Growth and yield responses of cowpeas (Vigna unguiculata L.) to water stress and defoliation.
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Date
2012
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Abstract
Cowpea (Vigna unguiculata L.) is an important legume, especially in the hot, dry tropics
and subtropics of sub-Saharan Africa. It has been widely reported to be drought tolerant.
Cowpea is a highly nutritious, multi-purpose crop, used as a leafy vegetable and grain
legume with potential to contribute to food security in marginal areas. However, the crop is
still classified as a neglected underutilised species; legume research focus has been mainly
devoted to established legumes such as common bean and soybeans. There is a need to
collect empirical information on cowpea which could be used to advise farmers on
management strategies. This study evaluated cowpea responses to water stress under
controlled and field conditions. Initially, two cowpea varieties (Brown and White birch)
were evaluated for seed quality using the standard germination that was laid out in a
completely randomised design and each variety was replicated for times. Electrolyte
conductivity test was also performed under laboratory conditions. Thereafter, a pot trial
was conducted to evaluate cowpea response to water stress imposed at different growth
stages under varying growth temperatures. The pot trial comprised three factors:
temperature [High (33/27ºC), Optimum (27/21ºC) and Low (21/15ºC)], water regimes (no
stress, terminal stress, intermittent stress – vegetative and intermittent stress - flowering)
and cowpea varieties. Lastly, a field trial was conducted to evaluate cowpea production as
well as the effect of sequential leaf harvesting on yield under irrigated and rainfed
conditions. The field trial was laid out as a split-plot design, with water regime (irrigation
vs. rainfed) as main factors, cowpea varieties as sub-factor and sequential harvesting (no
harvest, harvested once and harvested twice), replicated three times. All treatments were
arranged in a randomised complete block design. Results of the initial study showed that
germination capacity and vigour of cowpea varieties were significantly different (P <
0.001). White birch had higher electrolyte leakage than Brown birch. Pot trial results
showed that cowpea growth (leaf area, leaf number and plant height) was vigorous in the
high temperature regime compared with optimum and low temperature regimes.
Chlorophyll content index was higher under high temperature relative to optimum and low
temperature regimes, respectively. Under low and optimum temperature regimes, cowpea
growth was stunted; cowpea failed to flower and form yield. Whereas, under high
temperature regime, cowpea growth was vigorous hence flowered and formed yield.
Vegetative growth was more sensitive to water stress than flowering stage. Terminal stress
and stress imposed during flowering resulted in increased proline accumulation relative to
no stress and stress imposed during vegetative growth. Harvest index was lower when
water stress was imposed during vegetative relative to flowering stage. Field trial results
showed that cowpea growth was sensitive to water stress. Plant height, leaf number,
chlorophyll content index and stomatal conductance were lower under rainfed relative to
irrigated conditions. Sequential harvesting of leaves had no significant effect on cowpea
yield. It is concluded that tropical temperature conditions are most suitable for cowpea
production; the controlled environment study showed best crop performance under
33/27ºC. In the context of varieties used for the present study, vegetative growth was the
most sensitive stage to water stress. Cowpea performed better under rainfed relative to
irrigated conditions with respect to yield formation. Low temperature was found to be
more limiting to cowpea growth, development and productivity compared with water
stress. Whereas, under high temperature conditions, water stress was more limiting to plant
growth and productivity. White birch may be used as a dual purpose crop due to its ability
to produce reasonable grain yield regardless of defoliation.
Description
Thesis (M.Sc.Agric)-University of KwaZulu-Natal, Pietermaritzburg, 2012.
Keywords
Cowpea--Effect of drought on., Cowpea--Effect of temperature on., Cowpea--Effect of stress on., Cowpea--Growth., Cowpea--Varieties., Cowpea--Yields., Defoliation., Theses--Crop science.