THE ECOLOGY OF THE STRANDLINE ANNUALS CAKILE-MARITIMA AND SALSOLA-KALI .2. THE ROLE OF NITROGEN IN CONTROLLING PLANT PERFORMANCE

(1) The difference in performance of two annual halophytes, Cakile maritima and Salsola kali, was investigated in strandline habitats where plants are large and fecund and on foredunes where they are small and stunted. (2) The response of growth and photosynthetic gas exchange to nitrogen and phosphorus addition was investigated in the field. (3) Addition of nitrogen caused a substantial increase in growth on the foredune, confirming its limited availability. Phosphorus was present in the foredune sand in sufficient quantity to support the extra growth of nitrogen-fertilized plants. (4) Nitrogen addition increased the net photosynthetic rate of foredune plants to values comparable with those of unfertilized strandline plants. It is suggested that nitrogen deficiency limits growth on the foredune by restricting the rate of net photosynthesis. (5) Recalculation of photosynthesis as a nitrogen use efficiency (carbon fixed per unit nitrogen content of leaf) shows that the differences in photosynthetic rate in the field could be attributed wholly to differences in leaf nitrogen status.