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Field experiments were conducted in 2006/07 and 2007/08 in Botswana to study the response of bambara groundnut to heat stress (2006/07 season) and to heat stress and soil moisture deficit stress (2007/08 season). In both seasons selected bambara groundnut landraces were grown at different sowing dates to provide a range of field environmental conditions. While the effects of water stress were mitigated by irrigation throughout the growing season in the 2006/07 season, in the 2007/08 season plants were grown under two irrigation regimes: a control that was irrigated throughout and a rain-fed treatment, with irrigation withheld at 30 days after sowing (DAS). Owing to significant and above average rainfall in the 2007/08 season no significant (p=0.05) differences in soil moisture content were obtained between the irrigated and rain-fed treatments. The highest average maximum temperatures of 34.1 and 29.2°C achieved in the December sowings in the 2006/07 and 2007/08 cropping seasons, respectively, did not impact any of the parameters measured, including leaf tissue electrolyte leakage, light adapted chlorophyll fluorescence, crop growth and yield. However, there was a highly significant (p<0.001) decline in crop performance with decline in temperature associated with delay in sowing after December. The results obtained suggest a sensitivity of bambara groundnut landraces to both low temperatures and photoperiod under field conditions in a semi-arid environment.

A number of varieties of rice, a halophyte, Sesuvium portulacastrum and a glycophyte, Phaseolus vulgaris were grown in culture solution containing a range of concentrations of NaCI. Growth of the plants and internal sodium concentrations of the roots were measured after 14 days. The electrical potential difference (PD) between the external solution and the vacuole of the outer cells of the root was also measured. This enabled the driving force on sodium at the cell membranes to be calculated using the Nernst equation. It was found that Sesuvium and those varieties office that had previously shown salt tolerance generated relatively negative PDs and large driving forces tending to exclude sodium from the root. This suggested that a simple measurement of PD for plants grown in a given concentration of NaC1 over a given period of time would provide a fairly rapid screening method for salt tolerance in rice and possibly other species also.

Environmental pollution by potentially toxic elements (PTEs) has become a serious problem with increasing industrialization and the disturbance of natural biogeochemical cycles. Jatropha is an oilseed-bearing shrub with high potential for biodiesel production in arid regions. In this study, we examined the physiological responses of this plant to five representative PTEs (Cd, Cr, Cu, Ni, and Zn) in a hydroponic culture. Application of higher concentrations of Cd and Zn led to severe leaf chlorosis, and Cd, Cu, and Ni treatments resulted in significant growth retardation. Higher enrichment of the applied PTEs in the shoots was observed for Zn- and Cd-treated plants, with the latter reaching 24-fold enrichment in plants exposed to 10 μM Cd, suggesting that Jatropha can cope with relatively higher internal concentrations of toxic Cd. Although Cd stress led to the disturbance of essential mineral homeostasis and photosynthesis, this induced an increase in thiol compounds in the roots, suggesting defensive responses of Jatropha to PTEs. This study showed that Jatropha exhibits distinct sensitivities and physiological responses to different PTEs. This study also provides basic knowledge for diagnosing the physiological status of Jatropha trees for potential dual use in afforestation and as a sustainable energy supply.

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