Linking non-structural carbohydrate dynamics and carbon starvation to leaf hydraulic behavior in Pinus edulis and Juniperus monosperma

Background/Question/Methods

Discrepant mortality rates of piñon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma) in response to drought have been hypothesized to be related to their contrasting strategies for regulating xylem tension and avoiding hydraulic failure (isohydry for P. edulis and anisohydry for J. monosperma), and the impact of these strategies on the carbon balance of these trees. Despite intense interest in the role of carbon dynamics in tree survival and productivity, little is known of the short- and long-term consequences of drought on hydraulic function and associated impacts on carbohydrate storage and depletion in the context of these two contrasting strategies. Maintaining sufficient leaf hydraulic conductance (K_leaf) is critical for preventing transpiration-induced desiccation and subsequent stomatal closure that restricts carbon gain. Leaf hydraulic vulnerability plus diurnal and seasonal patterns of shoot water potential (ψ_l), K_leaf and concentrations of non-structural carbohydrates (NSC) were analyzed in P. edulis and J. monosperma during a growing season characterized by large variations in soil water availability to examine how leaf hydraulic function influences the storage, depletion and conversion of starch and sugars in these species at multiple points over time.

Results/Conclusions

Leaf hydraulic vulnerability was significantly lower in J. monosperma than P. edulis with a 50% loss of leaf hydraulic conductance (P₅₀) occurring at -3.2 MPa for J. monosperma and -1.54 MPa for P. edulis. ψ_l declined below P₅₀ in P. edulis in the morning and stayed below P₅₀ throughout the day in May, August and September resulting in substantial and sustained reductions in diurnal K_leaf. ψ_l in J. monosperma dropped below P₅₀ only at the peak of a dry period during August resulting in the sustained maintenance of diurnal K_leaf during much of the growing season.  Differences in mean total foliar NSC between species were negligible in May but substantial in August (p = 0.0053) and September (p = 0.00028) when NSC content in J. monosperma was two to three times greater than in P. edulis. By August, total NSC content in P. edulis was less than 2% and sucrose levels were close to 0%. The relatively low levels of total NSC in P. edulis suggest that its isohydric strategy pushed it towards carbon depletion/starvation during much of the 2012 growing season.