Agriculture Reference
In-Depth Information
The analysis of the relationship between relative yield and soil salinity indicates
clearly that soybean shows a higher salt tolerance if it is cultivated in loam soil.
Key-words:
nitrogen balance; salt balance; evapotranspiration; abiotic stress; pre-dawn
leaf water potential
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NTRODUCTION
According to the classification made by Ayers and Westcott (1985), soybean is
considered to be a moderately salt tolerant species. Katerji et al. (2000) classify soybean
among those species sensitive to salinity, like all grain legumes (Maas and Hoffman, 1977).
Since various varieties of soybean were considered in the studies carried out to classify the
species. These nuances in classification can probably be explained by the large range of
varietal response to soil salinity demonstrated by the soybean (Velagaleti and Schweitzer,
1993).
The effect of salinity on bacterial activity in nitrogen fixation is one of the hypotheses
that may explain grain legumes' sensitivity to salt (Pessarakli et al., 1989). In the case of
soybean, various authors (Bernstein and Ogata, 1966; Wilson, 1970; Tu, 1981) have noted a
decrease in the number and the dry weight of nodules at increasing levels of salinity.
However, nitrogen fixation as a percentage of the total nitrogen uptake varies considerably,
from 25% to 85% for soybean (Patterson and LaRue, 1983, Wagner and Zapata, 1982; Zapata
et al., 1987). According to Patterson and LaRue (1983), the large range of these estimations
may be explained by differences among varieties. This is probably one of the causes of the
variability in tolerance to salinity previously found in soybean by Velagaleti and Schweitzer
(1993).
Generally, environmental factors (climate, soil texture) can be considered among the
causes that vary crop tolerance to soil salinity, but in literature these factors have not been
studied in depth. More is known about the role of climate (temperature, air water deficit, and
evaporative demand) thanks to the review by Shalhavet (1994). The role of climate has also
been demonstrated by Katerji et al. (2000) with studies on the broad-bean. These authors have
noticed that the same variety of leguminous crop can be either tolerant or sensitive to soil
salinity, depending on the level of the evaporative demand during the crop cycle.
Additionally, the role of soil texture as a cause of variability in tolerance to salinity has not
yet been studied in the literature. However for leguminous crop and for the soybean in
particular, it is admitted that the physical soil properties interact with the symbiotic nitrogen
fixation (Bouniols et al., 1990).
The objective of this paper is to analyse the tolerance of soybean cultivated in clay and
loam soils, characterised by a wide range of salinity, so as to determine the possible role of
soil texture in salt tolerance. The analysis carried out was based on observations at court term
on the level of the plant water relationship, at mean term on the level of growth in leaf surface
and dry matter, and at long term on the level of yield, its components, seasonal
evapotranspiration and water use efficiency. The analysis was also based on observations
relative to the nitrogen balance carried out on soybean by van Hoorn et al. (2001), with the
same experimental set-up.
The three questions asked in this study are:
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