Partial defoliation and hydraulic integration in Ocimum basilicum (Lamiaceae): Testing a model for sectored xylem flow using 15 N labeling

 Premise of the study: Xylem sectoriality limits nutrient translocation throughout the plant, which may constrain growth following partial defoliation by herbivores. To date, the implications for nutrient allocation have not been assessed, and sectoriality studies lack a modeling framework for relating intersector transport to the hydraulic properties of the stem.  Methods: We present an Ohm ’ s law model for sectoriality of xylem transport in basil ( Ocimum basilicum ), which we parameterized and tested using hydroponically grown split-root basil, pruned to two branches. To evaluate xylem resistance, we forced KCl solution through excised stems along either direct or indirect pathways. To examine the effect of partial defoliation on nutrient allocation, we applied 15 N-NO 3 to one half of the root system after one of three defoliation treatments: uniform, orthostichous to label, or opposite the label.  Key results: In support of our model, we found a tight correlation between total water uptake and total leaf area and between the actual and predicted proportions of water taken up from the labeled container. Signifi cantly more 15 N accumulated in orthostichous than in opposite sector leaves for the uniform and opposite defoliation treatments, but not for the orthostichous defoliation treatment. Across individuals, 15 N distribution varied as predicted by the model, but there was generally 10% more 15 N crossover than predicted.  Conclusions: These results support our model and suggest high potential integration for O. basilicum . The fact that our model consistently underestimated the rate of crossover suggests that other mechanisms are also in play. Future research should evaluate possible mechanisms for this mixing, including the role of transporters in specialized transfer cells.