Effect of leaf age and position on net photosynthetic rates in maize (Zea Mays L.)

Greater ease in obtaining photosynthesis measurements has generated renewed interest in developing methods to compare leaf measurements among environments, stress treatments, species and varieties, and to extrapolate leaf measurements to estimate canopy photosynthetic rates through the growing season. In this study net photosynthetic rates were monitored for individual field-grown maize (Zea mays L.) leaves at a range of irradiances. Photosynthesis and irradiance measurements were fit to rectangular hyperbolic photosynthetic response curves and the curves were analysed as a function of plant and leaf age and leaf position. As plant and leaf age increased past full leaf expansion, Pmax (photosynthesis at light saturation) and P2000 (photosynthesis at irradiance (I) = 2000 μ E m−2 s−1) decreased and the curves flattened to approach an asymptote at lower irradiance levels. Linear regressions of P2000 against plant and leaf age had high correlations (r = 0.91 and 0.92, respectively). P2000 was not correlated with leaf position but most frequently the category containing the main ear, leaves 12–14, showed the highest P2000 values. Therefore, to reduce interplant variability, leaf photosynthesis comparisons should be made on the last fully expanded leaves, and once all leaves are fully expanded, on the ear leaf. Using a calculated value for P2000 and the coefficient α and previous measurements of maize respiration (D), photosynthesis (PN) can also be calculated as a function of irradiance for different aged leaves. Therefore, with available descriptions of canopy structure and irradiance levels within the canopy, leaf photosynthesis can be extrapolated over the whole canopy throughout the growing season.