173and stem morphology as observed in this study thus causing a reduction in growth,
resulting in lower WUE.
Increase in WUE of the plants in T2 and T4 indicates that less water was lost
from the soil through evaporation due to higher humidity in the heat tunnels. Lee
et al. ( 1972 ) recorded that increase in humidity from 50 to 95 % increased the flower
number, peg number and vegetative weight. Similar observations were made in this
study, notably in T2 where the RH was near saturation compared with 48 % under
ambient conditions. The use of heat tunnels resulted in clear temperature differ-
ences across treatments. These heat tunnels can thus be used in the field to screen
groundnut genotypes for high temperature tolerance, as humidity control can be
achieved with experience in using the heat tunnels (T2 vs. T4).
The effects of temperature and water stress on various components of groundnut
as recorded at final harvest are shown in the flow diagram (Fig. 8.5). The field study
also confirms the observations made under controlled environment studies (Kakani
2001 ) that the interaction for temperature and moisture stress is transient and disap-
pears with release of a stress treatment. The interaction between temperature and
water stress treatments was recorded in the harvests made immediately after the
withdrawal of high-temperature treatment (T4).
The interaction between water and temperature stress was significant only for
peg and pod number. This interaction is due to the sensitivity of the reproductive
processes such as pollen germination and fertilization to high temperature. In a
controlled environment with a maximum temperature of 37 °C for 10 days, a de-
crease in pod number of 43 % was recorded at 50 DAS (Kakani 2001 ). On the
other hand, in field, a temperature of 43.5 °C was imposed for 20 days that caused a
reduction of only 46 % in pod numbers. This lesser decrease in pod number can be
attributed to the greater tolerance to high temperature of the genotypes used in the
field (ICGS 11 and TMV 2) study compared to those in a controlled environment
(ICGV 86015 and ICG 796). Observations made on membrane thermostability and
cardinal temperatures for pollen germination and tube growth (Kakani et al. 2002 )
also show that genotypes tested in field were more tolerant than those tested in a
controlled environment.
The reasons for the existence or disappearance of the interaction can be attrib-
uted to the moisture level at that particular stage of crop growth. In the controlled
environment study, the interaction with high temperature occurred when the mois-
ture content in water stress treatment was 60 % available soil moisture (ASM).
Similar to controlled environment pots, WS plots in field were at 100 % ASM until
the initiation of water stress at 30 DAS. Time was required to bring down the mois-
ture level to 40 %, which can be seen from the trends in biomass and pod yields
(Fig. 8.4). Estimates of soil water content by simple water balance as shown below
in WS × T2 treatment; assuming water loss of ETC from soil, indicate that the water
content of soil at the end of the high-temperature treatment was about 62 % ASM.
It can also be seen that biomass or pod yields in the water stress treatments are
lower than irrigated treatments only after 50 DAS and remain less until the final
harvest. This suggests that the interaction of water stress with high temperature
would also have occurred at a moisture level of 60 % ASM, as observed from the
8 Effect of High Temperature and Water Stress on Groundnuts ...