Drought effect on nitrate reductase and sucrose-phosphate synthase activities in wheat (Triticum durum L.): role of leaf internal CO2

Abstract

In order to study the impact of a decline of leaf internal CO₂ molar ratio on nitrate reductase (NR) and sucrose-phosphate synthase (SPS) activities, leaves of wheat (Triticum durum) were submitted to different treatments: slow or rapid dehydration and decline in ambient CO₂ concentration and abscisic acid (ABA) supply. In agreement with the literature, NR activity of slowly dehydrated leaves was inhibited by about 50% when net CO₂ assimilation (A_n) decreased by 45%. NR activity of stressed leaves kept 4 h in air containing 5% CO₂ or after 2 d of re-watering was only partially restored. NR activity was slightly dependent on ambient CO₂ molar ratio, declining by 30% when non-stressed leaves were kept in CO₂-free air for 4 h. The decline of NR activity after ABA supply (through the transpiration stream) and after rapid dehydration of non-stressed leaves was comparable with the decrease observed under low CO₂ treatment. Overall, these data suggest that a drought-induced decrease of the leaf internal CO₂ concentration is only part of the signal triggering the decline of NR activity. In disagreement with most of the literature, SPS activity increased during slow dehydration, being stimulated by 30% when A_n declined by 40%. SPS activity of stressed leaves kept 4 h in air containing 5% CO₂ or 2 d after re-watering was slightly increased or unchanged, respectively. By contrast to NR activity, SPS activity of well-hydrated leaves was hardly affected by low CO₂. Increased SPS activity was mimicked, in non-stressed leaves, by a rapid dehydration within 4 h and by ABA fed through the transpiration stream. In durum wheat, the increase in SPS activity could be linked to ABA-based signalling during a drought stress.