RESUMEN
Photosynthesis, primary productivity, N content, and N(2) fixation were determined as a function of applied NH(4) (+) in peas (Pisum sativum L. cv. Alaska) which were inoculated or not inoculated with Rhizobium leguminosarum. Cabon dioxide exchange rate (CER) increased 10-fold, total N content 7-fold, and total dry weight 3-fold in 26-day-old uninoculated plants as applied NH(4) (+) was increased from 0 to 16 millimolar. In inoculated plants of the same age CER and dry weight were maximal at 2 millimolar NH(4) (+), and total N content increased between 0 and 2 millimolar NH(4) (+) but did not change significantly with higher NH(4) (+) applications. Per cent N content of uninoculated plants was significantly lower than that of inoculated plants except at the highest NH(4) (+) concentration (16 millimolar). Symbiotic N(2) fixation by inoculated plants was maximal in peas grown with 2 millimolar NH(4) (+); and apparent relative efficiency of N(2) fixation, calculated from C(2)H(2) reduction and H(2) evolution, was maximal in the 2 to 4 millimolar NH(4) (+) concentration range. The capacity to fix N(2) through the Rhizobium-legume symbiosis significantly enhanced the rate and efficiency of photosynthesis and plant N content when NH(4) (+) concentration in the nutrient solution was below 8 millimolar. Above 8 millimolar NH(4) (+) concentration uninoculated plants had greater CER, N content, and dry weight.
RESUMEN
Physiological responses to infection by strains of Rhizobium leguminosarum which differed in their capacity to reduce N(2) were determined in 26-day-old pea plants (Pisum sativum L. cv. Alaska) grown under uniform environmental conditions in the absence of combined N. The highest N(2) reduction rates, calculated from H(2) evolution and C(2)H(2)-dependent C(2)H(4) production measurements, were approximately 6-fold greater than the lowest. Higher N(2) fixation rates were associated with greater CO(2) exchange rates (R(2) = 0.92) and carboxylation efficiency (R(2) = 0.99). Increases in the apparent relative efficiency of N(2) fixation [1-(H(2) evolved in air/C(2)H(2) reduced)] (acteroid efficiency) were associated with increases in whole-plant N(2) fixation efficiency (N(2)/CO(2) reduction ratio) (R(2) = 0.95). Whole-plant dry weight and total N content were related by regression analysis (R(2) = 0.98); both parameters were enhanced by increased N(2) fixation in a manner analogous to previously reported increases caused by greater external applications of NH(4) (+). These data reveal that photosynthetic parameters in genetically uniform host plants grown under identical environmental conditions are affected by N(2) fixation characteristics of the rhizobial symbiont. The measured efficiencies of micro- and macrosymbiont are directly related under uniform environments.
RESUMEN
Delayed leaf senescence has been found in a soybean population which maintains its chlorophyll and ribulosebisphosphate carboxylase activity in leaves and nitrogen fixation (acetylene reduction) activity in root nodules throughout seed maturation. Incorporation of delayed leaf senescence into an agronomically desirable genetic background may help to increase seed yield and symbiotic nitrogen fixation during seed development.