María E. Otegui* and Martín Uribelarrea

1. Tolerant AX852 Tolerant AX852 10 pl m-2 15 pl m-2 10 pl m-2 15 pl m-2 Ear leaf pre-silking 50 Ear + husk 4 Ear leaf post-silking Ear without husk 40 3 30 2 20 1 10 0 0 Ethylene Production (nL g-1 h-1) Ethylene Production (nL g-1 h-1) Intolerant AX886 Intolerant AX886 10 pl m-2 10 pl m-2 15 pl m-2 15 pl m-2 60 y = 1.5903 e0.203 X r2= 0.725 4 50 50 40 40 30 3 20 30 10 2 0 -15 -10 -5 0 5 10 15 20 1 10 0 0 -15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15 Days from silking Days from silking Ethylene production around silking in maize: response of two hybrids with contrasting tolerance to increased stand density María E. Otegui* and Martín Uribelarrea Departamento de Producción Vegetal, Facultad de Agronomía UBA andIFEVA-CONICET. Buenos Aires, Argentina. *otegui@agro.uba.ar INTRODUCTION MATERIALS AND METHODS Ethylene production has been associated with different responses of crops to abiotic and biotic stress conditions [3]. In maize (Zea mays L.), there are reports of ethylene synthesis by endosperm and embryo tissues, and of their differential sensitivity to ethylene production [4]. Ethylene production in maize kernels increases under light stress [2], but there is no comparative study addressing ethylene production of vegetative (leaves) and reproductive (earshoot) tissues during the critical period for kernel number determination (i.e., around silking). We determined ethylene production of these tissues around silking for hybrids with contrasting response to increased stand density. Field experiment during 2008-2009 at the experimental unit of FAUBA (35°35´S and 59°29´W), Argentina, with no water or nutrient limitations. Treatments: a factorial combination of (i) two temperate, single cross maize hybrids of contrasting tolerance to increased stand density (AX852: tolerant, RM 117; AX886: intolerant, RM 118). (ii) two stand densities: 10 plants m-2 (near optimum for irrigated systems) and 15 plants m-2 (above optimum) Experimental design: Split plot, with stand density in the main plot and hybrids in the subplot. Three replicates. Subplot size: seven rows, at 0.7 m between rows and 8 m length. Measurements: ethylene production of ear leaf and uppermost earshoot tissues along period between ca. silking – 10 days and silking + 10 days. Sampling rate: every other day. RESULTS CONCLUSIONS Ethylene production along the presilking period was significantly (P<0.05) larger for earshoot (5.02 nL g-1 h-1 for AX 852 and 8.88 nL g-1 h-1 for AX886, averaged across stand densities) than for leaf tissues (0.63 nL g-1 h-1; averaged across treatments), but no significant difference was detected between stand densities for each organ (Figures 1 and 2). From silking until R2, all tested tissues exhibited low levels of ethylene production (0.825 nL g-1 h-1 for leaves and 0.822 nL g-1 h-1 for ears without husks; averaged across treatments). When earshoot data where corrected by tissue weight and stand density (i.e., expressed on a per land unit basis), significant differences (P<0.05) were detected in ethylene production between stand densities for each hybrid (Table). No difference was detected between hybrids at each stand density. For the tolerant hybrid, postsilking ethylene production was always larger than the presilking production (P≤0.07). Figure 2. Ethylene production evolution of the apical ear leaf around silking. Data correspond to two hybrids with contrasting tolerance to crowding stress cropped at two stand densities (10 and 15 pl m-2), and expressed on a fresh weight basis. Figure 1.Ethylene production evolution of the apical earshoot around silking. Data correspond to two hybrids with contrasting tolerance to crowding stress cropped at two stand densities (10 and 15 pl m-2), and expressed on a fresh weight basis. Data in the inset correspond to both hybrids in the current research (black symbols and fitted equation) and to the American hybrid Pioneer 34B23CS (red symbols) grown at 8 pl m-2 in Cárcova (2003). • Ethylene production per unit fresh weight of earshoot tissue was within the range reported in previous studies for temperate maize hybrids [1]. In agreement with previous findings, this production decreased exponentially from silking – 15 d to silking + 15 d. Before silking, production of reproductive tissues (whole earshoot) was up to tenfold larger than observed for vegetative tissues (ear leaf). • Contrary to shading experiments [2], increased light stress determined by enhanced plant population did not cause an increase in presilking ethylene production of earshoot tissues. Expected differences between stand densities (i.e., 15 pl m-2 > 10 pl m-2) were detected only for postsilking data expressed on a land unit basis. On this basis, postsilking ethylene production was larger than the presilking one only for the tolerant hybrid. • Hybrids did not differ in ethylene production per unit fresh weight of tissue. Large values registered for earshoots of the intolerant hybrid on a land unit basis were due to differences between hybrids in earshoot fresh weight (intolerant > tolerant; data not shown) Table. Ethylene production of the apical earshoot on a per land unit basis (nL m-2 h-1) References 1- Cárcova, J. 2003. Fundamentos reproductivos para incrementar y estabilizar la producción de granos en maíz. PhD Thesis. Graduate’s School, Fac. of Agronomy, UBA. 2- Cheng, C.Y. and H.S. Lur. 1996. Ethylene may be involved in abortion of maize caryopsis. Phys. Plantarum 98:245252. 3- Dangl, J.L., R.A. Dietrich, and H. Thomas. 2000. Senescence and programmed cell death. In: Buchanan et al. (eds), Biochemistry & molecular biology of plants. Am. Soc. of Plant Phys., pp: 1044-1101. 4- Gallie, D.R. and T.E. Young. 2004. The ethylene biosynthetic and perception machinery is differentially expressed during endosperm and embryo development in maize. Mol Gen Genomics 271:267–281. a Different lower-case letters within a column and hybrid indicate significant differences between stand densities (P<0.05). b Different upper-case letters within a row indicate significant differences between subperiods (P≤0.07) Acknowledgement This work was financed by AgroFresh – Rohm and Haas.