Shoot Fruiting Status, Leaf Nitrogen, and Photosynthesis During Kernel Fill in ‘Western' Pecan, Poster Board #198

Most leaf nitrogen is found within the photosynthetic apparatus, and, as such, nitrogen availability is closely tied with both photosynthetic function and productivity of fruit trees.  Reproductive growth in fruit trees also represents a strong nitrogen sink.  When soil nitrogen uptake by roots is low, fruit trees remobilize and translocate nitrogen from leaves to help meet nitrogen demand of developing fruit.  Our objective was to describe shoot-level impacts of pecan fruiting on leaf nitrogen and photosynthesis during kernel fill under a range of tree nitrogen status.  Our study was conducted in a mature pecan (cv. ‘Western’) orchard near Las Cruces, NM.  In 2009, fifteen trees exhibiting a range of nitrogen deficiency symptom severity (quantified using a SPAD 502 chlorophyll meter) were grouped into three symptom severity classes each with five trees: 1) ‘severe’, SPAD 26.4-31.4; 2) ‘moderate’, SPAD 33.6-37.2; and 3) ‘slight’, SPAD 39.7-46.7.  For the ‘slight’ class, 2.3 kg and 2.1 kg nitrogen (as urea) were broadcast-applied around each tree in 2009 and 2010, respectively.   For the ‘moderate’ class, 1.2 kg nitrogen was broadcast around each tree in 2009 and 1.0 kg nitrogen in 2010. Light-saturated leaf photosynthesis was measured with a Li-6400XT photosynthesis system on fruiting and non-fruiting shoots during kernel fill (2 October, 2009, 1 October, 2010, and 5 November, 2010). Following photosynthesis measurement, the leaflet measured and its leaflet pair partner were sampled, dried in an oven, and ground. Leaf tissue nitrogen concentration was measured with a ThermoFinnigan DeltaPlus XP Isotopic Ratio Mass Spectrometer.  Both leaf nitrogen concentration and leaf nitrogen per area were significantly lower on fruiting shoots than non-fruiting shoots on all three sampling dates.  For both variables, the nitrogen status main effect was also significant while the 2-way interaction of shoot fruiting status and tree nitrogen status was not.  Photosynthesis of leaves on fruiting shoots was also significantly lower than that of non-fruiting shoots on all sampling dates, especially on 2 November 2010, when fruiting shoot photosynthesis was reduced by 42.8%.  Our data suggest that pecan fruit nitrogen demand reduced nitrogen in nearby leaves on the same shoot during kernel fill and that photosynthesis of those leaves was negatively affected as a consequence. The effect of tree nitrogen status and shoot fruiting status might be best summarized with an additive model where there is a larger relative reduction in leaf nitrogen and photosynthesis for fruiting shoots on low nitrogen status trees.