Summary The Phyllobacterium brassicacearum STM196 strain stimulates Arabidopsis thaliana growth and antagonizes high nitrate inhibition of lateral root development. A previous study identified two STM196‐responsive genes, NRT2.5 and NRT2.6 (Mantelin et al., 2006, Planta 223: 591–603). We investigated the role of NRT2.5 and NRT2.6 in the plant response to STM196 using single and double Arabidopsis mutants. The single mutants were also crossed with an nrt2.1 mutant, lacking the major nitrate root transporter, to distinguish the effects of NRT2.5 and NRT2.6 from potential indirect effects of nitrate pools. The nrt2.5 and nrt2.6 mutations abolished the plant growth and root system architecture responses to STM196. The determination of nitrate content revealed that NRT2.5 and NRT2.6 do not play an important role in nitrate distribution between plant organs. Conversely, NRT2.5 and NRT2.6 appeared to play a role in the plant response independent of nitrate uptake. Using a nitrate reductase mutant, it was confirmed that the NRT2.5/NRT2.6‐dependent plant signalling pathway is independent of nitrate‐dependent regulation of root development. Our findings demonstrate that NRT2.5 and NRT2.6, which are preferentially expressed in leaves, play an essential role in plant growth promotion by the rhizospheric bacterium STM196.

Senescence was evaluated at different stages of the grain‐filling period in eight durum wheat varieties using numerical image analysis (NIA). The varieties were grown under early, severe drought conditions on the high plains of Sétif in Algeria. After flowering, three different irrigation treatments were applied. Treatment effect was small, while a genotypic effect was noted for most of the senescence parameters. Senescence correlated to biomass, while the maximal rate of senescence, Vsmax, correlated to thousand‐kernel weight. The potential of the method of numerical image analysis for monitoring flag leaf senescence, detecting genotypic variability and selecting genotypes with delayed senescence is discussed.

Water deficiency brings about a marked limitation in soybean (Glycine max. L. Merr.) yield by impairing photosynthesis and symbiotic N2 fixation. The objective of this study was to determine the photosynthetic and N2‐fixation response to short‐ and long‐duration water status variations in leaves and nodules. Plants were grown in pots in a greenhouse. Carbon dioxide exchange rate was measured by gas analysis and N2 fixation by the acetylene reduction method. Leaf water status was determined with a pressure bomb and nodule water potential with a psychrometer. Dinitrogen fixation decreased steadily throughout the water deficiency period whereas photosynthesis first decreased only slightly, and then dropped dramatically. After a severe water stress, partial recovery was slower for N2 fixation than for photosynthesis. The N2‐fixation response appeared to be directly related to a reduction in nodule mass, which affected nodule structural constituents after a severe stress. During the water deficiency period, the water status of the nodules responded only partially to water status variations within the plant during the day. Dinitrogen fixation depended more on the water status of the nodules than on the changes in nodule mass. There was a higher dependence of N2 fixation on nodule water content than on nodule water potential, particularly for values <‐1.2 MPa. The N2‐fixation activity maintained during the severe phase of water stress, when photosynthesis was zero, resulted from the relative independence of the nodules on the daily water variations within the plant and from the strong binding of the nodule water.

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