Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30-40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N 2 fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineral N (intercropping legumes with cereals) can also be critical. Much of the N 2 fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legume grain may be relatively small. Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. While some of these rotational effects may be associated with improvements in availability ofN in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogen emitted from nodules as a by-product of'N, fixation.

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Rice is among the 3 most important crops worldwide. While much of the world's rice harvest is based on modern high-yield varieties, traditional varieties of rice grown by indigenous groups have great importance as a resource for future crop improvement. These local landraces represent an intermediate stage of domestication between a wild ancestor and modern varieties and they serve as reservoirs of genetic variation. Such genetic variation is influenced both by natural processes such as selection and drift, and by the agriculture practices of local farmers. How these processes interact to shape and change the population genetics of landrace rice is unknown. Here, we determine the population genetic structure of a single variety of landrace rice, Bue Chomee, cultivated by Karen people of Thailand. Microsatellite markers reveal high level of genetic variation despite predominant inbreeding in the crop. Bue Chomee rice shows slight but significant genetic differentiation among Karen villages. Moreover, genetically determined traits such as flowering time can vary significantly among villages. An unanticipated result was the overall pattern of genetic differentiation across villages which conforms to an isolation by distance model of differentiation. Isolation by distance is observed in natural plant species where the likelihood of gene flow is inversely related to distance. In Karen rice, gene flow is the result of farmers' seed sharing networks. Taken together, these data suggest that landrace rice is a dynamic genetic system that responds to evolutionary forces, both natural and those imposed by humans.Oryza sativa ͉ geographical differentiation ͉ farmers' management ͉ local adaption ͉ crop genetic diversity

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This study evaluated how zinc (Zn) concentration of rice (Oryza sativa L.) seed may be increased and subsequent seedling growth improved by foliar Zn application. Eight foliar Zn treatments of 0.5% zinc sulfate (ZnSO 4 Á 7H 2 O) were applied to the rice plant at different growth stages. The resulting seeds were germinated to evaluate effects of seed Zn on seedling growth. Foliar Zn increased paddy Zn concentration only when applied after flowering, with larger increases when applications were repeated. The largest increases of up to ten-fold were in the husk, and smaller increases in brown rice Zn. In the first few days of germination, seedlings from seeds with 42 to 67 mg Zn kg À1 had longer roots and coleoptiles than those from seeds with 18 mg Zn kg À1 , but this effect disappeared later. The benefit of high seed Zn in seedling growth is also indicated by a positive correlation between Zn concentration in germinating seeds and the combined roots and shoot dry weight (r ¼ 0.55, p < 0.05). Zinc in rice grains can be effectively raised by foliar Zn application after flowering, with a potential benefit of this to rice eaters indicated by up to 55% increases of brown rice Zn, and agronomically in more rapid early growth and establishment.