Xylem structure of roots is linked to leaf habit in southern California chaparral shrubs

Background/Question/Methods

Plants transpire water in order to assimilate carbon and this links the plant vascular system to leaf function.  Thus, differences in leaf function should also be reflected in differences in vascular function.  Deciduous and evergreen species should differ in the percentages of cell types (vessels, parenchyma, and fibers) within their vascular tissue due to different hydraulic and metabolic needs. We hypothesized that deciduous and evergreen species would differ in stem and root vascular structure.  Deciduous species were expected to have greater vessel and parenchyma areas compared with co-occurring evergreen species.  This is because the leaves of deciduous species may need to have higher rates of transpiration and deciduous stems may need to store more carbohydrates to flush new leaves early in the growing season.  As a corollary to this we predicted that deciduous species would have lower amounts of fibers than evergreens species as a tradeoff for increased hydraulic demands.  Percentage cell type per unit cross sectional area data were collected from thin sections of xylem tissue of stems and roots of 36 deciduous and evergreen species in southern California.

Results/Conclusions

Deciduous species had significantly greater percentage of vessels (mean ±1SE evergreen = 22.8 ± 1.4 and deciduous = 28.4 ± 1.6; P = 0.017) and significantly lower percentage of fibers (evergreen = 56.1 ± 2.3 and deciduous = 46.6 ± 2.5; P = 0.01) in roots than evergreen species.  Parenchyma percentages were not different in the roots (evergreen = 21.1 ± 1.5 and deciduous = 25.1 ± 1.8; P > 0.05).  Percentage of cell types in stems did not differ between deciduous and evergreen species.  These data support the prediction that deciduous species may need a more efficient vascular system compared to evergreen species within the same chaparral environment.  The increased efficiency may come at the cost of mechanical strength (reduced percentage of fibers) in roots.  Differences in storage between deciduous and evergreen species have not lead to differences in parenchyma area.  Despite similar parenchyma per unit cross section for evergreen and deciduous species in roots and stems, there still may be differences in packing efficiency of parenchyma carbohydrates.