A Model for C3Leaves Describing the Dependence of Net Photosynthesis on Irradiance

Abstract

Measurements of the photosynthesis-light response of flag leaves in a winter wheat crop were made during the period from maximum elongation until complete senescence. Immediately following leaf elongation, the maximum rates of photosynthesis and the values of efficiency at low light were in the range 2-8-3-6 g C02 m~2 h-1 and 8-11 //g C02 J-1 respectively. The shape of the photosynthesis light response remained constant throughout and was close to a 'Blackman' type response rather than a rectangular hyperbola. The results were analysed, therefore, using a more recent model which is a non-rectangular hyperbola. Stomatal and internal resistances were equally important in limiting the maximum rate of photosynthesis. INTRODUCTION The model, derived by Marshall and Biscoe (1980), describes the photosynthesis light response (PLR) as a non-rectangular hyperbola which has four parameters: Pn ma* (maximum rate of net photosynthesis), Rd (rate of dark respiration), a (photochemical efficiency of photosynthesis at low light intensity), and d (ratio of physical to total diffusion resistance to C02). In this paper the effect of age on flag leaf photosynthesis is assessed in terms of these individual parameters. Com parisons and investigations need not be limited to effects of age alone but may also include effects of treatments, for example, differing levels of nitrogen fertilization or water stress. However, such comparisons are valid only if the calculated values of the parameters are reliable and representative of the actual measurements. This was shown to be the case in Marshall and Biscoe (1980) and further proof is given in the Results section of this paper. Having applied the model as an analytical tool to summarize information about the PLRs, the causes of changes with age in calculated parameter values are then investigated with reference to the physical and chemical processes they represent. 1 Present address: Broom's Barn Experimental Station, Higham, Bury St. Edmunds, Suffolk, IP28 6NP. This content downloaded from 207.46.13.15 on Fri, 26 Aug 2016 05:36:03 UTC All use subject to http://about.jstor.org/terms 42 Marshall and Biscoe—Flag Leaf Photosynthesis MATERIALS AND METHODS Using an infrared gas analysis system (Marshall and Biscoe, 1977), detailed measurements of PLRs were made on the flag leaf in a winter wheat crop in 1977, from maximum elongation (5 June 1977) until complete senescence, a period of 65 d. The crop underwent normal farm management and the season was both cooler and wetter than average. The model was fitted to all sets of measurements with the aid of a computer program from the NAG library (fitting routine E04JAF) as described in Marshall and Biscoe (1980). RESULTS AND DISCUSSION Photosynthesis-light response curves Figure 1 shows three typical sets of flag leaf PLR measurements made 17, 31, and 51 d after maximum elongation (DAME). These curves show that as the leaf ages the maximum rate of net photosynthesis decreases but there appears to be relatively little change in the initial slope at low light intensity, consistent with other results where detailed PLR measurements have been presented (Ludlow and Wilson, 1971; McPherson and Slatyer, 1973). The solid lines in Fig. 1 are non-rectangular hyperbolae and it can be seen that they are an acceptable fit. The calculated values of the parameters are shown in Table 1.