Drought‐Induced Nitrogen Retranslocation in Perennial C4 Grasses of Tallgrass Prairie

We determined if drought—induced nitrogen retranslocation occurs in perrenial grasses of tallgrass prairie, as suggested from studies of annual changes in plant N content. To test this, we analyzed six C₄ 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 ³ ⁵S—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 of herbivory, fire, and volatilization during periods when soil N uptake and carbon assimilation are limited by water availability.