Effects of soil moisture deficits on the water relations of bambara groundnut (Vigna subterranea L. Verdc.)
Journal of Experimental Botany
Effects of soil moisture deficits on the water relations of bambara groundnut (Vigna subterranea L. Verde.)
S.T. Collinson 1
E.J. Clawson 1
S.N. Azam-Ali
C.R. Black 0
0 Department of Physiology and Environmental Science, University of Nottingham , Sutton Bonington Campus, Nottingham LE12 5RD , UK
1 Department of Agriculture and Horticulture, University of Nottingham , Sutton Bonington Campus, Nottingham LE12 5RD , UK
The components of leaf water potential (V,) and relative water content (RWC) were measured for stands of bambara groundnut (Vigna subterranea) exposed to three soil moisture regimes in controlled-environment glasshouses at the Tropical Crops Research Unit, Sutton Bonington Campus. Treatments ranged from fully irrigated (wet) to no irrigation from 35 days after sowing (DAS) (dry). RWC values varied between 92-96% for the wet treatment, but declined from 93% to 83% in the dry treatment as the season progressed. V, at midday decreased in both the wet and dry treatments, but the seasonal decline was more pronounced in the latter: seasonal minimum values were -1.19 and - 2 . 0 8 MPa, respectively. Plants in the wet treatment maintained turgor (Vp) at about 0.5 MPa throughout the season, whereas values in the dry treatment approached zero towards the end of the season. There was a linear relationship between Vp and ¥*j, with <PP approaching zero at a V, of -2.0 MPa. Mean daily leaf conductance was consistently higher in the wet treatment (0.46-0.79 cm s"1) than in the intermediate and dry treatments (0.13-0.48 cm s*1). Conductances in the intermediate and dry treatments were similar, and the lower evapotranspirational water losses in the latter were attributable to its consistently lower leaf area indices (/.): L at final harvest was 3.3, 3.3 and 1.9 for the wet, intermediate and dry treatments. Bambara groundnut was apparently able to maintain turgor through a combination of osmotic adjustment, reductions in leaf area index and effective stomatal regulation of water loss.
Vigna subterranea; water relations; soil moisture
Introduction
Bambara groundnut (Vigna subterranea L. Verde.) is grown
as a subsistence crop in semi-arid regions of Africa where
the success of other legumes is uncertain because of poor
soils, drought and disease (Vietmeyer, 1978; Haq, 1983).
However, previous research on the crop has been limited,
largely because its commercial value is restricted by a low
lipid content (5-8%; Deshpande and Damodaran, 1990;
Brough and Azam-Ali, 1992), even though it is an
important source of protein at the subsistence level. Previous
controlled environment experiments have shown that
bambara groundnut is capable of producing a worthwhile yield
under conditions where groundnut (Arachis hypogaea L.)
may fail completely (Babekir, 1989). The mechanisms
which enable bambara groundnut to produce at least some
yield during severe drought are poorly understood, but
may be linked with its relatively high root: shoot biomass
ratio and small leaf area, which restricts transpirational
loss of water (Collinson et al., 1996).
Crops grown in drought-prone areas display varied
responses to water stress which may be divided into three
categories; escape, avoidance and tolerance (Turner,
1979). Drought escape is demonstrated by desert
ephemerals and some short duration dryland crops which have
a condensed growth cycle and reach maturity before
drought occurs; their short life-cycle is often combined
with developmental plasticity. Crop species displaying
this type of adaptation are often photoperiod-sensitive so
that flowering coincides with the average date of the end
of the rainy season (Ludlow and Muchow, 1988).
Drought-avoiding species, such as cowpea (Vigna
unguiculata (L.) Walp.) and sorghum (Sorghum bicolor
(L.) Moench), avoid water deficits by maximizing water
uptake and minimizing water loss. This response is
typified by the tropical legume, Siratro, which has deep roots
to maximize water uptake, stomata which are sensitive to
decreased leaf water potential and increased leaf-to-air
vapour pressure deficit, and exhibits paraheliotropic leaf
movements to reduce interception of incident solar
radiation (Ludlow, 1989). Other species, such as groundnut,
pigeonpea (Cajanus cajan (L.) Millspaugh) and cotton
(Gossypium hirsutum), have developed mechanisms to
survive drought through dehydration tolerance. In these
species, osmotic adjustment often assists in turgor
maintenance, hence allowing stomatal opening, photosynthesis
and leaf expansion to be maintained over a wider range
of soil moisture stress than in more susceptible species.
Although bambara groundnut has clearly evolved
mechanisms to withstand drought, the lack of quantitative
evidence regarding the nature of its responses means that
it has not yet been categorized into one of the three
groups outlined above. Begemann (1988) examined 72
landraces of bambara groundnut grown under two water
stress regimes in Ibadan, N (...truncated)