40 C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47
Total water use by intercrops is frequently little differ- ent from monocrops, particularly when water losses
from land left bare after harvesting the shorter duration component are taken into account Morris and Garrity,
1993. These workers reviewed several experiments in which seasonal rainfall varied between 84 and 575 mm
and concluded that water use by intercrops was gen- erally within ±7 of equivalent monocrops, although
larger benefits were occasionally observed. For exam- ple, Natarajan and Willey 1980a,b found no differ-
ence in water use between monocrops and intercrops of pigeonpea and sorghum up to the point when the
shorter duration sorghum was harvested. However, the longer duration pigeonpea extracted a further 170 mm
prior to harvest 10 weeks later, utilising residual water and late-season rains that would otherwise have been
lost. Nevertheless, although total water use by the in- tercrop greatly exceeded that of monocrop sorghum,
there was no advantage over monocrop pigeonpea. In a similar study, Reddy and Willey 1981 showed that
water use by milletgroundnut intercrops exceeded that of the monocrops, primarily because a larger leaf area
index was maintained for longer. Substantial improve- ments in water use may also occur when species with
complementary root distributions are used, as in the ricepigeonpea system described by Jena and Misra
1988 in which the deep rooting pigeonpea extracted substantial quantities of water from below the rooting
zone of rice Oriza sativa.
The beneficial effect of intercropping generally originates from improvements in e
w
rather than sea- sonal water use, to which several factors may con-
tribute. Intercropping may increase the proportion of available water that is used for transpiration because
the presence of a C4 cereal results in more rapid canopy development and reduced soil evaporation;
however, this potential benefit must be offset against the risk of premature depletion of available water prior
to maturity. Fast growing C4 species with inherently high e
w
values may also capture more of the avail- able water, thereby increasing the overall yield and
e
w
value for the system. The dominant and subsidiary components of many intercrops are, respectively, C4
and C3 species with relatively high and low e
w
val- ues. Finally, intercropping may confer microclimatic
benefits including partial shade and reduced evapora- tive demand on the shorter component, frequently a
C3 species with a relatively low photosynthetic light saturation point; in such instances, partial shade may
have little effect on assimilation, with the result that e
w
is improved by the concurrent reductions in tran- spiration. These comments are less applicable to agro-
forestry, in which the overstorey trees are invariably C3 species and the understorey species may be C4
cereals which do not respond favourably to shading.
4. Thermal time
years ago, at an international conference on soil physical properties and crop production in the tropics
held at Ibadan, Nigeria, JLM remarked that the litera- ture on soil temperature published during the previous
20 years had extended existing evidence that crops respond to soil temperature without adding much
to our understanding of the underlying mechanisms Monteith, 1979. At an earlier symposium on the
ecophysiology of tropical crops in Manaus, Brazil, he proposed that plant responses to temperature should
be analysed in terms of the reciprocal of the duration of specific stages of development, rather than sim-
ply examining the rates of growth and development Monteith, 1977b. He postulated a linear relationship
between a base temperature, T
b
, at which specific processes such as germination or primordial initia-
tion begin, and an optimum, T
o
at which the process proceeds at its maximum rate, and another linear but
declining relationship between T
o
and a maximum temperature, T
m
, beyond which development ceases. These relationships have been confirmed by several
studies of germination in both tropical and temperate species Garcia-Huidobro et al., 1982a for pearl mil-
let; Mohamed et al., 1988 for millet and groundnut. There is now general agreement that values of T
b
and T
o
lie within the range 9–13 and 27–32
◦
C, respec- tively, for a range of mainly tropical species Angus et
al., 1981, but the precise range for T
m
41–47
◦
C has been defined for only a few processes. The concept of
thermal time which arose from these relationships, in contrast to earlier suggestions that plants may ‘accu-
mulate temperature’, has since been used to describe the phenological responses to temperature of many
crops, with particular success in terms of defining the rate of progression towards key events such as flow-
ering and maturity Ong and Monteith, 1985; Roberts and Summerfield, 1987.
C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47 41
In his 1977b paper, JLM also stressed the need to understand the relation between the time of exposure
and the effects of high soil temperature on the growth of tropical plants. Unfortunately, such information
is still limited to a few studies of germination. The work of Garcia-Huidobro et al. 1982a,b; 1985 on
the germination of pearl millet seeds was among the first to examine the effects of supra-optimal tem-
peratures; substantial reductions in both the rate of germination and final percentage germination were
obtained when seeds were exposed to 50
◦
C for 2–4 h Garcia-Huidobro et al., 1982a,b; Garcia-Huidobro
et al., 1985. Subsequent work confirmed that germi- nation in pearl millet was delayed by heat treatment,
and that substantial genotypic variation in responses to supra-optimal temperatures 40–45
◦
C existed Khalifa and Ong, 1990.
In his Manaus paper on climate, JLM 1977b also postulated that the maximum number of organs which
differentiated or the maximum size achieved by a set of organs may either increase or decrease as temper-
ature increases depending on whether the duration or rate of growth is more sensitive to temperature. He
suggested that the observed reduction in the number of spikelets in rice as temperature decreased might be
because T
b
for the duration of initiation was higher than that for the rate of initiation. New insights into
the major determinants of spikelet final numbers came following analysis of results obtained in Kenya
for maize Hawkins and Cooper, 1981 and in India and the controlled environment glasshouses at Sutton
Bonington for pearl millet Ong and Squire, 1984. A strong correlation was found between spikelet or
grain number and ‘thermal growth rate’, expressed as intercepted radiation per degree day MJ
◦
C per day or the increase in plant biomass per degree day g per
plant
◦
C per day; Fig. 5. Similar interactions between growth or radiation interception and development
or thermal time in determining spikelet and grain numbers have been demonstrated for other species; for
example, the thermal growth rate concept has recently been used to explain large differences in groundnut
yield between the humid tropics of Indonesia and the subtropical regions of Australia Bell and Wright,
1998.
The concept of thermal time is now being used to examine crop responses to microclimatic modifi-
cations in tropical agroforestry systems. The earliest
Fig. 5. The relation between the final number of grains per plant G
n
and thermal interception rate TIR per plant for pearl millet. TIR is calculated as intercepted radiation divided by accumulated
thermal time during the corresponding period and is expressed as MJ
◦
C per day. Different symbols represent data from separate experiments carried out under controlled environmental glasshouse
conditions modified from Ong and Squire, 1984.
example is that described by Corlett et al. 1992a for leucaenapearl millet alley cropping systems in India.
These workers used the ratetemperature relationship described by Ong and Monteith 1985 and hourly
measurements of leaf and soil temperatures to calcu- late the potential delay in flowering for millet growing
the agroforestry systems. They reported that a maxi- mum delay of 2–3 days could be expected since tem-
peratures within the alleys were generally below T
o
for millet, and concluded that beneficial effects of shade
can only be expected when temperatures exceed T
o
for understorey crops. Recent studies at Machakos, Kenya
of maize and cowpea grown under Grevillea robusta Lott, 1998 and in Sapone, Burkina Faso, of pearl
millet grown under nere Parkia biglobosa and karite Butyrospermum parkii; Jonsson, 1995 confirm that
flowering and maturity were delayed by 10–12 days when soil temperature at a depth of 5 cm exceeded
40
◦
C for several hours during the day. Fig. 6 shows that the weekly mean daily soil temperature at a depth
of 5 cm under both karite and especially nere trees was consistently lower than in the open during the
rainy season at Sapone. Millet seedlings growing in the open experienced several hours of supra-optimal
temperatures on a regular daily basis, whereas those
42 C. Black, C. Ong Agricultural and Forest Meteorology 104 2000 25–47
Fig. 6. Weekly daily soil temperatures under Parkia biglobosa
j
and Butyrospermum parkii
h
or in the open
s
during the cropping season at Sapone, Burkina Faso modified from Jonsson, 1995.
grown under shade rarely experienced temperatures above 40
◦
C. The influence of such temperature effects on crop growth and yield needs to be separated from
that of drought, although there is strong evidence from field experiments in northern Nigeria that high soil
temperatures reduce both leaf growth and the radia- tion conversion coefficient for pearl millet McIntyre
et al., 1993.
5. Progress and challenges ahead
