Photosynthate and Dry Matter Distribution in Effectively and Ineffectively Nodulated Alfalfa1

Abstract

Ineffectively nodulated alfalfa (Medicago sativa L.) grown with combined N [In(+N)] was compared with symbiotically‐grown, effectively‐nodulated alfalfa (Sar) to determine whether ineffective nodulation had correlated effects on photosynthate partitioning into dry matter. Ineffectively nodulated alfalfa grown with [In( + N)] and without [In( − N)] combined N were compared to determine the effects of N stress on photosynthate partitioning. Either the topmost or lowest fully‐expanded, intact green leaf on the main stem was labeled with 14CO2 in pulse‐chase experiments. After a 24 h chase period, the plants were divided into source leaf, main stem, unexpanded leaves and shoot apex on the main stem, other fully‐expanded leaves on the main stem, shoots growing from axillary buds on the main stem, shoots growing from crown buds, crown, nodules, and root. The 14C partitioning was measured as relative specific activity (RSA) and percent of total plant recovered radioactivity (%TPR). The dry mass, %TPR, and RSA of aerial organs were similar in In( + N) and Sar. However, the dry mass and %TPR of the belowground organs (roots + crown + nodules) were 46% and 25% less, respectively, in In(+N) than in Sar. This differential partitioning of dry matter and photosynthate to roots and crowns between the populations indicates that the single recessive gene for ineffective nodulation may have correlated effects on growth or development. The allocation of photosynthate from the topmost and lowest fully‐expanded leaf was significantly (P<0.05) different to all organs except nodules and roots. The topmost fullyexpanded leaf was the primary supplier of photosynthate to the unexpanded leaves and shoot apex on the main stem, the main stem, and fully‐expanded leaves on the main stem. The lowest fully‐expanded leaf was the primary supplier to the shoots growing from axillary buds on the main stem, the shoots growing from crown buds, and the crown. The dry mass, RSA, and %TPR of the main stem, shoots growing from axillary buds on the main stem, fully‐expanded leaves on the main stem, and shoots growing from the crown were less in the In(−N) than in the In(+N). The converse was true for the unexpanded leaves and shoot apex on the main stem, crown, roots, and nodules. The carbon exchange rate of the topmost fully‐expanded leaf was reduced 45%, but the export of photosynthate significantly increased, under N stress. The dry mass, %TPR, and RSA of the nodules were greater by 19‐, 36‐, and 6‐fold, respectively, in In(−N) than in In( + N). These results indicate that the nitrate inhibition of nodulation and photosynthate partitioning to alfalfa nodules is unrelated to rhizobial effectiveness or to the functioning of the bacteroids.