Leaf Water Relations and Gas Exchange in Relation to Forage Production in Four Asiatic Bluestems1

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The Old World bluestems (OWB, Bothriochloa spp.) comprise broad germplasm reservoir showing extreme variability in phenology, morphology, seed production, cold hardiness, and water‐use efficiency (WUE). Previous research has demonstrated their potential for use in forage‐based beef production systems of the Southern Great Plains. Four accessions representing a range in water stress tolerance as measured by forage production were grown under optimum soil water conditions in the glasshouse for purposes of comparing their gas exchange characteristics and leaf water relations. At a leaf temperature of 25 C, leaf CO2 exchange rate (CER) ranged from 40 to 56 mg dm−2hour−1 (highest/WW‐477, WW‐765, WW‐ 573, WW‐604/lowest). However, the ratio of stomatal CO2 resistance (r′1) to total CO2 resistance (r′1), an index of WUE based solely on plant parameters, ranked the accessions: WW‐573, WW‐765, WW‐477, WW‐604. The higher r′2r′1 of WW‐573 indicates that it was realizing a greater proportion of its potential maximum CER (the CER which would occur if limited only by r′2) than the other accessions and therefore had better coupling between CO2 uptake and stomatal opening. At 35 C (near optimum for CER) mean values of CER and r′2r′1 increased 41 and 33%, respectively, while r′2 decreased only 10%. Stomatal control over CER increased relatively more in the less water‐use efficient accessions so that there were no differences in r′2r′1 at 35 C. While differences in osmotic potential among accessions were minimal, the relatlve water content at which turgor potential = 0 (RWC0) ranged from 0.84 (WW‐477) to 0.78 (WW‐573). Differences in field drought performance and measurable variation in CER, WUE, and turgot maintenance among these OWB's for selected conditions encourages the pursuit of screening techniques based on these physiological parameters.