DROUGHT AND POT SIZE EFFECTS ON TRANSPIRATION EFFICIENCY AND CARBON-ISOTOPE DISCRIMINATION OF COWPEA ACCESSIONS AND HYBRIDS

Carbon isotope discrimination (Delta) has been proposed as a selection criterion for improving adaptation to water-limited environments beca use it provides a measure of seasonal transpiration efficiency (WUE). In cowpea (Vigna unguiculata (L.) Walp.), consistent genotypic and dro ught-induced differences in Delta and WUE have been observed which wer e correlated as expected based on theory. Values of Delta and WUE for reciprocal hybrids grown under field conditions indicated nuclear inhe ritance for both characters. High WUE and low Delta were partially dom inant under dry field pot conditions, whereas high a was partially dom inant under natural wet soil conditions. Studies were conducted to tes t whether differences in rooting environment and xylem ABA levels are responsible for this change in dominance relations. Cowpea accessions and hybrids were grown in the field and subjected to wet or dry treatm ents with three different pot sizes. The experiment was conducted twic e, giving similar results. The dry treatment resulted in decreases in Delta, and increases in WUE and ABA concentration in the xylem sap. Un der drought, genotypes with higher WUE had higher xylem ABA, and the h ybrids exhibited greater increases in ABA concentration in response to the dry treatment than either parent. Partial confounding was present in that the hybrids had substantial leaf area and water-use rate, and may have experienced greater soil drought in some conditions than som e parents, with interacting effects of pot size. Plants in larger pots produced more biomass and leaf area but with no changes in xylem ABA. Concentration of ABA in the xylem sap was correlated with Delta and W UE for genotypic and drought treatment effects but not for pot size ef fects. Hybrids tended to have higher a and lower WUE in relation to mi d-parent means when grown in large wet pots than in small dry ones. Ch anges in hybrid performance with respect to Delta and WUE were more co nsistent with changes in xylem ABA for drought treatment effects than for pot size effects. Another chemical signal might be involved in med iating pot size effects.