The identification of QTL for several physiological traits in sunflower is described. Traits related to photosynthesis (leaf chlorophyll concentration, net photosynthesis and internal CO(2) concentration) and water status (stomatal conductance, transpiration, predawn leaf water potential, and relative water content) were evaluated in a population of recombinant inbred lines under greenhouse conditions. Narrow-sense heritabilities were low to average. Using an AFLP linkage map, 19 QTL were detected explaining 8.8-62.9% of the phenotypic variance for each trait. Among these, two major QTL for net photosynthesis were identified on linkage group IX. One QTL co-location was found on linkage group VIII for stomatal movements and water status. Coincident locations for QTL regulating photosynthesis, transpiration and leaf water potential were described on linkage group XIV. These results lead to the first description of the organization of genomic regions related to yield in sunflower.

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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.