CANOPY DEVELOPMENT, PHOTOSYNTHESIS AND RADIATION-USE EFFICIENCY IN SUNFLOWER IN RESPONSE TO NITROGEN

Sunflower (Helianthus annuus L.) was sown at two densities (5.7 and 2. 9 plants m(-2)) at Cordoba, Spain (lat. 38 degrees N) on May 4, 1992 o n a site of low nitrogen status. A high nitrogen comparison was establ ished with 25 g m(-2) applied in a split dressing of 15 and 10 g m(-2) at 15 and 65 days after sowing (das). The growth of the four treatmen ts (2 densities X 2 N levels) was measured on three occasions, 41 and 56 das and finally at flowering 71 das, as above-ground biomass in lam inae and in stems plus petioles. Leaf nitrogen concentration was measu red on every third leaf in those canopies. The areas of all leaves on 4 plants per plot was measured every 10 days from 33 das. On those day s and many intervening occasions, canopy light interception was measur ed at noon and on some occasions integrated values were obtained over periods of several days. Frequent measurements of leaf photosynthesis were made in response to irradiance and leaf nitrogen content. Over th e experimental period to 71 das, nitrogen promoted growth from 200 to 800 g m(-2). This response was almost equally the result of doubling i nterception (1.9) and radiation-use efficiency (2.1). In the first sta ge of growth, up to 41 das, interception increased more (2.5) than did RUE (1.6). Maximum leaf photosynthesis (at high irradiance) increased from 10 to 40 mu mol m(-2) s(-1) over the range of specific leaf nitr ogen concentration from 1.0 to 2.8 g N m(-2). Analyses with a simulati on model of canopy photosynthesis showed that only a small part (14%) of the observed response in RUE could be attributed to greater photosy nthesis resulting from greater N content and its distribution in the c anopies. The major part therefore comprised undefined responses of par titioning of biomass to roots and to losses by respiration.