Relationships among xylem transport, biomechanics and storage in stems and roots of nine Rhamnaceae species of the California chaparral.

Here, hypotheses about stem and root xylem structure and function were assessed by analyzing xylem in nine chaparral Rhamnaceae species. Traits characterizing xylem transport efficiency and safety, mechanical strength and storage were analyzed using linear regression, principal components analysis and phylogenetic independent contrasts (PICs). Stems showed a strong, positive correlation between xylem mechanical strength (xylem density and modulus of rupture) and xylem transport safety (resistance to cavitation and estimated vessel implosion resistance), and this was supported by PICs. Like stems, greater root cavitation resistance was correlated with greater vessel implosion resistance; however, unlike stems, root cavitation resistance was not correlated with xylem density and modulus of rupture. Also different from stems, roots displayed a trade-off between xylem transport safety from cavitation and xylem transport efficiency. Both stems and roots showed a trade-off between xylem transport safety and xylem storage of water and nutrients, respectively. Stems and roots differ in xylem structural and functional relationships, associated with differences in their local environment (air vs soil) and their primary functions.

Mechanisms for tolerating freeze-thaw stress of two evergreen chaparral species: Rhus ovata and Malosma laurina (Anacardiaceae).

The response to freeze-thaw stress was examined for two co-occurring evergreen species, Malosma laurina and Rhus ovata. Laboratory and field experiments on adults and seedlings were made in the spring and winter in 1996 and again on adults in 2003 and 2004. Laboratory and field results indicated that the stem xylem for adults of M. laurina and R. ovata were similarly susceptible to freezing-induced cavitation (percentage loss of conductivity = 92 ± 2.6% for R. ovata and 90 ± 4.2% for M. laurina at ≤ -6°C). In contrast, leaves of M. laurina were more susceptible to freezing injury than leaves of R. ovata. Among seedlings in the field, leaves of M. laurina exhibited freezing injury at -4°C and total shoot mortality at -7.2°C, whereas co-occurring seedlings of R. ovata were uninjured. Surprisingly, R. ovata tolerates high levels of freezing-induced xylem embolism in the field, an apparently rare condition among evergreen plants. Rhus ovata avoids desiccation when xylem embolism is high by exhibiting low minimum leaf conductance compared to M. laurina. These results suggest a link between minimum leaf conductance and stem hydraulics as a mechanism permitting the persistence of an evergreen leaf habit in freezing environments.

Stem diameter variations and cold hardiness in walnut trees.

The effect of freezing temperatures on stem diameter was measured in the field and in climatic chambers using linear variable differential transformers (LVDT sensors). In acclimated stems, there was reversible stem shrinkage associated with freeze-thaw cycles. The maximum shrinkage correlated with stem diameter (thickness of the bark). The wood was responsible for only 15% of the shrinkage associated with a freeze event, and experiments with isolated bark showed that connection with the wood was not necessary for most of the freeze-induced shrinkage to occur. Considering the amount of stem shrinkage associated with summer drought in walnut, the amount of contraction of the bark with freezing was actually much less than might be predicted by water relations theory. Reversible stem shrinkage occurred in living tissues, but not in autoclaved tissues. For the latter, swelling was observed with freezing and this swelling could be explained by the bark alone. Similar swelling was observed during September and October for non-acclimated plants. Water was lost with each freeze-thaw cycle starting with the first, and freezing injury of the bark, with discoloration of tissues, was also observed in non-acclimated plants. Given that the diameter fluctuation patterns were dramatically different for acclimated versus non-acclimated plants, and for living versus autoclaved tissues, LVDT sensors could represent a novel, non-invasive approach to testing cold hardiness.

Seasonal variation in xylem pressure of walnut trees: root and stem pressures.

Measurements of air and soil temperatures and xylem pressure were made on 17-year-old orchard trees and on 5-year-old potted trees of walnut (Juglans regia L.). Cooling chambers were used to determine the relationships between temperature and sugar concentration ([glucose] + [fructose] + [sucrose], GFS) and seasonal changes in xylem pressure development. Pressure transducers were attached to twigs of intact plants, root stumps and excised shoots while the potted trees were subjected to various temperature regimes in autumn, winter and spring. Osmolarity and GFS of the xylem sap (apoplast) were measured before and after cooling or warming treatments. In autumn and spring, xylem pressures of up to 160 kPa were closely correlated with soil temperature but were not correlated with GFS in xylem sap. High root pressures were associated with uptake of mineral nutrients from soil, especially nitrate. In autumn and spring, xylem pressures were detected in root stumps as well as in intact plants, but not in excised stems. In contrast, in winter, 83% of the xylem sap osmolarity in both excised stems and intact plants could be accounted for by GFS, and both GFS and osmolarity were inversely proportional to temperature. Plants kept at 1.5 degrees C developed positive xylem pressures up to 35 kPa, xylem sap osmolarities up to 260 mosmol l(-1) and GFS concentrations up to 70 g l(-1). Autumn and spring xylem pressures, which appeared to be of root origin, were about 55% of the theoretical pressures predicted by osmolarity of the xylem sap. In contrast, winter pressures appeared to be of stem origin and were only 7% of the theoretical pressures, perhaps because of a lower stem water content during winter.

Winter stem xylem pressure in walnut trees: effects of carbohydrates, cooling and freezing.

Pressure transducers were attached to twigs of orchard trees and potted trees of walnut (Juglans regia L.) to measure winter stem xylem pressures. Experimental potted trees were partially defoliated in the late summer and early autumn to lower the amount of stored carbohydrates. Potted trees were placed in cooling chambers and subjected to various temperature regimes, including freeze-thaw cycles. Xylem pressures were inversely proportional to the previous 48-h air temperature, but positively correlated with the osmolarity of the xylem sap. Defoliated trees had significantly lower concentrations of stored carbohydrates and significantly lower xylem sap osmolarities than controls. Plants kept at 1.5 degrees C developed xylem pressures up to 40 kPa, just 7% of the theoretical osmotic pressure of the xylem sap. However, exposure to low, nonfreezing temperatures followed by freeze-thaw cycles resulted in pressures over 210 kPa, which was 39% of the theoretical osmotic pressure. A simple osmotic model could account for the modest positive winter pressures at low, nonfreezing temperatures, but not for the synergistic effects of freeze-thaw cycles.

Seasonal conductivity and embolism in the roots and stems of two clonal ring-porous trees, Sassafras albidum (Lauraceae) and Rhus typhina (Anacardiaceae).

Seasonal xylem (wood) conductivity and embolism (air blockage) patterns were monitored in roots vs. stems of two clonal ring-porous tree species, Sassafras albidum and Rhus typhina, throughout 1996 and 1997. Stems of both species were 100% embolized in the early spring and became conductive by late June following leaf expansion and maturation of new earlywood vessels. Dyes indicated the stem conduction was restricted almost exclusively to the current year's growth ring. Stems became totally embolized again by early October, before the first freezing temperatures. In contrast, woody roots of both species maintained low embolism values, many conductive growth rings, and high conductivity values regardless of the season. No positive root pressures were detected in either species. The mean frost depth (204 ± 11 mm) was deeper than all sampled roots of Rhus and 47% of sampled roots of Sassafras. The roots that had been in frozen soil either avoided embolism altogether or they were able to reverse embolism by a mechanism other than positive root pressures.

Root pressure and specific conductivity in temperate lianas: exotic Celastrus orbiculatus (Celastraceae) vs. native Vitis riparia (Vitaceae).

The exotic temperate liana (woody vine) Celastrus orbiculatus has become a weed in Michigan, occurring in many of the same habitats as the native liana Vitis riparia. However, C. orbiculatus frequently develops into extensive monospecific infestations, while V. riparia does not. Freezing-induced embolism may be responsible for limiting liana distribution. Root pressure has been observed in numerous tropical lianas and temperate species of Vitis and has been implicated as vital to the recovery of xylem function in wide vessels following winter freezes. For both of these co-occurring lianas we investigated root pressure and water conductance as possible explanatory factors for their differential spread. According to our hypothesis, C. orbiculatus should have produced greater or more frequent root pressures than V. riparia. However, the reverse proved true, indicating that root pressure is not a prerequisite for weedy proliferation of C. orbiculatus. Additionally, the seasonal patterns of specific conductivity of stem xylem indicate that each species responds differently to environmental constraints. Vitis riparia establishes conductivity early in the growing season, before the leaves emerge, using root pressure to reverse embolism, but loses conductivity with the first freeze in early autumn. Celastrus orbiculatus is slow to establish conductivity, depending on new wood production, but leafs out sooner than V. riparia and maintains green leaves after the first freeze. Vulnerability curves of xylem to cavitation caused by water stress for the two species indicate that they respond similarly to dehydration. These results indicate that root pressures are not responsible for the invasive success of C. orbiculatus and suggest that other factors must be key to its prolific invasion.

Degradation of the upper pulvinus in modern and fossil leaves of Cercis (Fabaceae).

Identification of fossil leaf impressions as Cercis has been questioned based upon the presence or absence of a pulvinus at the base of the lamina (upper pulvinus). In the present study, leaves of Cercis canadensis were examined before and after abscission to explore the degradation processes that could occur prior to fossilization, and the North American record for fossil foliage of Cercis was revised accordingly. Results for C. canadensis indicate that: (1) the pulvinus consists largely of tissues with nonlignified cells (a wide cortex, a nonlignified fiber sheath, phloem, and pith) that degrade rapidly after leaf abscission, (2) the lignified xylem tissue that remains in the pulvinus after degradation is in brittle strands, (3) the pulvinus degrades at a faster rate than the lamina or the petiole, and (4) the degraded pulvinus cushion leaves a semicircular pattern on the lamina. From examination of fossils as well as extant species, we: (1) demonstrated that in fossils, the upper pulvinus can show a greater degree of degradation than the adjoining petiole or lamina tissue, suggesting the degradation of upper pulvinus tissue is similar in modern vs. fossil specimens, (2) defined numerous other laminar characters that can be used in conjunction with, or in the absence of, an upper pulvinus to confirm the presence of Cercis in the fossil record, and (3) showed from those criteria that the earliest known North American fossil leaf record for Cercis, from a specimen newly reported in the present study, is from the middle Miocene Succor Creek flora of Oregon.

Response of chaparral shrubs to below-freezing temperatures: acclimation, ecotypes, seedlings vs. adults.

Leaf death due to freezing was examined for four, co-occurring species of chaparral shrubs from the Santa Monica Mountains of southern California, Rhus laurina (= Malosma laurina), R. ovata, Ceanothus megacarpus, and C. spinosus. Measurements were made on seedlings vs. adults for all species, and for Rhus spp. in winter vs. summer, and at a warm vs. a cold site. We used four methods to determine the temperature for 50% change in activity or cell death (LT(50)) of leaves: (1) electrical conductivity (electrolyte leakage into a bathing solution), (2) photosynthetic fluorescent capacity (Fv/Fm), (3) percentage of palisade mesophyll cells stained by fluorescein diacetate vital stain, and (4) visual score of leaf color (Munsell color chart). In all four species seedlings were found to be more sensitive to freezing temperatures than were adults by 1°-3°C. For adults the LT(50) ranged from -5°C for Rhus laurina in the summer to -16°C for Rhus ovata in the winter. The LT(50) of R. ovata located at a colder inland site was 4C lower than R. ovata at the warmer coastal site just 4 km apart, suggesting ecotypic differences between R. ovata at the two sites. Both R. laurina and R. ovata underwent significant winter hardening. At the cold site, R. ovata acclimated by 6°C on average, while R. laurina acclimated by only 3°C. These results were consistent with species distributions and with field observations of differential shoot dieback between these two congeneric species after a natural freeze-thaw event in the Santa Monica Mountains.

The mode of origin of root buds and root sprouts in the clonal tree Sassafras albidum (Lauraceae).

The developmental anatomy of root buds and root sprouts was examined in the clonal tree Sassafras albidum. Root samples from 13 clones that varied widely in age and vigor were sectioned and two types of buds were found, "additional" buds and "reparative" buds. Additional buds form during the early growth of uninjured roots and they perennate by growing outwards in concert with the vascular cambium such that bud traces are produced in the secondary xylem. Reparative buds form de novo in response to senescence, injuries, or other types of disturbance. Reparative buds were found on the roots of seven of the clones, whereas additional buds were found on the roots of all 13 clones. The reparative buds had originated in the proliferated pericycle, where they were subtended by sphaeroblasts, or spherical nodules of wood. Few of the reparative buds were vascularized and none were connected with the vasculature of their parent roots. In contrast, most of the additional buds were vascularized, and the leaf traces of several of the additional buds appeared to be contiguous with the conducting xylem of their parent roots. To determine whether both bud types were functional, 82 field-collected root sprouts and 44 incubation-induced sprouts were sectioned at the root-sprout junction and examined for evidence relating to their mode of origin. None of the sprouts were subtended by sphaeroblasts, but 98% were subtended by bud traces, which indicated that they had originated from additional buds. Although reparative buds were more common than additional buds on some of the root samples, they appear to be dysfunctional at sprouting. Additional buds, on the other hand, are able to sprout both as a normal part of clonal spread and from root cuttings.

Mechanisms of deposition of epicuticular wax in leaves of broccoli, Brassica oleracea var. capitata L.

Broceeli (Brassika oleraceo L. var. capitata L.) leaf waxes were labelled with [2-14 Cjacetate and examined during synthesis and deposition using microautoradiography. Waxes were viewed with light microscopy and scanning electron microscopy (SEM), including cryo-SEM. In microautoradiography, silver grains resulting from radioactive decay of labelled waxes seemed to be distributed randomly throughout the cuticle. This random distribution supports the hypothesis of diffusion or complex anastomosing pores as the mechanism of epicuriculur wax position. The hypothesis of wax-depositing pores running perpendicular to the leaf surface is not supported.

Water Transport in the Liana Bauhinia fassoglensis (Fabaceae).

To determine the efficiency of xylem conductance in the liana (woody vine) Bauhinia fassoglensis Kotschy ex Schweinf., we measured hydraulic conductance per unit stem length (measured K(h)), leaf-specific conductivity (LSC = K(h)/distal leaf area), transpiration rate (E), xylem water potential (epsilon), vessel number, and vessel diameter. The measured K(h) was 49% (se = 7%) of the predicted K(h) from Poiseuille's law. The mean LSC for unbranched stem segments was 1.10 x 10(-8) square meters per megapascal per second (se = 0.07). LSCs were much lower (about 0.2) at branch junctions. At midday, with E at 7 x 10(-8) meters per second, the measured drop in epsilon was about 0.08 megapascal per meter along the stems and branches and about 0.27 megapascal in going from stem to leaf. In addition, there was a drop of about 0.20 megapascal at branch junctions as predicted by E/LSC. In diurnal measurements leaf epsilon never dropped below about -1.2 megapascal. For long (e.g. 16 meters) stems, the predicted mid-day drop in epsilon through the xylem transport system might be great enough to have substantial physiological impact.