Root Organic Reserve Accumulation and Defoliation Stress Tolerance of Alfalfa (Medicago sativa L.) Seedlings

N. E. Kalengamaliro, J. J. Volenec, S. M. Cunningham, and B. C. Joem
Dept. of Agronomy, Purdue University, West Lafayette, IN 47907-1150

Previous results have suggested that defoliation tolerance of alfalfa (Medicago sativa L.) depends on root organic reserves. Our seedling development studies have shown that deposition of starch and vegetative storage proteins (VSP) in alfalfa roots begins approximately 40 days after planting (DAP). We hypothesized that seedlings defoliated prior to 40 DAP would have impaired regrowth. Our objectives were: (1) to determine if combined inorganic N would stimulate early VSP deposition in alfalfa roots; and (2) to determine if defoliation tolerance of seedlings depends on root organic reserve accumulation. In Exp. 1, normal and ineffective-nodulating Saranac were grown in coarse sand with and without combined N. Roots were sampled 1S, 25, 39, 55, and 67 DAP and analyzed for sugars, starch, and VSPs. Root starch and VSP concentrations were low 15 and 25 DAP. There was rapid accumulation of root starch and VSPs between 25 and 55 DAP in Saranac without N and Saranac and ineffective-nodulating Saranac with N. Roots of ineffective Saranac without N did not accumulate starch or VSPs during this period. Combined N increased herbage growth, but did not stimulate early deposition of starch or VSPs. This suggested that initial VSP deposition may be controlled by factors other than N availability.

In Exp. 2 seedlings were completely defoliated 14, 28, 42, and 56 DAP. Results showed that 78% of seedlings survived complete defoliation 14 DAP. At this time root starch levels were very low (20 1lg/mg dry wt.) and VSPs could not be detected using immunoblotting. In Harvest 3 and 4, when root starch and VSP accumulation began, 80% of the seedlings survived. However, at Harvest 2 (28 DAP) only about 50% of seedlings survived complete defoliation. This low survival rate was associated with the largest defoliation-induced declines in root protein and sugar concentrations. This suggests that there may be threshold sugar and protein concentrations in roots required for seedling survival.

The results from these experiments show that starch and VSPs were not abundant in roots of young alfalfa seedlings (<30 DAP). In addition, VSP accumulation was coordinated with root starch deposition. The onset of starch and VSP accumulation coincided with declines in buffer-soluble protein and sugar concentrations in roots. Addition of fertilizer N increased herbage growth but did not lead to early VSP deposition. High levels of starch and VSPs may not be an absolute necessity for defoliation tolerance of young alfalfa seedlings.

1995 Central Alfalfa Improvement Conference Proceedings