Fall Dormancy Response of Alfalfa Investigated with Reciprocal Cleft Grafts

Control of the fall dormancy response in alfalfa ( Medicago sativa L.) has been associated with cool temperatures and short days typical of autumn, but the physiological bases of the phenomenon are poorly understood. The purpose of these glasshouse and growth chamber investigations was to develop a model system based on reciprocal cleft grafts between shoots and roots of contrasting cultivars to determine whether a specific graft component is associated with the dormancy response and whether graft‐transmissible factors might be implicated in the response. The experimental approach involved reciprocal cleft grafts between fall‐dormant ‘Norseman’ and fall nondormant ‘African’. The degree of prostrate growth of the canopy (as height‐to‐width ratio), mean internode length, mean area per leaf, herbage dry weight, and crown freezing tolerance of various graft combinations were measured. The ratio of canopy height to width, mean internode length, and mean area per leaf were primarily determined by the genotype of the shoot plus crown, with mediation by root genotype. In contrast, herbage production per plant indicated that the fall‐dormant member of the grafted plant conditioned herbage growth regardless of whether fall dormancy was a characteristic of the shoot genotype or the root genotype. The results further indicate that one or more graft‐transmissible factors may be involved in the fall dormancy response. This model system provides an approach to implicate specific plant organs in the fall dormancy response of alfalfa, and may provide new insights for investigating the metabolic and genetic basis of fall dormancy.