at most time points of the day Fig. 5 on both
the south and the north plants in N–S row canopy than those in E–W row canopy were direct reasons for the
higher normalized daily canopy irradiance in N–S row canopy compared with the E–W row canopy in ev-
ery latitude region. On the winter solstice and vernal equinox, however, the reasons for the differences in
normalized daily canopy irradiance between row ori- entations could be a little more complicated. Higher
normalized daily canopy irradiance was contributed by the higher average of normalized daily leaf irra-
diance i.e., normalized daily plant irradiance under either of the row orientations — as shown in Figs. 6
and 7, curves of normalized daily leaf irradiance for E–W and N–S row canopies were crossed each other
at several time points.
In addition, analytical results on the winter solstice and the vernal equinox also showed that, regardless of
the row orientation and latitude, the azimuthal direc- tion of the leaf on the south plant has bigger influence
on normalized daily leaf irradiance than the leaf po- sition on the plant. That is, for the plant on the south
edge of the canopy, a leaf with an azimuthal direction to south or east showed a higher value of normalized
daily leaf irradiance than those with a azimuthal di- rection to north or west in the morning. However, for
those plants located in the central and northern parts in the canopy, leaf position has more important effect
than its azimuthal direction on the normalized daily leaf irradiance; generally, leaves on the upper nodes
gave higher values of normalized daily leaf irradiance than those on lower nodes. This is in agreement with
the results observed within a 2 m high cucumber row canopy by Nederhoff 1984. Her results showed that
80 of the leaves were in shade at only 0.25 m depth below the top of the canopy under direct solar radia-
tion condition.
S. Li et al. Agricultural and Forest Meteorology 100 2000 243–253 253
The process of solar radiation penetration into the crop canopy is quite complicated and in many cases,
it is nearly impossible to understand all the details. Results presented in this paper were based on the av-
erage value of the direct solar radiation intercepted by leaf surface of the model crop. They help to ex-
plain the preference of the N–S row orientation for lean-to greenhouses by Chinese growers. The results
were obtained under certain conditions. So, there must be limitation for their applications. Detailed analysis
and experimental observation based on the improved model crop canopies await further investigation.
5. Conclusions
