The low SLP and photosynthetic rate of these early leaves is supported by the data of Jacob and Lawlor (1991) for young maize plants with ∼3 leaves (crosses in Fig. 4). The photosynthetic rates were low in these initial leaves of the 0 kg P ha−1 crops. However, for most of the crop duration they were in the lower part of the canopy and there fore, made only a small contribution to crop assimilation rate.