Leaf Morphology, Shoot Growth, and Gas Exchange of Multifoliolate Alfalfa Phenotypes M. G. Etzel, J. J. Volenec,* and J. J. Vorst ABSTRACT Herbage yield of alfalfa (Medicago sativa L.) is closely associated with leaf area. The objective of this study was to examine the re- lationships among leaf morphology, herbage growth, gas exchange, and root carbohydrate metabolism of alfalfa genotypes having tri- foliolate or multifoliolate leaves. Plants selected for consistent pro- duction of five, seven, or nine leaflets per leaf were compared with genotypes having trifoliolate leaves that were selected for slow (SSER) or rapid (RSER) shoot elongation rate. Plants were grown in a con- trolled environment chamber at 23°C. In Exp. 1, multifoliolate se- lections averaged between 4.1 and 7.3 leaflets per leaf and had up to threefold greater area per leaf than trifoliolate phenotypes. Her- bage regrowth of multifoliolate phenotypes was equal to phenotype RSER and greater than that of phenotype SSER. After Day 10 of regrowth, plant leaf area development was greater for phenotypes selected for seven or nine leaflets per leaf than trifoliolate pheno- types. Dark respiration rates of recently expanded leaves of plants were similar, while net photosynthetic rates of phenotypes selected for seven or nine leaflets per leaf were slightly lower than that of phenotype RSER. In Exp. 2, removal of lateral leaflets of multifo- liolate leaves during early leaf expansion, converted them to trifo- liolate leaves, increased net photosynthesis and dark respiration of trimmed leaves an average of 20% over that of intact multifoliolate leaves. Yield per shoot and herbage dry weight were reduced by removal of lateral leaflets, indicating that the multifoliolate leaf mor- phology was an important determinant of herbage yield of these multifoliolate phenotypes. In both experiments, selection for seven or more leaflets per leaf increased plant leaf area, a factor that may enhance herbage regrowth and forage yield of alfalfa. Additional Index Words: Medicago sativa L., Regrowth, Photo- synthesis, Nonstructural carbohydrate, Dark respiration. A LFALFA herbage yield is positively associated with leaf area per plant because leaves constitute be- tween 30 and 60% of forage yield (10, 11, 22, 26). In addition, plants with large leaf areas also exhibit high rates of photosynthesis per plant and thus tend to be high yielding (6). Therefore, understanding factors in- fluencing leaf area development of alfalfa may provide valuable insight into yield-determining factors of this species. Increasing the number of leaflets per leaf may be one method of modifying plant leaf area. In alfalfa, this can be accomplished genetically using the multi- foliolate leaf characteristic (1). Work by Bingham and Murphy (3) and Brick et al. (5) indicated that the mul- tifoliolate leaf trait was readily transmitted and highly heritable. However, segregation ratios and differences in extent of expression indicated that the trait was under complex genetic control. The influence of the multifoliolate trait on whole- plant morphology and herbage yield has been exam- ined. Bingham and Murphy (3) reported that plant height and internode length of genotypes with multi- foliolate leaves were greater than that of the trifoliol- ate cultivar, 'Narragansett'. However, subsequent field studies by Ferguson and Murphy (9) indicated no con- sistent effect of the multifoliolate trait on shoot and internode length. Multifoliolate genotypes did have fewer nodes per shoot when compared to plants with trifoliolate leaves. Brick et al. (5) also reported incon- sistent effects of the multifoliolate trait on internode length. They reported higher specific leaf weight (SLW, g m- 2 ) for leaves of multifoliolate genotypes when compared to the cultivar 'Mesa-Sirsa'. Herbage yields of widely spaced multifoliolate genotypes have been Department of Agronomy, Purdue Univ., West Lafayette, IN 47907. Contribution from the Purdue Univ. Agric. Exp. Stn. Journal Paper no. 11038. Received 12 June 1987. Corresponding author. Published in Crop Sci. 28:263-269 (1988).