Amaranthus (Amaranthus spp) and pearl millet (Pennisetum glaucum [L.] R. Br.) are drought tolerant
crops with much potential that has not been well exploited as they can be cultivated under semi-arid
climatic conditions. This study was carried out to characterize their water use and model their growth and
yield in response to water. Experiments were carried out under a field line source sprinkler irrigation
system for both crops for two seasons, as well as in a greenhouse with a pot experiment for amaranthus
and in the lysimeter facility for pearl millet studies, each for one growth cycle. One genotype of
amaranthus (Amaranthus crentus ex Arusha) and two lines of pearl millet (GCI 17, improved line and
Monyaloti, local variety) were used in the trials with these crops in a semi-arid area near Bloemfontein,
South Africa.
The influence of water application on growth of amaranthus was contrary to the expectation that fully
irrigated plants will perform better than the plants receiving less water. Fully irrigated plants produced
shorter plants with less leaves and branches. However, irrigation improved the plant height in both lines
of the pearl millet. A large amount of irrigation resulted in taller plants for both lines while the shortest
plants were found in the rainfed plots. Another millet crop parameter that was affected by irrigation was
flower emergence. Flower emergence was earlier in irrigated plots of both lines of pearl millet and during
the two seasons. In both lines of pearl millet, irrigation increased leaf area index and biomass
accumulation during both seasons.
The two crops were able to exhibit the ability to tolerate water stress with different coping mechanisms
and this influenced their water uptake and invariably also water use. Amaranthus was able to manage
water stress in rainfed plots through the closure of stomata in the field and during the pot trials. Stomatal
closure reduces water loss as a response to water deficit in the soil-crop-atmosphere continuum. Daily
water use of amaranthus ranged from 1.2 to 6.5 mm day-1 while the seasonal water use was 437 mm for the first season and 482 mm for the second season. Higher water use in the second season was attributed
to higher atmospheric evaporative demand recorded during the second amaranthus growing season
compared to the first. It was observed that while water application can increase the production of
amaranthus, it should also not be too much or it could have a detrimental effect on biomass production of
the crop. This conclusion is due to the fact that the lowest irrigated plots produced higher fresh and dry
mass of amaranthus during both seasons while production in the fully irrigated plots was low for the two
seasons. The response of pearl millet to water deficit stress was to lower the leaf water potential (more
negative) and also gradually decrease the leaf stomatal conductance.
Pearl millet demonstrated a response to the water stress condition by closing of the stomata as leaf water
potential declined (towards more negative) so as to conserve water and prevent water loss. This
minimized water loss through transpiration when the soil water available is limited. The crop adjusted to
severe water stress conditions by maintaining a leaf water potential that keeps the leaf turgid in order to
avoid wilting when the stomata closes so as to prevent excessive water loss. The daily evapotranspiration
of the two lines of pearl millet for the two seasons were between 2 and 8 mm day-1 for the first season and
1 and 6 mm day-1 for the second season. The difference could also be attributed to a higher atmospheric
evaporative demand in the first pearl millet growing season than the second season. Overall, the improved
(GCI 17) and the local variety (Monyaloti) of pearl millet had water use of 309 and 414 mm in 2008/2009
season. The water use for the two lines was higher in the 2009/2010 season with GCI 17 having water use
of 401 mm and Monyaloti 457 mm which was probably due to high availability of water. High soil water
content coupled with a higher amount of rainfall in the second season than the first season could be the
reason for difference of the water use of the two lines of pearl millet for the two seasons. However, the
water use of the plants of the two lines of pearl millet from the rainfed plots and water stressed treatments
showed that the crop was able to reduce water use under water stress conditions as a coping mechanism
and hereby increase water use efficiency of the crop.
With the aid of the data from the field experiment, greenhouse and lysimeter trials, calibration and
validation of AquaCrop crop model was performed successfully for both crops. Simulation of biomass
production and cumulative evapotranspiration of both crops were performed adequately. The good
performance in simulating these crop parameters were illustrated with a high index of agreement that was
higher than 0.9 except for 2 cases of CC excluding the soil water comparisons. However, it was observed
that more effort is needed to accurately simulate early canopy cover in amaranthus and also the soil water
content and depletion patterns for both crops. Following successful validation, the model was also applied
to predict the performance of both crops under a range of proposed planting dates and choice of varieties
in pearl millet as possible adaptation strategies under two climate change scenarios. The model was able to predict the production of the two crops under predicted climate change for the period between the year
2046 and 2065 and the most appropriate adaptation strategy as a recommendation is to delay planting for
two months until the first half of January for both crops under the two future climate change scenarios
(A2 and B1).
In conclusion, the two crops under investigation can adjust to water limited conditions but through
different mechanisms. Amaranthus can avoid water stress through restricting growth, while the pearl
millet crop escapes water stress through speedy completion of growth stages before the water stress
condition sets in. It was also revealed that there are possibilities of cultivating these crops in central South
Africa. However, more studies should be carried out on the effect of interaction of nutrient and irrigation
on amaranthus production to reveal the reasons for the unexpected response of amaranthus to water
application. Studies on root development of the two crops are hereby recommended to aid in accurate
simulation of water balance of the two crops in the field situations. The calibration and validation of
AquaCrop for these two crops can also be improved by using datasets of more varieties or genotypes of
the crops and from other agro-ecological regions. In general, underutilised crops provide means of food
security and source of income for farmers. Due to the fact that they are drought tolerant, they require
minimum amount of input which is a desirable quality for low resource farmers and can be used as
alternative crops in semi-arid areas.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-07192013-085510 |
| Date | 19 July 2013 |
| Creators | Bello, Zaid Adekunle |
| Contributors | Prof S Walker |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-07192013-085510/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
Page generated in 0.0034 seconds