Climate models predict a reduction in precipitation and an increase in evapotranspiration rates in many regions of the world in coming decades, resulting in increased drought. In addition to decreasing plant growth and reproduction, drought also decreases the concentration (%) of nitrogen (N) and phosphorous (P) in plant tissues, but the reasons for this decrease are not fully understood. We investigated if decreases in the nutrient status of plant tissues during drought may be caused in part by decreases in the concentration of nutrient-uptake proteins in roots, which has never been studied. Corn (Zea mays) and big bluestem (Andropogon gerardii), drought-sensitive and -tolerant warm-season grasses, respectively, were subjected to a gradual dry-down lasting ca. 3.5 weeks. Plants were harvested at mid and late drought (ca. 50% and 70-95% stomatal closure), at which times we measured biomass, tissue nutrient concentrations, nutrient uptake rates, and the concentration of nutrient-uptake proteins in roots (NO3: NRT1, NH4: AMT1, P: PHT1, B: BOR1, NIP5;1).
Results/Conclusions
Drought reduced the concentration of most nutrients in roots and shoots, indicating that (1) drought reduced the acquisition of nutrients more than it did plant growth, and (2) decreases in nutrient acquisition cannot be explained simply by decreases in water uptake. The rate of nutrient uptake per gram of root decreased with drought, providing an explanation for decreases in nutrient concentration. Decreases in nutrient uptake rate were correlated with decreases in the concentrations of the major nutrient-uptake proteins in roots. Drought-related decreases in the levels of nutrient-uptake proteins per gram root occurred despite compensatory increases in levels per unit total root protein, because of larger decreases in the concentration of total protein per gram root. The negative effects of drought on nutrient concentration, uptake, and uptake proteins were greater for late vs. mid drought. Hence, decreases in plant nutrient concentration, and hence food quality, with drought are caused, at least in part, by decreases in the concentration of nutrient-uptake proteins in roots, and this will increase with climate-change related increases in drought frequency and severity.