Tree Ecophysiologist University of Florida Lake Alfred, Florida
Body of Abstract: Sugar transport from source to sink tissues is a fundamental challenge of vascular plant life. Source-sink attenuation of photosynthesis exemplifies this challenge: Plants down-regulate carbon fixation when they are sink-limited. Source-sink gradients drive transport processes, thus increasing carbon use to supply ratio (high sink:source) may increase carbon fixation, a key goal in agriculture under increasing ambient CO2. However, source-sink movement is mitigated by transport tissues, which has been largely overlooked, in part because of technical challenges of study. We hypothesize that hydraulic constraints of long distant transport have led trees to develop a constrained carbohydrate transport system. We will present evidence from radiolabeling experiments in pine and citrus trees to support the hypothesis that transport itself is a major mitigating factor in delivery of carbohydrates to sinks, and that trees manage their own transport limitations through strategies of carbohydrate storage. Long-leaf pine (Pinus pallustris) leaves show parallel phloem files running the length of the needle, with loading occurring all along that length, leading to competition between basal and distal leaf portions, preventing distal regions from exporting when the basal segments are active. The leaf manages this process by varying export from different leaf segments at different portions of the diel cycle. Citrus trees (Citrus spp.) are evergreen angiosperm trees that exhibit short periods of rapid whole-canopy shoot growth with high carbon demand. Despite wide fluctuation in total sink demand, citrus trees do not exhibit changes in carbon transport speed, but show signs of reserve accumulation and remobilization, storing starch near the points of future stem growth to meet fluctuating demand. We show that this highly constrained and buffered system mitigates the photosynthetic dynamics of such tree species, with implications for the whole-plant manipulation of photosynthetic efficiency.