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ABSTRACT Freezing air temperatures kill most leaves, yet the leaves of some species can survive these events. Tracking the temporal and spatial dynamics of freezing remains an impediment to characterizing frost tolerance. Here we deploye time‐lapse imaging and image subtraction analysis, coupled with fine wire thermocouples, to discern the in situ spatial dynamics of freezing and thawing. Our method of analysis of pixel brightness reveals that ice formation in leaves exposed to natural frosts initiates in mesophyll before spreading to veins, and that while ex situ xylem sap freezes near 0°C, in situ xylem sap has a freezing point of −2°C in our model freezing‐resistant species ofLonicera. Photosynthetic rates in leaves that have been exposed to a rapid freeze or thaw do not recover, but leaves exposed to a slow, natural freezing and thawing to −10°C do recover. Using this method, we are able to quantify the spatial formation and timing of freezing events in leaves, and suggest that in situ and ex situ freezing points for xylem sap can differ by more than 4°C depending on the rate of temperature decline. 

Abstract PremiseHydraulic segmentation, caused by the difference in embolism resistance across plant organs, provides a sacrificial layer of cheaper plant organs, like leaves, to protect more costly organs, such as stems, during drought. Within‐leaf hydraulic segmentation has been observed in two compound‐leaved tree species, with leaflets being more vulnerable than the rachis or petiole. Many herbaceous species have compound leaves, and some species have leaflets that are associated with pulvini at the base of the lamina, which could provide an anatomical means of preventing embolism from spreading within a leaf because of the higher number of vessel endings in the pulvinus. MethodsWe used the optical vulnerability method to investigate whether differences in embolism resistance were observed across the leaf tissues of six herbaceous species and one deciduous tree species with compound leaves. Our species selection included both palmately and pinnately‐compound leaved species, one of each with a pulvinus at the base of the leaflets. ResultsWe found considerable variation in embolism resistance across the species measured, but no evidence of variation in embolism resistance within the leaf. In two species with pulvini, we observed major embolism events crossing the pulvinus, spreading from the rachis or petiole into the lamina, and embolizing both tissues at the same water potential. ConclusionsWe conclude that within‐leaf hydraulic segmentation, caused by variation in embolism resistance, is not a universal phenomenon to compound‐leaved species and that the presence of a pulvinus does not provide a barrier to embolism spread in compound leaves. 

Abstract By regulating carbon uptake and water loss by plants, stomata are not only responsible for productivity but also survival during drought. The timing of the onset of stomatal closure is crucial for preventing excessive water loss during drought, but is poorly explained by plant hydraulics alone and what triggers stomatal closure remains disputed. We investigated whether the hormone abscisic acid (ABA) was this trigger in a highly embolism‐resistant tree speciesUmbellularia californica. We tracked leaf ABA levels, determined the leaf water potential and gravimetric soil water content (gSWC) thresholds for stomatal closure and transpiration decline during a progressive drought. We found thatU. californicaplants have a peaking‐type ABA dynamic, where ABA levels rise early in drought and then decline under prolonged drought conditions. The early increase in ABA levels correlated with the closing of stomata and reduced transpiration. Furthermore, we found that transpiration declined before any large decreases in predawn plant water status and could best be explained by transient drops in midday water potentials triggering increased ABA levels. Our results indicate that ABA‐mediated stomatal regulation may be an integral mechanism for reducing transpiration during drought before major drops in bulk soil and plant water status. 

Summary The onset of stomatal closure reduces transpiration during drought. In seed plants, drought causes declines in plant water status which increases leaf endogenous abscisic acid (ABA) levels required for stomatal closure. There are multiple possible points of increased belowground resistance in the soil–plant atmospheric continuum that could decrease leaf water potential enough to trigger ABA production and the subsequent decreases in transpiration.We investigate the dynamic patterns of leaf ABA levels, plant hydraulic conductance and the point of failure in the soil–plant conductance in the highly embolism‐resistant speciesCallitris tuberculatausing continuous dendrometer measurements of leaf water potential during drought.We show that decreases in transpiration and ABA biosynthesis begin before any permanent decreases in predawn water potential, collapse in soil–plant hydraulic pathway and xylem embolism spread.We find that a dynamic but recoverable increases in hydraulic resistance in the soil in close proximity to the roots is the most likely driver of declines in midday leaf water potential needed for ABA biosynthesis and the onset of decreases in transpiration. 

Abstract The phytohormone abscisic acid (ABA) is synthesised by plants during drought to close stomata and regulate desiccation tolerance pathways. Conifers and some angiosperms with embolism‐resistant xylem show a peaking‐type (p‐type) response in ABA levels, in which ABA levels increase early in drought then decrease as drought progresses, declining to pre‐stressed levels. The mechanism behind this dynamic remains unknown. Here, we sought to characterise the mechanism driving p‐type ABA dynamics in the coniferCallitris rhomboideaand the highly drought‐resistant angiospermUmbellularia californica. We measured leaf water potentials (Ψ_l), stomatal conductance, ABA, conjugates and phaseic acid (PA) levels in potted plants during a prolonged but non‐fatal drought. Both species displayed a p‐type ABA dynamic during prolonged drought. In branches collected before and after the peak in endogenous ABA levels in planta, that were rehydrated overnight and then bench dried, ABA biosynthesis was deactivated beyond leaf turgor loss point. Considerable conversion of ABA to conjugates was found to occur during drought, but not catabolism to PA. The mechanism driving the decline in ABA levels in p‐type species may be conserved across embolism‐resistant seed plants and is mediated by sustained conjugation of ABA and the deactivation of ABA accumulation asΨ_lbecomes more negative than turgor loss.