Background/Question/Methods
Sectoriality - the degree to which long distance transport in plants is constrained by vascular anatomy - has been shown to vary considerably among woody species, and research indicates that sectoriality in temperate woody species is negatively correlated with shade-tolerance. However, the adaptive significance of this correlation is unknown. One hypothesis suggests that sectoriality constrains growth by limiting the ability of shaded plants to make use of patchy light. To test this idea, we adapted the Ohm's law analogy for xylem hydraulics to simulate water flow to leaves of sectored and unsectored plants under patchy light conditions. We selected hydraulic parameters typical of temperate woody species, and to simulate the effects of xylem embolism we varied the axial resistance from baseline. We simulated photosynthetic potential as a function of either patchy or uniform soil water potential.
Results/Conclusions
According to our model, photosynthesis is unaffected by sectoriality when water is plentiful, but as soil water potential becomes more negative, sectoriality can constrain stomatal conductance and therefore photosynthesis. The simulated importance of sectoriality is dependent on both the patchiness of soil water and on the whole plant axial resistance. When soil water is uniform and axial resistance is low, sectoriality presents only a minor constraint to photosynthesis in patchy light, even at low soil water potential. However, when soil water potential is patchy, the simulated effect of sectoriality on photosynthetic potential increases dramatically. Furthermore, at elevated stem axial resistance, such as can result from drought-induced embolism, sectoriality becomes a more important constraint, even when soil water is uniform. We conclude that xylem sectoriality can limit the photosynthetic potential of shaded plants, but only when water is a limiting resource. These effects are expected to be particularly strong under conditions that favor soil moisture heterogeneity and for species vulnerable to embolism.
