Email: eisrat.jahan@sydney.edu.au Does mesophyll conductance of wheat change with leaf age and water availability Eisrat Jahan, PhD Candidate Margaret Barbour Faculty of Agriculture and Environment, The University of Sydney, Australia Aim Background To understand the effect of leaf age, plant age and water limitation on photosynthesis, gm and leaf intrinsic water use efficiency in wheat (Triticum aestivum L.) Inadequate water availability is considered the main environmental factor to limit plant photosynthesis, plant growth and ultimately yield production, especially in semi-arid areas like Australia. Understanding the effect of leaf age on photosynthetic capacity is necessary to estimate the long-term carbon budget of the leaf and of the whole plant. Mesophyll conductance (gm) is now recognized as an important limiting process for photosynthesis, and decreased gm is the main factor involved in early age-induced photosynthetic decline (Flexas et al. 2007). Methodology Results Plant material and growth conditions Sixteen plants of wheat (cultivar ‘Tasman’) were grown in a controlled-environment growth cabinet at the University of Sydney, Centre for Carbon Water and Food. The environment inside the cabinet was; Daytime temperature 25 °C, nighttime temperature 17 °C, relative humidity 75%, light period 14-h, PPFD 800 µmol m-2s-1 at the top of the plants. Drought treatment started at sixth week after plant emergence by suspending water to half of the pots until temporary wilting point, then maintaining soil water gravimetrically. Figure 2.Light-saturated photosynthetic rate (A), stomatal conductance to CO2 (gsc), mesophyll conductance (gm) and leaf-intrinsic water use efficiency (A/gsw) with increasing plant age. The bar represents the average standard error, n = 7. Figure 1.Light-saturated photosynthetic rate (A), stomatal conductance to CO2 (gsc), mesophyll conductance (gm) and leaf-intrinsic water use efficiency (A/gsw) with increasing leaf age. The bar represents the average standard error, n = 5. Measurement and estimation of gm Leaves were labeled on the week of their full expansion (leaf number 5 on week 5, up to leaf 9 on week 9). Mesophyll conductance was measured using a coupled of leaf gas exchange system (Li-Cor 6400) and stable carbon isotope tunable-diode laser absorption spectrometer (TDLAS) system. The value of gm was calculated from the difference between predicted carbon isotope discrimination assuming infinite mesophyll conductance (∆i), and measured discrimination (∆obs). Figure 5.The relationships between mesophyll conductance (gm) and light-saturated photosynthetic rate (A), and between mesophyll conductance (gm) and stomatal conductance to CO2 (gsc). A: (Irrigated) gm = .028A - 0.078, r = 0.78, P < 0.001 & (Drought) gm = .027A - 0.095, r = 0.58, P < 0.001 B: (Irrigated) gm = 1.90gsc + 0.027, r = 0.69, P < 0.001 & (Drought) gm = 1.59 gsc + 0.19, r = 0.45, P < 0.001 Figure 3.Light-saturated photosynthetic rate (A), stomatal conductance to CO2 (gsc), mesophyll conductance (gm) and leaf-intrinsic water use efficiency (A/gsw) with lower leaf position in both irrigated and drought conditions at the 9th week of measurement. Values are mean ± standard error, n = 5. Conclusion: gm declined as leaves aged and plants aged. Overall, gm was not significantly affected by drought, but tended to be slightly reduced in droughted plants for leaves that had recently finished expanding, and slightly higher in droughted plants for older leaves. Droughted plants had higher leaf intrinsic water-use efficiency, but this was driven by reduced stomatal conductance not by changes in photosynthetic rate or mesophyll conductance. Contact details: Email: eisrat.jahan@sydney.edu.au Mobile: +61452517403 Reference: Flexas J, Ortuno MF, Ribas-Carbo M, Diaz-Espejo A, Florez-Sarasa ID, Medrano H (2007) Mesophyll conductance to CO2 in Arabidopsis thaliana. New Phytologist 175, 501-511.