Modeling the influence of differential sectoriality on the photosynthetic responses of understory saplings to patchy light and water availability

Abstract

Exploitation of patchy light is a key determinant of plant performance in the forest understory. While many adaptive traits are known, the role of stem vasculature in understory photosynthesis is not established. Sectoriality-the degree of vascular constraint to long distance transport-has been hypothesized to limit growth in heterogeneous light. We simulated the photosynthetic potential of sectored and integrated plants in patchy light, as a function of soil water potential (patchy or uniform). We used hydraulic parameters typical of temperate woody species in an Ohm's law model including a tangential resistance parameter, and simulated cavitation by varying axial resistance of leaves, leaves and roots, or the whole plant. Our results suggest that differential sectoriality will not affect photosynthesis when water is plentiful, but can constrain stomatal conductance at more negative soil water potentials, especially when only a small portion of the crown receives light. This effect is strongest just below the turgor loss point, and depends on axial resistance and soil water heterogeneity. Increased resistance in high light leaves decreases photosynthesis regardless of sectoriality. However, when resistance is increased for leaves and roots or the whole plant, photosynthesis decreases more for sectored than for integrated plants. Moreover, the simulations suggest that sectoriality can further depress photosynthesis when water availability is asymmetrical. These results might explain why integrated species, such as Betula lenta, B. alleghaniensis, and Acer saccharum thrive in the forest understory and grow rapidly into canopy gaps, while sectored species, such as Quercus rubra, do not.

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

“…Our results support the usefulness of this model for understanding nitrogen distribution following a defoliation event, and similar methods can easily be applied to any process expected to change xylem water potential differences, such as patchy nutrients ( Gloser et al, 2007 ) or light ( Thorn and Orians, 2011a ). Our results support the usefulness of this model for understanding nitrogen distribution following a defoliation event, and similar methods can easily be applied to any process expected to change xylem water potential differences, such as patchy nutrients ( Gloser et al, 2007 ) or light ( Thorn and Orians, 2011a ).…”

“…Future directions -The quantitative approach used in this study provides a new perspective on sectoriality of nutrient transport, with general applicability for studies on xylem sectoriality. Our results support the usefulness of this model for understanding nitrogen distribution following a defoliation event, and similar methods can easily be applied to any process expected to change xylem water potential differences, such as patchy nutrients ( Gloser et al, 2007 ) or light ( Thorn and Orians, 2011a ). The availability of quantitative a priori expectations for nutrient distribution under passive fl ow provides a baseline to which observations can be compared.…”

“…In the circuit, the stem is subdivided into n segments, and the resistance components for r a and r t for each component are scaled to represent just the contribution of that component (modifi ed from Thorn and Orians, 2011a ). Hydroponic containers received a 50% dilution of the same nutrient solution used for plants grown in zeolite (see above), which was fully replaced once every 2 d. Once a day, deionized water was added to maintain constant solution volume.…”

“…The parameter r t designates the tangential resistance to fl ow between the two hydraulic sectors of the plant. In the circuit, the stem is subdivided into n segments, and the resistance components for r a and r t for each component are scaled to represent just the contribution of that component (modifi ed from Thorn and Orians, 2011a ). Fig.…”

“…Similar to the model presented by Thorn and Orians (2011a) , our sectoriality model adapts the Ohm ' s Law analogy for hydraulic conductance to simulate the rate of resource transfer between parallel xylem streams. The model simulates the whole plant xylem as two direct root-to-leaf pathways, connected to each other by a set of tangential resistors that represent the combined effects of indirect xylem pathways.…”