The role of water relations in the maintenance of hydraulic function in mangroves

Abstract

Severe drought, heat and increasing salinity are causing an increase in large-scale mangrove death. Yet the relative importance of plasticity in water relations, hydraulic vulnerability and uptake of atmospheric water that underpin hydraulic function during drought remain poorly resolved. This thesis addresses the following questions: 1. How does acclimation of mangrove leaf water relations to a severe dry season drought affect leaf water relations in the following dry season? 2. How do interactions between branch water relations and stem hydraulic vulnerability confer salinity and drought tolerance in mangroves? 3. Can bark water uptake in mangroves increase stem hydration and contribute to stem water storage recharge?

Little is understood of the capacity of mangroves to show long-term acclimation of leaf water relations to severe drought. I tested for differences between mid-dry season leaf water relations before and after an extreme drought in two co-occurring mangroves: Aegiceras corniculatum and Rhizophora stylosa. Consistent with ecological stress memory, the legacy of severe drought enhanced salinity tolerance in the subsequent dry season through coordinated adjustments that reduced the leaf water potential at the turgor loss point and increased cell wall rigidity. Thus, "memory" of drought conditions in the previous dry season would enable greater leaf function with minimal adjustments in response to future drought conditions. However, declining turgor safety margins (i.e., difference between soil water potential and water potential at turgor loss), imply increased vulnerability of living tissues to dehydration.

Coping with water stress depends on maintaining both cellular function and hydraulic conductance. Yet measurements of vulnerability to water stress rarely focus on water storage and release in branch organs that buffer hydraulic function during water stress. I investigated the relationship between branch water relations and stem hydraulic vulnerability in two mangrove species growing at high and low salinities. Hydraulic redistribution from the foliage to the stem increased stem relative water content by up to 21%, while the water potentials at which 12% and 50% loss of stem hydraulic conductivity occurred decreased by ~1.7 MPa between low and high salinity sites, enabling substantial increases in hydraulic safety at high salinity. These results emphasise the importance of combining water relations and hydraulic vulnerability parameters to understand vulnerability to water stress across the whole plant.

Foliar water uptake can recharge water storage tissue and enable hydration above that possible from root water alone; however, few studies have investigated the role of the bark in water uptake. I investigated the pathways for, and the dynamics of bark water uptake in Avicennia marina stems. A novel function for lenticels as a pathway for bark uptake of liquid water was identified, thus providing the first evidence that targeted entry points control dynamics of water uptake through the outer bark surface. Last, it was established that bark water uptake was sufficient to increase stem water content by up to 3.7% and cause swelling of stem tissue layers, implicating it as a contributor to diel patterns of water storage recharge that buffer xylem water potential, and maintain hydration of living tissue. Show more