DROUGHT-INDUCED NITROGEN RETRANSLOCATION IN PERENNIAL C-4 GRASSES OF TALLGRASS PRAIRIE

We determined if drought-induced nitrogen retranslocation occurs in perennial grasses of tallgrass prairie, as suggested from studies of annual changes in plant N content. To test this, we analyzed six C-4 grasses representing a wide range of drought tolerance for shoot, rhizome, and root N before and after controlled drought. Shoot N concentration decreased in all species during drought (31-41%), including in recently expanded leaves (23-38%). No consistent pattern with respect to drought tolerance was apparent in these decreases or in observed changes in distribution of whole-plant N, although there was some suggestion of a mesic-to-xeric gradient in the magnitude of retranslocation. For example, the proportion of total plant N allocated to shoots decreased during drought 20-29% in the most mesic species over three experiments, 2-12% in the three intermediate species, and 4-6% in the two most xeric species, for pre- vs. post-drought comparisons. However, when drought-stressed plants were compared to well-watered age controls, the respective values were 20-21%, 12-20%, and 0-19%, the apparent result of size-related changes in N allocation in control plants in one experiment. In most cases, shoot N was moved primarily into rhizomes, though in one species with intermediate drought tolerance, evidence suggested that much of the retranslocated shoot N was apparently lost through fine-root turnover. Retranslocation of shoot N to rhizomes and roots, confirmed by monitoring movement of S-35-methionine, was in response to drought stress rather than phenology and involved the entire shoot (e.g., blades, culms, recently expanded leaves). Post-drought photosynthesis and leaf N concentration remained well below predrought levels 6 d following rewatering. Thus decreases in leaf N status during drought as a consequence of retranslocation likely result in lower photosynthetic capacity and decreased whole-plant carbon gain following relief of water stress after rain. Drought-induced retranslocation may serve to protect plant N from loss to herbivory, fire, and volatilization during periods when soil N uptake and carbon assimilation are limited by water availability.