Nitrate reduction in Solanum tuberosum L.: development of nitrate reductase activity in field-grown plants

Nitrate reductase activity (in vivo method, substrate non-limiting) in unshaded leaves from the top of the canopy has been determined for field-grown potato plants over the course of the growing season. The pattern of change was almost identical for plants receiving no added fertilizer and those receiving 24 g N m−2. Activity increased to a peak at about 90 days after planting and declined thereafter. On a fresh weight basis activity was always higher in fertilized plants. Nitrate reductase activity was positively and significantly correlated with leaf protein content in high N plants (r2 = 0.71; P = 0.05), but poorly correlated with both the nitrate content of the leaf lamina and the nitrate concentration in petiole sap. Up until 90 days after planting (mid-July) there appeared to be a positive relationship between increased activity of nitrate reductase and solar radiation. However, results obtained over two seasons showed that the decline in activity after this time was not consistently linked with a fall in the level of solar radiation. Remobilization of reduced-N and stored nitrate from leaves and stems accompanied this decline in nitrate reductase activity and in the latter part of the season appeared to account for all of the N gained by growing tubers. In unfertilized plants nitrate-N accounted for 5 per cent or less of total plant N. Fertilized plants contained up to 25 per cent nitrate-N. While nitrate availability limited growth in unfertilized plants, sub-optimal rates of nitrate assimilation in fertilized plants, particularly during the early stages of post-emergence growth, may contribute to inefficient use of acquired nitrate. The carbohydrate status of leaf lamina and petiole sap were modified by N supply. The soluble sugar and starch contents of low N leaves were higher than in their high N counterparts. By contrast, the concentration of soluble sugars in petiole sap increased to a higher value in high N samples. Although sap sugar levels declined in both treatments towards the end of the season, N application delayed this decline for several weeks.