Adaptation of photosynthesis in marama bean Tylosema esculentum (Burchell A. Schreiber) to a high temperature, high radiation, drought-prone environment

Mitchell, R. A C and Keys, A. J. and Madgwick, P. J. and Parry, M. A J and Lawlor, D. W. (2005) Adaptation of photosynthesis in marama bean Tylosema esculentum (Burchell A. Schreiber) to a high temperature, high radiation, drought-prone environment. Plant Physiology and Biochemistry, 43 (10-11). pp. 969-976. ISSN 0981-9428

Abstract

Marama bean, Tylosema esculentum, is a tuberous legume native to the Kalahari region of Southern Africa where it grows under high temperatures (typical daily max 37°C during growing season) and radiation (frequently in excess of 2000 μmol m-2s-1) in sandy soils with low rainfall. These conditions might be expected to select for increased water-use efficiency of photosynthesis. However, marama was found to give similar leaf photosynthetic rates to other C3 plants for a given internal leaf CO2 concentration and Rubisco content. Under conditions of increasing drought, no increase in water-use efficiency of photosynthesis was observed, but stomata closed early and preceded any change in leaf water potential. The possibility of subtle adaptations of photosynthetic characteristics to its natural environment were investigated at the level of Rubisco kinetics. The specificity factor of marama Rubisco was slightly lower than that of wheat, but the apparent Km for CO2 in air (Km′) was about 20% lower than that of wheat. This is consistent with better adaptation for efficient photosynthesis at high temperatures in marama compared to wheat, although the net benefit is predicted to be very small (< 0.5% at 35°C). The sequence of marama rbcL gene shows 27 deduced amino acid residue differences from that for wheat, and the possibility that one or more of these cause the difference in Rubisco Km′ is discussed.