PARENT SESSION
Posters P1D Photosynthesis and global change. Abstracts (699-710)

Up-regulation of photosynthesis in young leaves of the C₄ sugarcane by elevated growth CO₂. Joseph Vu^*,1, Leon Allen Jr.¹, ¹ USDA-ARS & Agronomy Department, 304 Newell Hall, Gainesville, FL, USA

ABSTRACT- Sugarcane was grown for a season at ambient (C_A) and double-ambient (C_H) CO₂ levels. Midday CO₂ exchange rates (CER) and activities of key photosynthesis and sucrose metabolism enzymes were determined on the sixth leaf at 7, 14, 32, and 48 days after leaf emergence (DAE). Leaf CER, measured at the [CO₂] used for growth, were highest at 14 DAE for both C_A and C_H plants. However, leaf CER of the C_H plants were 15% greater at 7 DAE. In addition, C_H plants had lower stomatal conductance (18-51% less), lower transpiration (11-40% less) and higher water use efficiency (WUE) (38-75% more) at all DAE. Concentrations of chlorophyll and soluble protein and activities of Rubisco, PEP carboxylase (PEPC), NADP-malate dehydrogenase (NADP-MDH), pyruvate, P_i dikinase (PPDK) and sucrose-P synthase (SPS), expressed on a leaf area basis, generally followed leaf CER patterns during leaf ontogeny. At 14 DAE, chlorophyll and soluble protein of the C_H plants were 31 and 15% greater. PEPC of the C_H plants was 4-21% lower at all DAE. In contrast, Rubisco was enhanced by 21%, NADP-MDH by 174%, PPDK by 117% and SPS by 37% at 14 DAE for the C_H plants. Furthermore, leaf sucrose of the C_H plants was 31 and 19% greater at 7 and 14DAE. Thus, there was an up-regulation of photosynthesis in the young expanding leaves of sugarcane by elevated growth [CO₂]. In addition, a reduction in stomatal conductance under elevated growth [CO₂] led to a conservation of water resources and an improvement in WUE, and this could also account for the observed enhancement in sugarcane biomass accumulation.

KEY WORDS: C₄ photosynthesis, Rising atmospheric CO₂, global climate change