Varietal differences in the nodulation of subterranean clover

Abstract

The pattern of nodule formation (time of formation, number, distribution, and size) was examined in 15 varieties of subterranean clover and in some autotetraploid and hybrid lines, with the following results: (1) The time at which nodules first appeared on the seedling differed significantly between varieties by 2-3 days but was not affected by ploidy. (2) Selection in hybrids for early- or late-nodulating habit beyond parental limit failed, except in the cross Tallarook x Morocco, which segregated dwarfs that nodulated late. Except in this cross, the time at which nodules first formed was inherited polygenetically. (3) Bacterial strains differed (by several days) in the time at which they initiated nodulation; strain and host differences were generally independent. (4) The average number of nodules formed differed up to sixfold between varieties; autotetraploids formed 32% fewer nodules than diploids. Differences between individual plants of one variety were small. (5) The habit of abundant nodule formation in hybrids was dominant over sparse nodulation, and in the F2 generation the range in nodule number usually exceeded that of the parents, which indicated complex inheritance. Dwarfs segregating in the Tallarook x Morocco cross bore half to one-third as many nodules as other plants. (6) After hybridization, selection for nodule abundance in subsequent selfed generations increased mean nodule number above that of the abundant parent. Selection for sparseness, except in one hybrid, failed to decrease nodule number below that of the sparse parent. (7) Nodule number also depended upon bacterial strain. With some notable exceptions host and strain differences tended to be independent. (8) Varieties with few nodules usually also had few lateral roots, and vice versa. In some crosses, selection for nodule number similarly affected lateral root number; in others, sparse and abundant selections bore intermediate numbers of roots. Young seedlings formed more lateral roots when uninoculated than when inoculated. (9) The proportion of nodules borne on lateral roots was greatest (tending towards 50%) on abundantly nodulating plants. (10) Nodule number and average nodule length were related hyperbolically; aggregate nodule length tended to a constant value that was host-dependent. Abundantly nodulating varieties and lines had larger aggregate nodule lengths, but with effective strains of bacteria these ranged less than twofold. Similar relationships connected average nodule diameter and number. (11) Aggregate nodule length also depended on bacterial strain, usually independently of host effects. One hybrid, inoculated with a particular strain of bacteria, showed simple segregation of aggregate nodule length in the F2 generation. Elsewhere this characteristic showed blending inheritance. (12) With effective strains, the aggregate whole-nodule volume per plant varied by less than 50% between varieties and lines, and was larger for abundantly nodulating plants. (13) Further anatomical analysis showed that the effectively nodulated plant contained substantially the same amount of persistent nitrogen-fixing bacteroid tissue irrespective of the number of nodules formed. Selected lines differing by several hundred per cent. in nodule number differed by only 4% in the product of bacteroid volume and its duration. (14) With fully effective strains of bacteria, the amount of nitrogen fixed per unit of active bacteroid tissue was independent of nodule number (22 µg N per mm3 bacteroid tissue per day). (15) Whole nodule volume was strongly influenced by bacterial strain, but the relative volumes of the nodule's meristem, cortex, and total bacteroid tissue (active and degenerate) were independent of bacterial strain and only slightly influenced by selection for nodule number. (16) The total amount and proportion of degenerate bacteroid tissue was much larger with the ineffective strain than with the effective strain.