Two critical terrestrial ecosystem processes in the context of global change - the rate of forest carbon uptake and the occurrence of drought-induced tree mortality - are both linked to carbon transport in the phloem. Yet, despite this importance, many questions about how carbon and water interact for mature forest trees remain unanswered. Here, we use diurnal variation in leaf water status to address the following questions: (1) Do midday depressions in water potential limit the rate of leaf carbon export due to the reliance of the phloem on the xylem for water supply and subsequent carbon movement? (2) Do sugars then accumulate in the leaf during the middle of the day and assist in turgor maintenance? (3) Are the concentrations in the source phloem sap high enough to generate turgor pressure, but low enough to permit passive loading? To answer these questions, we measured diurnal and seasonal patterns of non-structural carbohydrates, net assimilation, solute and water potentials, and estimated carbon export in leaves of five red oak trees. Additionally, we developed a novel method for quantifying phloem sap sugar concentrations using Raman spectroscopy and subsequently measured these concentrations at midday and in the evening.
Results/Conclusions
We found that carbon export in the phloem occurred throughout the day at equal or higher rates than at night despite water potentials falling to -1.8 MPa at midday. Sucrose and starch accumulated over the course of the day, with sucrose contributing approximately 50% of the 0.4 MPa diurnal active osmotic adjustment. As a result of this diurnal osmotic adjustment, estimates of midday turgor were always greater than 0.7 MPa. Additionally, we show that sugar concentrations in the phloem sieve elements in the leaf petiole of mature red oak trees are conserved at 1535 mmol throughout the day. This is the first record of sugar concentration in sieve elements in source tissues of mature forest trees. These concentrations are high enough to generate at least 2.5 MPa of phloem turgor during midday, and 4 MPa at night, and thus to allow Munch flow of carbon to occur despite diel variation in water potential. However, these concentrations are too high to allow for the passive loading of sucrose from the sites of production to the transport cells. By increasing our fundamental understanding of phloem physiology, these findings will improve mechanistic modeling of important forest ecosystem processes dependent on carbon transport.