CAN CARBON ISOTOPE VALUES IN THE SEEDS OF A C4 PLANT (PEARL MILLET, PENNISETUM GLAUCUM) SERVE AS A PROXY FOR PAST WATER MANAGEMENT PRACTICES?

Stable carbon isotope discrimination has been shown to correspond with water conditions during C₃ plant growth and has therefore been developed into a proxy for past water management practices. Many important cereals grown in semiarid regions are however C₄, and because C₄ plants are less sensitive to environmental variables than C₃ plants, the effects environmental variables have on their carbon isotope (δ¹³C) values have received comparatively less attention. We investigated the influence of water availability on pearl millet (Pennisetum glaucum) seed carbon isotope ratios by first examing the relationship between cumulative annual precipitation and δ¹³C values measured in USDA seeds grown outdoors at the Germplasm Introduction and Research Unit, Kingshill, St. Croix, United States Virgin Islands between 1994 and 2013. We observed that δ¹³C values were significantly positively related to cumulative annual precipitation, though over a fairly small range of δ¹³C values (< 2‰). We subsequently grew 75 millet plants from five regional accessions in the greenhouse at Washington University in St. Louis and subjected them to three different water treatments (maintained at average soil moisture levels of 0.43 m³/m³, 0.21 m³/m³, and 0.16 m³/m³, respectively). Pearl millet δ¹³C values were again significantly positively correlated with water availability; seeds were enriched in ¹³C by 0.23 ± 0.03‰ for each 0.1 m³/m³ increase in soil moisture. Sample origin (accession) was also a significant predictor of seed δ¹³C values. These results suggest that both environment (water availability) and genetics (accession) are responsible for variation in the isotopic values of pearl millet seeds. Because seed δ¹³C values differed by just 0.6‰ between the well-watered and draught treatments, it is unlikely that δ¹³C values in C₄ plants are sensitive enough to serve as proxies for past water management practices, although isotope data may still be informative when combined with morphological information (e.g. seed size).