Interspecific competition in a pecan–cotton alleycropping system in the southern United States: Production physiology Diomides S. Zamora, Shibu Jose, P.K.R. Nair, and Craig L. Ramsey Abstract: A study was conducted on a Red Bay sandy loam soil (Rhodic Paleudult) in Jay, Florida, USA, to investigate how interspecific interactions between pecan (Carya illinoensis K. Koch) and cotton (Gossypium hirsutum L.) would affect cotton leaf morphology and gas exchange and thereby biomass and lint yield. We quantified specific leaf area (SLA), spe- cific leaf nitrogen (SLN), net photosynthesis (A), transpiration, stomatal conductance, and net canopy photosynthetic index (CNPI) from cotton with and without aboveground and belowground interactions. To separate roots of cotton and pecan, polyethylene-lined trenches were installed (barrier treatment) parallel to tree rows in half the number of plots. Results showed that SLA for barrier and nonbarrier plants was 61% and 47% higher, respectively, compared with the monoculture cotton. Monoculture plants exhibited higher CNPI (70.7 mmolÁm –2 Ás –1 ) compared with the barrier (52.7 mmolÁm –2 Ás –1 ) and nonbarrier plants (18.3 mmolÁm –2 Ás –1 ). SLN was similar for both the barrier and nonbarrier plants; however, it was lower than the monoculture. A positive curvilinear relationship between A and SLN was observed, with peak A (28 mmolÁm –2 Ás –1 ) observed between 2.2 and 2.4 mg NÁm –2 . Significant curvilinear relationships between CNPI and aboveground biomass and lint yield were also observed for all treatments. These findings indicate that competitive interactions in alleycropping regu- late leaf level traits such as SLA and SLN by altering water and light availability, which in turn exert a profound influence on aboveground biomass and lint yield for cotton plants. Key words: aboveground and belowground competition, net canopy photosynthetic index (CNPI), net photosynthesis, pho- tosynthetically active radiation (PAR), specific leaf area, specific leaf nitrogen. Re ´sume ´: Les auteurs ont conduit une e ´tude dans un loam sableux (paleudulte rhodique) a ` Red Bay, dans la re ´gion de Jay, en Floride, aux USA, afin d’examiner comment les interactions interspe ´cifiques entre le pacanier (Carya illinoensis K. Koch) et le coton (Gossypium hirsutum L.) affectent la morphologie foliaire du coton et ses e ´changes gazeux, et conse ´- quemment sa biomasse et son rendement en fibre. Ils ont quantifie ´ la surface foliaire spe ´cifique (SLA), l’azote foliaire spe ´- cifique (SLN), la photosynthe `se nette (A), la transpiration, la conductance stomatale ainsi que l’index photosynthe ´tique de la canope ´e (CNPI), chez le coton avec ou sans interactions au-dessus ou au-dessous du sol. Afin de se ´parer les racines du coton et du pacanier, on a installe ´ des tranche ´es de ´limite ´es par du polye ´thyle `ne (traitement isolement), paralle `lement aux range ´es d’arbres, dans la moitie ´ des parcelles. Les re ´sultats montrent que la SLA chez les plantes isole ´es et non isole ´es est de 61 % et 47 % plus e ´leve ´e, respectivement, comparativement au coton en monoculture. Les plants en monoculture mon- trent un CNPI plus e ´leve ´ (70.7 mmol m –2 Ás –1 ) comparativement aux traitements isole ´s (52.7 mmol m –2 Ás –1 ) et non isole ´s (18.3 mmolÁm –2 Ás –1 ). Le SLN est semblable chez les plantes isole ´es ou non; cependant, il est plus faible qu’en monoculture. On observe une corre ´lation curviline ´aire positive entre A et SLN, avec un pic en A (28 mmol m –2 Ás –1 ) observe ´ avec 2.2 a ` 2.4 mg NÁm –2 . On observe e ´galement des relations curviline ´aires significatives entre le CPNI et la biomasse et fibre au- dessus du sol, chez tous les traitements. Ces constatations indiquent que des interactions compe ´titives, dans la culture en alle ´es, re `glent des caracte `res foliaires comme la SLA et la SLN, en alte ´rant les disponibilite ´s en eau et en lumie `re, lesquel- les a ` leur tour exercent une profonde influence sur les rendements en biomasse et en fibre. Mots cle ´s : compe ´tition e ´pige ´e et hypoge ´e, index photosynthe ´tique net de la canope ´e (CNPI), photosynthe `se nette, radia- tion photosynthe ´tique active (PAR), surface foliaire spe ´cifique, azote foliaire spe ´cifique. [Traduit par la Re ´daction] Introduction It is well known that canopy level mechanisms influence growth and yield in plants. Leaf-level traits such as specific leaf area (SLA), specific leaf nitrogen (SLN), and net photo- synthesis (P net ) have all been explored in explaining growth and yield in agronomic and forestry systems (Reich et al. 1998a, 1998b, 1999; Zhao and Oosterhuis 1998; Gillespie et al. 2000; Gazal and Kubiske 2004). These traits, which influence carbon fixation and allocation patterns in plants (Evans 1989; Sinclair et al. 1993; Muchow and Sinclair 1994; Pettigrew et al. 2000; Milroy and Bange 2003), are Received 8 June 2006. Published on the NRC Research Press Web site at http://canjbot.nrc.ca on 17 January 2007. D.S. Zamora, 1 S. Jose, 2 and P.K.R. Nair. School of Forest Resources and Conservation, Newins-Zeigler Hall, Gainesville, FL 32611, USA. C.L. Ramsey. USDA-APHIS-PPQ-CPHST, National Weed Management Laboratory, 2301 Research Boulevard, Suite 108, Fort Collins, CO 80526-8117, USA. 1 Present address: University of Minnesota Extension Service, 708 Maple Street, Brainerd, MN 56401, USA. 2 Corresponding author (e-mail: sjose@ufl.edu). 1686 Can. J. Bot. 84: 1686–1694 (2006) doi:10.1139/B06-130 # 2006 NRC Canada