Radiation use efficiency and leaf photosynthesis of sweet corn in response to phosphorus in a cool temperate environment

Phosphorus (P) deficiency has been shown to decrease accumulated intercepted solar radiation (RIcum) for sweet corn and in this paper the effects on radiation use efficiency (RUE) and leaf photosynthetic rate at 2000 μmol m−2 s−1 PPFD (P2000) are examined. Data from two consecutive field experiments on a low P site at Lincoln, New Zealand, were analyzed. In the first experiment (2001/2002) 0, 50, 100, 150 or 200 kg P ha−1 was applied to sweet corn followed by an additional 0, 0, 10, 20 or 40 kg P ha−1 in 2002/2003 applied to the same plots. Thus, total P applications were 0, 50, 110, 170, or 240 kg P ha−1.There were no differences in RUE between P treatments but RUE changed with crop ontogeny. The RUE was 0.66 g MJ−1 before each crop had 10 fully expanded leaves and RUE was 1.34 g MJ−1 after this. The cause of this difference between development stages was unclear, but it was not related to air temperature. In contrast to RUE, there were clear differences in P2000 due to differences in specific leaf phosphorus (SLP). At a SLP of 0.12 g P m−2 or greater, P2000 was a constant value of 34 μmol CO2 m−2 s−1. When SLP was less than 0.12 g P m−2P2000 was reduced. The differences in P2000 between P treatments occurred when the plants were young (≤10 fully expanded leaves) and after this, SLP of all leaves exceeded 0.12 g P m−2 and there were no differences in P2000. These early leaves were unimportant in determining crop RUE but are likely to have been important in establishing the hierarchies that led to the changes in RIcum reported previously. These results demonstrate the importance of an adequate supply of P early in crop growth.