TEMPORAL EXPRESSION OF EFFECTS OF VARYING NITROGEN SUPPLY ON CANOPY GROWTH, PHOTOSYNTHESIS AND FRUIT PRODUCTION FOR ACTINIDIA-DELICIOSA VINES IN THE FIELD

The effects of varying nitrogen supply on canopy leaf area, response o f leaf net photosynthesis (A(n)) to quantum flux density (Q), and frui t yields of kiwifruit vines (Actinidia deliciosa var. deliciosa) were examined in a two-year field experiment. Vines were grown with 0, 250 or 750 kg N ha-1 year-1. The responses to nitrogen supply were compare d with responses to shade, to examine the impact of reduced carbon ass imilation on canopy leaf area and fruit yields. Nitrogen supply did no t affect significantly any of the measured variables during the first season of the experiment. In the second season, canopy leaf area was r educed significantly where nitrogen supply was limited. The quantum ef ficiency of photosynthesis (phi(q)) increased from 0.03 mol CO2 mol-1 Q soon after leaf emergence to more than 0.05 mol CO2 mol-1 Q during t he middle of the growing season. The quantum saturated rate of A(n)(A( sat)) also increased during the season, from 7-10 mumol CO2 m-2 s-1 so on after leaf emergence, to 15-20 mumol CO2 m-2 s-1 during the middle of the growing season. Phi(q) and A(sat) increased significantly with nitrogen supply at all measurement times during the second season. For vines with high nitrogen, fruit yields in both seasons were similar, averaging 3.05 kg m-2. Fruit yields in the second season were reduced significantly where nitrogen supply was limited, due to reduced fruit numbers. The relative effects of reduced leaf area and reduced leaf ph otosynthesis for carbon assimilation by nitrogen deficient vines were examined using a mathematical model of canopy photosynthesis for kiwif ruit vines. Simulations of canopy photosynthesis indicated that effect s on leaf area and on leaf photosynthesis were of similar importance i n the overall effects of nitrogen deficiency on carbon assimilation. T he effects of nitrogen supply on fruit numbers (i.e. flower developmen t) preceded the measured effects on carbon assimilation, indicating th at the nitrogen supply affected carbon partitioning to reserves in the first season.