THE RELATION BETWEEN THE NITROGEN CONCENTRATION AND PHOTOSYNTHETIC CAPACITY OF POTATO (SOLANUM-TUBEROSUM L) LEAVES

Measurements of the rate of light-saturated photosynthesis (Pmax) were made on terminal leaflets of potato plants growing in crops supplied with 0, 3, 6, 12, 24 and 36 g N m-2. Measurements were made between 100 and 154 d after planting. Two types of leaf were selected-the fourth leaf on the second-level branch (L4, B1) and the youngest terminal leaflet that was measurable (LYM). Later, the total nitrogen concentration of each leaflet (NL) was measured. A linear regression between Pmax and NL, common to both leaf positions, explained 68.5% of the total variation. With L4, B1 leaves there was a significant improvement in the proportion of variation explained when regressions with separate intercepts and a common slope were fitted to individual fertilizer treatments. These results suggest that an increasing proportion of leaf nitrogen was not associated with the performance of the photosynthetic system with increasing nitrogen supply. This separation between nitrogen treatments was not as clear for LYM leaves. Stomatal conductance to transfer of water vapour was neither influenced by leaf position nor directly by nitrogen supply. Rather conductance declined in parallel with the decline in photosynthetic capacity. © 1991 Annals of Botany Company.