Two current hypotheses to explain nitrate inhibition of nodule function both involve decreased O, supply for respiration in support of N, fixation. This decrease could result from either (a) decreased O, permeability (P,) of the nodule cortex, or (b) conversion of leghemoglobin (Lb) to an inactive, nitrosyl form. These hypotheses were tested using alfalfa (Medicago sativa L. cv WeevIchek) and birdsfoot trefoil (Lotus corniculatus L. cv Fergus) plants grown in growth pouches under controlled conditions. Nodulated roots were exposed to 1 O mM KNO, or Nitrate exposure can inhibit both nodulation of legumes and N, fixation by existing nodules (Streeter, 1988). Modification of the response of crop and forage legumes to nitrate may be desirable in some agricultura1 contexts, e.g. to increase the nitrogen contribution from a green manure crop or to decrease total nitrogen supply where nitrate leaching is a problem. The mechanism by which nitrate inhibits the activity of existing nodules has been controversial, despite considerable research on the topic.Early hypotheses involving a direct carbohydrate limitation of nodule metabolism or a direct nitrite toxicity are no longer widely accepted (Streeter, 1988;Vessey and Waterer, 1992). Instead, recent reviews summarize the evidence that nitrate inhibition of N, fixation involves a decreased O, supply for respiration in the nodule central zone (Vessey and Waterer, 1992;Hunt and Layzell, 1993). This evidence includes decreased FOL in nitrate-treated nodules (Layzell et al., 1990) and partia1 alleviation of nitrate inhibition by elevated external O, (Vessey et al., 1988). A decrease in FOL is not predicted by the direct carbohydrate limitation or nitrite toxicity hypotheses. In fact, inhibition of respiration by carbohydrate limitation or nitrite toxicity would be expected to decrease the capacity to consume O, diffusing into the nodule, thereby increasing central zone O, concentration and FOL.Decreased nodule O, permeability has been implicated in legume responses to several forms of stress (Hunt and Layzell, 1993). Past estimates of P, in nitrate-treated nodules were based on attempts to calculate the O, flux into nodules from net CO, exchange rates of nodulated roots. Although several variations on this approach (Schuller et al., 1988;Vessey et al., 1988;Minchin et al., 1992) a11 appeared to show a decrease in P, with nitrate exposure, a reexamination of nitrate effects by an independent method seemed worthwhile. Therefore, in this paper we report nitrate effects on P, in alfalfa (Medicago sativa) and birdsfoot trefoil (Lotus corniculatus) as measured by nodule oximetry (Denison and Layzell, 1991). This method estimates P, of intact, attached nodules by spectrophotometric monitoring of changes in FOL in response to changes in external O, concentration.Nodule O, permeability is defined here explicitly by Fick's law of diffusionwhere F is the total inward O, flux (mol/s) across the gas diffusion barrier in the nodule cortex (Tjepkema and Yocum, 1973), P , is the O, perm...

Despite the optimism of some molecular biologists, natural selection among the wild ancestors of crops is unlikely to have missed simple genetic improvements that would consistently have enhanced individual fitness. Tradeoff-free opportunities for further improvement of crop traits like photosynthetic efficiency or drought tolerance may therefore be elusive. Opportunities linked to acceptable tradeoffs may be abundant, however. Tradeoffs between individual competitiveness and the collective productivity of plant communities (e.g. those linked to height) have been key to past increases in yield potential. Solar tracking by leaves could involve such tradeoffs, if photosynthetic benefits to tracking leaves are outweighed by increased shading of leaves lower in the canopy. This hypothesis was tested using rotation in the horizontal plane to disrupt solar tracking in alfalfa. In sparse canopies, solar tracking increased net canopy photosynthesis, but rarely by more than 3%. As leaf area increased, solar tracking tended to decrease net canopy photosynthesis, despite edge effects in our 1-m2 artificial communities, which probably exaggerated net photosynthetic benefits of tracking. Computer modeling suggested that the season-long effects of solar tracking on community productivity can be negative. Solar tracking may have persisted, nonetheless, because individuals whose leaves track the sun increase shading of competitors.