Rapid leaf xylem acclimation diminishes the chances of embolism in grapevines.

Under most conditions tight stomatal regulation in grapevines (Vitis vinifera) avoids xylem embolism. The current study evaluated grapevine responses to challenging scenarios that might lead to leaf embolism and consequential leaf damage. We hypothesized that embolism would occur if the vines experienced low xylem water potential (Ψx) shortly after bud break or later in the season under a combination of extreme drought and heat. We subjected vines to two potentially dangerous environments: (i) withholding irrigation from a vineyard grown in a heatwave-prone environment, and (ii) subjecting potted vines to terminal drought 1 month after bud break. In the field experiment, a heatwave at the beginning of August resulted in leaf temperatures over 45 °C. However, effective stomatal response maintained the xylem water potential (Ψx) well above the embolism threshold, and no leaf desiccation was observed. In the pot experiment, leaves of well-watered vines in May were relatively vulnerable to embolism with 50% embolism (P50) at -1.8 MPa. However, when exposed to drought, these leaves acclimated their leaf P50 by 0.65 MPa in less than a week and before reaching embolism values. When dried to embolizing Ψx, the leaf damage proportion matched (percentage-wise) the leaf embolism level. Our findings indicate that embolism and leaf damage are usually avoided by the grapevines' efficient stomatal regulation and rapid acclimation of their xylem vulnerability.

Seasonal adjustment of leaf embolism resistance and its importance for hydraulic safety in deciduous trees.

Embolism resistance is often viewed as seasonally stable. Here we examined the seasonality in the leaf xylem vulnerability curve (VC) and turgor loss point (ΨTLP ) of nine deciduous species that originated from Mediterranean, temperate, tropical, or sub-tropical habitats and were growing on the Volcani campus, Israel. All four Mediterranean/temperate species exhibited a shift of their VC to lower xylem pressures (Ψx ) along the dry season, in addition to two of the five tropical/sub-tropical species. In three of the species that exhibited VC seasonality, it was critical for avoiding embolism in the leaf. In total, seven out of the nine species avoided embolism. The seasonal VC adjustment was over two times higher as compared with the seasonal adjustment of ΨTLP , resulting in improved hydraulic safety as the season progressed. The results suggest that seasonality in the leaf xylem vulnerability is common in species that originate from Mediterranean or temperate habitats that have large seasonal environmental changes. This seasonality is advantageous because it enables a gradual seasonal reduction in the Ψx without increasing the danger of embolism. The results also highlight that measuring the minimal Ψx and the VC at different times can lead to erroneous estimations of the hydraulic safety margins. Changing the current hydraulic dogma into a seasonal dynamic in the vulnerability of the xylem itself should enable physiologists to understand plants' responses to their environment better.

Productivity versus drought adaptation in olive leaves: Comparison of water relations in a modern versus a traditional cultivar.

The physiological traits that set the tradeoff between productivity and drought adaptation in plants are still under debate. To reveal these traits, we compared the water relations of two olive (Olea europaea) cultivars: "Barnea"-a highly productive modern cultivar; and "Souri"-a drought-adapted traditional cultivar. We hypothesized that Souri has lower hydraulic conductivity and lower hydraulic vulnerability. The hypothesis was tested at the leaf level. The soil volumetric water content (θ), stem water potential (ΨS ), and gas exchange were measured in both cultivars while they dried until a significant reduction in their maximal photochemical potential (Fv /Fm  < 0.6) was obtained. Additionally, pressure-volume relations, leaf hydraulic vulnerability, and the petiole xylem architecture were evaluated. To our surprise, Souri's leaf hydraulic conductivity was more vulnerable to low ΨS , approaching zero at -8 MPa compared with <-10 MPa in "Barnea." At the same time, Souri's higher osmotic content and cell rigidity enabled it to sustain 1.4 MPa lower ΨS , while maintaining near optimal (Fv /Fm ). However, both cultivars significantly reduced their Fv /Fm (<0.6) at the same θ, suggesting that the capability to sustain a low θ is not the issue. Instead, Souri's lower transpiration enabled it to withstand a longer drought while avoiding low θ. Barnea's larger xylem vessels and hydraulic conductivity supported higher stomatal conductance (gs ) and assimilation rate, which nurtured its higher productivity but resulted in quick depletion of θ. These results suggest that hydraulic resistance or the ability to sustain low θ do not set the tradeoff between productivity and drought adaptation in olive leaves.

An increase in xylem embolism resistance of grapevine leaves during the growing season is coordinated with stomatal regulation, turgor loss point and intervessel pit membranes.

Although xylem embolism resistance is traditionally considered as static, we hypothesized that in grapevine (Vitis vinifera) leaf xylem becomes more embolism-resistant over the growing season. We evaluated xylem architecture, turgor loss point (ΨTLP ) and water potentials leading to 25% of maximal stomatal conductance (gs25 ) or 50% embolism in the leaf xylem (P50 ) in three irrigation treatments and at three time points during the growing season, while separating the effects of leaf age and time of season. Hydraulic traits acclimated over the growing season in a coordinated manner. Without irrigation, ΨTLP , gs25 , and P50 decreased between late May and late August by 0.95, 0.77 and 0.71 MPa, respectively. A seasonal shift in P50 occurred even in mature leaves, while irrigation had only a mild effect (< 0.2 MPa) on P50 . Vessel size and pit membrane thickness were also seasonally dynamic, providing a plausible explanation for the shift in P50 . Our findings provide clear evidence that grapevines can modify their hydraulic traits along a growing season to allow lower xylem water potential, without compromising gas exchange, leaf turgor or xylem integrity. Seasonal changes should be considered when modeling ecosystem vulnerability to drought or comparing datasets acquired at different phenological stages.

Shoot stem cell specification in roots by the WUSCHEL transcription factor.

The WUSCHEL homeobox transcription factor is required to specify stem-cell identity at the shoot apical meristem and its ectopic expression is sufficient to induce de novo shoot meristem formation. Yet, the manner by which WUS promotes stem-cell fate is not yet fully understood. In the present research we address this question by inducing WUS function outside of its domain. We show that activation of WUS function in the root inhibits the responses to exogenous auxin and suppresses the initiation and growth of lateral roots. Using time lapse movies to follow the cell-cycle marker CYCB1;1::GFP, we also show that activation of WUS function suppresses cell division and cell elongation. In addition, activation of WUS represses the auxin-induced expression of the PLETHORA1 root identity gene and promotes shoot fate. Shoot apical meristem formation requires a high cytokinin-to-auxin ratio. Our findings provide evidence for the manner by which WUS specifies stem-cell identity: by affecting auxin responses, by reducing the cell mitotic activity and by repressing other developmental pathways. At the meristem, the stem-cells which are characterized by low division rate are surrounded by the highly proliferative meristematic cells. Our results also provide a model for WUS establishing the differential mitotic rates between two cell populations at the minute structure of the meristem.