Leaf gas exchange characteristics of red raspberry germplasm in a hot environment

Net CO2 assimilation (A), evapotranspiration (ET), and stomatal conductance (g(s)) were determined In two experiments for 14 and 18 raspberry (Rubus sp.) genotypes, respectively, grown in 4-L containers and exposed to 35 degrees C daytime temperatures 2 weeks and 4 weeks after placement in growth chambers. Measurements were taken on two successive leaves on the same primocane between the third and seventh node (approximate to 75% to 85% of full leaf expansion). In Expt. 1, selections from Louisiana exhibited higher A (3.10-5.73 mu mol.m(-2).s(-1)) than those from Oregon (0.50-2.65 mu mol.m(-2).s(-1)). In Expt, 2, the genotype x time interactions were nonsignificant, and, time of measurement did not affect A or ET (P less than or equal to 0.05). Assimilation ranged from 2.08 to 6.84 mu mol.m(-2).s(-1) and varied greatly among genotypes, indicating that diverse A levels exist at high temperatures in raspberry germplasm, NC 296, a selection of R. coreanus Miq. from China, and 'Dormanred', a southern-adapted raspberry cultivar with R. parvifolius Hemsl. as a parent, had the highest A rates. Evapotranspiration and g(s) did not differ among genotypes. Average g(s) for all genotypes declined from 234 mmol.m(-2).s(-1) in week 2 to 157 mmol.m(-2).s(-1) in week 4. Our findings, coupled with plant performance under hot conditions, can be used to identify potential parental raspberry germplasm for breeding southern-adapted cultivars.