Shoot Water Potential Research Articles

Evolution of drought tolerance and defense: dependence of tradeoffs on mechanism, environment and defense switching

Plants evolve defenses against herbivores and pathogens in stressful environments; however, plants that evolve tolerances to other environmental stressors may have compromised defenses. Such tradeoffs involving defenses may depend on limited resources or otherwise stressful environments; however, the effect of stressful environments on defense expression might be different for different genotypes (G×E). To test these predictions, we studied genetic variation and co‐variation of drought stress tolerance and defenses at two levels of genetic variation: between and within closely related species. We did this across an experimental drought stress gradient in a growth room for species for which genetic variation in drought tolerance was likely. In apparent contrast to predictions, the species Boechera holboellii (Brassicaceae) from lower and dryer elevations had slower inherent growth rates and correspondingly higher total defensive glucosinolate concentrations than the closely related species B. stricta from higher elevations. Thus, B. holboellii was both drought tolerant and defended; however, optimality theory does predict tradeoffs between defense and growth. Differences between species in the direct effect of water deficiency on glucosinolate production did not obscure the grow‐or‐defend tradeoff. B. holboellii may also have been more resistant to the specialist herbivore Plutella xylostella; a trend that was less clear because it depended on plant development and water deficient conditions. At finer scales of genetic variation, there was significant variation among families and naturally occurring inbred lines of B. stricta in drought tolerance measured as inherent growth, the reaction norm of growth across drought treatments, shoot water potential, and transpiration rates. Evidence for tradeoffs was also found within B. stricta in genetic correlations between resistance and transpiration rates, or glucosinolates and growth rates. No G×E was detected at these finer scales of genetic variation, although sometimes the tradeoff was dependent on drought conditions. Direct effects of drought stress resulted in an apparent plastic switch between resistance and tolerance to damage, which might be a cost avoidance mechanism because tradeoffs never involved tolerance to damage. Thus, when drought tolerance is manifest as slow inherent growth rates, plants may also have relatively high defense levels, especially in stressful environments. Otherwise, defenses may be compromised by drought‐coping mechanisms, although plastic switches to less costly defenses may alleviate constraints in stressful environments.

New approach for using trunk growth rate and endocarp development in the irrigation scheduling of young olive orchards

Trunk diameter variation (TDV) is considered one of the most promising tools for automating the scheduling of fruit tree irrigation, and trunk growth rate (TGR) a possible indicator of TDV values. The use of TGR in commercial orchards is less common in olive trees than in other species, possibly because the influence of the environment, orchard age and the presence of developing fruit on the olive tree seasonal TGR pattern is poorly understood. In this study the trunk growth rate (TGR) seasonal progression was characterized in fully irrigated young olive trees during fruiting and non-fruiting years. In both years, at the beginning of the season, a period with an almost linear increase was found, mainly determined by temperature. In the non-fruiting year TGR remained almost constant after that initial increase, while during the fruiting year a significant decrease occurred at approximately 25 days, from values around 0.2 mm day −1 to values around 0.1 mm day −1. Since midday shoot water potentials were not affected, this variation was likely not produced by water stress conditions. In addition, the lack of relationship between trunk growth rate and air temperature indicated that the TGR decrease in the fruiting year was not determined either by air temperature. The period of decreasing TGR values, however, coincided with the time that the endocarp reached its maximum transverse area, a significant moment for fruit development which precedes pit hardening. These results suggest that the traditional period where regulated deficit irrigation is done, “the pit hardening”, may be indicated easier and more accurate for TGR decrease and endocarp expansion.

Ecophysiology of tea

Tea [Camellia sinensis (L.) O. Kuntze] is one of the most important beverage crops in the world. The major tea-growing regions of the world are South-East Asia and Eastern Africa where it is grown across a wide range of altitudes up to 2200 m a.s.l.. This paper reviews the key physiological processes responsible for yield determination of tea and discusses how these processes are influenced by genotypic and environmental factors. Yield formation of tea is discussed in terms of assimilate supply through photosynthesis and formation of harvestable sinks (i.e. shoots). The photosynthetic apparatus and partial processes (i.e. light capture, electron transport and carboxylation) of tea show specific adaptations to shade. Consequently, maximum light-saturated photosynthetic rates of tea are below the average for C3 plants and photoinhibition occurs at high light intensities. These processes restrict the source capacity of tea. Tea yields are sink-limited as well because shoots are harvested before their maximum biomass is reached in order to maintain quality characters of made tea. In the absence of water deficits, rates of shoot initiation and extension are determined by air temperature and saturation vapour pressure deficit, with the former having positive and the latter having negative relationships with the above rates. During dry periods, when the soil water deficit exceeds a genotypically- and environmentally-determined threshold, rates of shoot initiation and extension are reduced with decreasing shoot water potential. Tea yields respond significantly to irrigation, a promising option to increase productivity during dry periods, which are experienced in many tea-growing regions.

Elk browsing increases aboveground growth of water-stressed willows by modifying plant architecture

In the northern elk wintering range of Yellowstone National Park, USA, wolf (Canis lupus) removal allowed elk (Cervus elaphus) to overbrowse riparian woody plants, leading to the exclusion of beaver (Castor canadensis) and a subsequent water table decline in many small stream valleys. Reduced elk browsing following wolf reintroduction may or may not facilitate willow (Salix sp.) recovery in these areas. To determine if the effect of elk browsing on willow interacts with that of beaver abandonment, we manipulated elk browsing and the water table in a factorial experiment. Under the condition of an ambient (low) water table, elk browsing increased shoot water potential (Psis), photosynthesis per unit leaf area (A), stomatal conductance per unit leaf area (gs), and aboveground current annual growth (CAG) by 50%. Elk browsing occurred entirely during dormancy and did not affect total plant leaf area (L). Improved water balance, photosynthetic rate, and annual aboveground productivity in browsed willows appeared to be due to morphological changes, such as increased shoot diameter and decreased branching, which typically increase plant hydraulic conductivity. An elevated water table increased Psis, A, gs, CAG, and L, and eliminated or lessened the positive effect of browsing on CAG for most species. Because low water tables create conditions whereby high willow productivity depends on the morphological effects of annual elk browsing, removing elk browsing in areas of water table decline is unlikely to result in vigorous willow stands. As large willow standing crops are required by beaver, a positive feedback between water-stressed willow and beaver absence may preclude the reestablishment of historical conditions. In areas with low water table, willow restoration may depend on actions to promote the re-establishment of beaver in addition to reducing elk browsing.

Early drought-induced changes to the needle proteome of Norway spruce

To elucidate early drought responses in needles of Norway spruce (Picea abies (Karst.) L.), we subjected 1-year-old seedlings to gradual desiccation for 6 weeks. Four weeks of drought treatment caused a small but significant decrease in photosystem II quantum yield of light-adapted needles (phi(a)) compared with that of well-watered controls. Six weeks of drought treatment reduced phi(a) and the photosystem II quantum yield of dark-adapted needles (phi) by 50 and 8%, respectively, and reduced shoot water potential by 0.7 MPa, but had no measurable effect on needle relative water content. After two weeks of drought treatment, and before there was a discernible effect of drought on phi or a statistically significant effect on shoot water potential, needles were analyzed for changes in protein composition. Five out of several hundred detected proteins in needles of drought-treated plants showed consistent changes compared with control leaves. The proteins were identified by LC-MS/MS as components of the oxygen-evolving complex (oxygen evolving enhancer protein 2), ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit, and one protein of unknown function, whose mRNA was found in a previous screen of wound- and methyl-jasmonate-induced bark proteins.

Measuring water stress in Eucalyptus grandis Hill ex Maiden seedlings planted into pots

A pot trial incorporating various watering regimes was initiated to assess: 1) water stress in Eucalyptus grandis seedlings, and 2) the efficacy of different types of research equipment in quantifying these levels of water stress for applied research. There were two dry soil treatments differing in terms of seedling root plug moisture at transplanting, dry (DD) and wet (WD), respectively, and three treatments consisting of well watered seedlings transplanted into wet soil (WWD, WWW and control). Treatment WWW was re-watered when seedlings were water stressed. The control was maintained at field capacity for the entire trial period. Seedling physiology was assessed by shoot water potential, stomatal conductance and chlorophyll fluorescence. Seedlings with dry root plugs, planted into dry soil were dead one to two days after planting. A wet root plug at the time of transplanting increased seedling shoot water potential and survival for up to three days in dry soil. Planting into wet soil increased shoot water potential for the duration of the trial and was also associated with new root growth. This study indicated that both the pressure chamber and the porometer provided simple and easy to interpret measures of water stress in E. grandis seedlings. Measurements of chlorophyll fluorescence did not significantly reflect treatment effects.

The north-eastern distribution range of European beech a review

Today, European beech (Fagus sylvatica L.) seems to be a markedly successful tree species in the north-east of its distribution range. The distribution area may be larger than originally assumed; past forest management is probably the main cause of the contraction in the postglacial European beech range. Numerous attempts consistently have failed to locate a distinct distribution edge for European beech. Therefore, we define northern and eastern Poland and the southern regions of the Baltic States as margins of European beech distribution. Ecophysiological approaches have identified the drought constraints for European beech in terms of (1) the critical limit for xylem cavitation and loss of hydraulic conductivity, reached at a shoot water potential of –1.9 MPa, and (2) a reduction in gross primary production and total ecosystem respiration when relative extractable soil water reaches 40 and 20 per cent, respectively. However, it is difficult to correlate European beech distribution margins with single macro-climatic factors. Moreover, the adaptation of European beech populations and provenances to drought and frost varies. The phenotypic plasticity and evolutionary adaptability of European beech appear to be underestimated. These characteristics may counteract a further contraction of the European beech range arising from climate change in the future.

Leaf morphological and physiological adjustments to the spectrally selective shade imposed by anthocyanins in Prunus cerasifera

Prunus domestica L. has green leaves, whereas Prunus cerasifera Ehrh. var. atropurpurea has red leaves due to the presence of mesophyll anthocyanins. We compared morphological and photosynthetic characteristics of leaves of these species, which were sampled from shoots grafted in pairs on P. domestica rootstocks, each pair comprising one shoot of each species. Two hypotheses were tested: (1) anthocyanins protect red leaves against photoinhibition; and (2) red leaves display shade characteristics because of light attenuation by anthocyanins. Parameters were measured seasonally, during a period of increasing water stress, which caused a similar drop in shoot water potential in each species. As judged by predawn measurements of maximum PSII yield, chronic photoinhibition did not develop in either species and, despite the anthocyanic screen, the red leaves of P. cerasifera displayed lower light-adapted PSII yields and higher non-photochemical quenching than the green leaves of P. domestica. Thus, it appears that, in this system, anthocyanins afford little photoprotection. As predicted by the shade acclimation hypothesis, red leaves were thinner and had a lower stomatal frequency, area- based CO2 assimilation rate, apparent carboxylation efficiency and chlorophyll a:b ratio than green leaves. However, red leaves were similar to green leaves in conductivity to water vapor diffusion, dry-mass-based chlorophyll concentrations and carotenoid:chlorophyll ratios. The data for red leaves indicate adaptations to a green-depleted, red-enriched shade, rather than a neutral or canopy-like shade. Thus, green light attenuation by anthocyanins may impose a limitation on leaf thickness. Moreover, the selective depletion of light at wavelengths that are preferentially absorbed by PSII and chlorophyll b may lead to adjustments in chlorophyll pigment ratios to compensate for the uneven spectral distribution of internal light. The apparent photosynthetic cost associated with lost photons and reduced leaf thickness, and the absence of a photoprotective advantage, suggest that there are other, yet to be identified, benefits for permanently anthocyanic leaves of P. cerasifera.

The effect of irrigation schedules on the water relations and growth of a young olive ( Olea europaea L.) orchard

In recent years there has been a notable worldwide increase in the amount of land devoted to olive orchards. Most of these new orchards are irrigated and represent large financial investments. The irrigation of young olive trees should reduce the period during which their production is small or non-existent. Although the water requirements of young olive orchards are thought to be low, little is in fact known in this regard. In the present work, three irrigation treatments (100, 75 and 50% coverage of water needs) were designed using the Orgaz method, and their effects on young olive trees tested in different plots over a period of 3 years. The 50% deficit treatment was designed to provide the trees with an amount of water in the region of that stipulated by the FAO method, the most commonly used irrigation scheduling system for olive orchards. No significant differences in shoot water potential nor abaxial leaf conductance were seen between the trees receiving the different treatments. However, canopy volume and shoot growth were affected. These results indicate that the traditional FAO model, which would have supplied about 35% of the water supplied by the Control treatment, may well reduce the economic benefits to be derived from young olive orchards.

Growth and physiological response of eastern white pine seedlings to partial cutting and site preparation

We examined how white pine ( Pinus strobus L.) seedlings planted under a mature cover of white and red ( Pinus resinosa Ait.) pine in eastern Ontario (Canada) responded to treatments aimed at improving light and soil conditions for seedling growth. The treatments were: (a) three levels of partial cutting (no cut or CS0, cut to one-crown spacing between residual trees or CS1, cut to two-crown spacing or CS2); (b) two levels of vegetation control (without herbicide or H0, with herbicide or H1); and (c) two levels of soil scarification (S0 and S1). On the third growing season after planting, total growth of seedlings was lowest in CS0 treatment and similar in CS1 and CS2 treatments. The CS2 created better growing light conditions than the CS1, with and average of 50% of full light at seedling height, which corresponded to the maximum height and diameter growth rates of seedlings. However, CS2 also stimulated the growth of competing woody vegetation (both understory trees and shrubs), and resulted in greater microsite heterogeneity of light availability. Scarification warmed the soil (approximately 1–3 °C in the middle of the growing season), decreased the density of competing trees, but increased the shrub density, with no impact on white pine seedling growth. The treatments had no effect on light-saturated photosynthetic rate ( A) of current-year foliage of seedlings, nor on their midday shoot water potential. Leaf N was higher in partial cuts and with vegetation control, but the relationship between N and A was weak to non-existent for the different foliage classes. Measures of the proportion of aboveground biomass allocated to foliage (leaf-mass ratio) suggest an acclimation response of young white pine that improves growth under moderate light availability and compensates for the lack of leaf-level photosynthetic plasticity. We suggest a combination of soil scarification under a one-crown spacing partial cut (corresponding to 14 m 2 ha −1 of residual basal area, or an average of 32% of available light at seedling height) as an establishment cut. This should provide optimum growth conditions for planted understory white pine, while also favoring natural regeneration and providing some protection against damage from insects and disease.

Mercury hinders recovery of shoot hydraulic conductivity during grapevine rehydration: evidence from a whole‐plant approach

This experiment aimed to test whether recovery of shoot hydraulic conductivity after drought depends on cellular metabolism in addition to xylem hydraulics. We rehydrated droughted grapevines (Vitis vinifera) after treating intact plants through the root with 0.5 mm mercuric chloride (a metabolic inhibitor) at the end of the stress period, before rehydration. The contribution of mercury-inhibited water transport in both shoot and root, and the extent of shoot vessel embolization, were assessed. Drought stress decreased plant water potential and induced embolization of the shoot vessels. The rehydration in Hg-untreated plants re-established both shoot water potential and specific shoot hydraulic conductivity (Kss) at levels comparable with watered controls, and induced recovery of most of the embolisms formed in the shoot during the drought. In contrast, in plants treated with HgCl2, recovery of Kss and root hydraulic conductance were impaired. In rehydrated, Hg-treated plants, the effects of Hg on Kss were reversed when either the shoot or the root was treated with 60 mM beta-mercaptoethanol as a mercuric scavenger. This work suggests that plant cellular metabolism, sensitive to mercuric chloride, affects the recovery of shoot hydraulic conductivity during grapevine rehydration by interfering with embolism removal, and that it involves either the root or the shoot level.

Interspecific Competition Between Coleogyne ramosissima Seedlings and Bromus rubens

Abstract Interactive effects of red brome grass (Bromus rubens) density and time of establishment on the early survival and growth of blackbrush (Coleogyne ramosissima) seedlings were quantitatively investigated. Seeds of Coleogyne and Bromus were collected from Cold Creek of the Spring Mountains in southern Nevada. A series of pot trial experiments were conducted in a controlled environmental glasshouse. In mixed culture pots when Coleogyne seedlings planted four weeks later than Bromus at medium and high density levels, survival of Coleogyne seedlings (experimental populations) was greatly reduced compared to single Coleogyne seedlings that grew alone (control population). Significant interactions were detected between neighboring Bromus density and time of planting for shoot height, root/shoot ratio, leaf length, and shoot water potential of experimental Coleogyne populations. When Bromus density was examined independently, all measured growth parameters of experimental Coleogyne populations were signi...

Enhanced monsoon precipitation and nitrogen deposition affect leaf traits and photosynthesis differently in spring and summer in the desert shrub Larrea tridentata

Leaf-level CO2 assimilation (A(area)) can largely be predicted from stomatal conductance (g(s)), leaf morphology (SLA) and nitrogen (N) content (N(area)) in species across biomes and functional groups. The effects of simulated global change scenarios, increased summer monsoon rain (+H2O), N deposition (+N) and the combination (+H2O +N), were hypothesized to affect leaf trait-photosynthesis relationships differently in the short- and long-term for the desert shrub Larrea tridentata. During the spring, +H2O and +H2O +N plants had lower A(area) and g(s), but similar shoot water potential (Psi(shoot)) compared with control and +N plants; differences in A(area) were attributed to lower leaf N(area) and g(s). During the summer, +H2O and +H2O +N plants displayed higher A(area) than control and +N plants, which was attributed to higher Psi(shoot), g(s) and SLA. Throughout the year, A(area) was strongly correlated with g(s) but weakly correlated with leaf N(area) and SLA. We concluded that increased summer monsoon had a stronger effect on the performance of Larrea than increased N deposition. In the short term, the +H2O and +H2O +N treatments were associated with increasing A(area) in summer, but also with low leaf N(area) and lower A(area) in the long term the following spring.

Shoot water status and ABA responses of transgenic hybrid larch Larix kaempferi x L. decidua to ectomycorrhizal fungi and osmotic stress

It has been postulated that osmotic effects on plant tissue are mediated by abscisic acid (ABA). Hybrid larch (Larix kaempferi (Lambert) Carr. x L. decidua Mill.) plantlets, transformed with the ABA-inducible wheat Em promoter associated with the Gus reporter gene, were axenically inoculated with two ectomycorrhizal fungi: Cenococcum geophilum Fr., considered tolerant to water stress, and Laccaria bicolor (Marie) Orton, considered less tolerant to drought. The mycorrhizal and non-mycorrhizal transgenic plantlets were subjected to osmotic stress by adding polyethylene glycol (PEG) to the culture medium. In the presence of PEG, L. bicolor and C. geophilum reduced shoot water potential and turgor potential, but increased host osmotic potential. Treatment of plantlets with PEG induced a significant increase in endogenous ABA concentrations. Laccaria bicolor and C. geophilum behaved similarly and significantly decreased the ABA response of plantlets to PEG treatment. Moreover, inoculation with either fungus regulated the ABA response of the plantlets even when the fungus was separated from the host by a cellophane sheet that prevented mycorrhiza formation. Although the wheat Em promoter was inducible in larch plantlets, it was not regulated by endogenous ABA. Induction of the wheat Em promoter in larch plantlets depended on organ type, with maximum induction in the root apex. Induction of the Em promoter was significantly decreased by mycorrhizal inoculation.

Seasonal changes in apparent hydraulic conductance and their implications for water use of European beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Matt.) Liebl] in South Europe

The water status of Fagus sylvatica L. and Quercus petraea (Matt) Liebl. was analysed during a cycle of progressive natural drought in southern Europe. Predawn (Ψpd) and midday water potential were measured in transpiring (Ψleaf) and non-transpiring leaves (Ψxyl). Furthermore, photosynthesis (A), stomatal conductance to water vapour (gs) and sap flow (Fd) were recorded on the same dates. Apparent leaf specific hydraulic conductance in the soil–plant–air continuum (Kh) and whole tree hydraulic conductance (Khsf) were calculated by using the simple analogy of the Ohm’s law. Kh was estimated at different points in the pathway as the ratio between transpiration (E) in the uppermost canopy leaves at midday and the gradient of water potential in the different compartments of the continuum soil–roots–stem–branches–leaves. There was a progressive decrease in water potential measured on non-transpiring leaves at the base of tree crown in both species (Ψlxyl) from the beginning of the growing season to the end of summer. A similar decrease was shown in shoot water potential (Ψuxyl) at the uppermost canopy. Predawn water potential (Ψpd) was high in both species until late July (28 July); afterwards, a significant decrease was registered in F. sylvatica and Q. petraea with minimum values of −0.81±0.03 and −0.75±0.06 MPa, respectively, by 15 September. In both species, leaf specific hydraulic conductance in the overall continuum soil–plant–air (Kh) decreased progressively as water stress increases. Minimum values of Kh and Khsf were recorded when Ψpd was lower. However, Q. petraea showed higher Kh than F. sylvatica for the same Ψpd. The decrease in Kh with water stress was mainly linked to its fall from the soil to the lowermost canopy (Ksrs). Nevertheless, a significant resistance in the petiole–leaf lamina (Kpl) was also recorded because significant differences in all dates were found on Ψ between transpiring and non-transpiring leaves from the same shoot. The decline in Kh was followed by an increase in stomatal control of daily water losses through the decrease of stomatal conductance to water vapour (gs) during the day. It promoted a seasonal increase in the stomatal limitation to carbon dioxide uptake for photosynthesis (A). These facts were more relevant in F. sylvatica, which had concurrently a higher decline in water use at the tree level than Q. petraea. The results showed a strong coupling in F. sylvatica and Q. petraea between processes at leaf and tree level. It may be hypothesised a role of specific hydraulic conductance not only in the regulation of water losses by transpiration but also of carbon uptake.

Plant Functional Groups in Alpine Fellfield Habitats of the White Mountains, California

An alpine fellfield community on granite substrate (elevation 3750 m) near the University of California Barcroft Laboratory in the White Mountains of eastern California was studied during the 2000 growing season to determine whether classic perennial life forms can be treated as plant functional groups. A series of 1-m2 quadrat samples were measured to determine common species. The four species with greatest cover were Penstemon heterodoxus var. heterodoxus, Trifolium andersonii var. beatleyae, Poa glauca subsp. rupicola, and Eriogonum ovalifolium var. nivale. These and four additional common perennial species were selected for ecophysiological studies representing four distinct ecological life forms: chamaephytes, cushion plants (including mat formers), herbaceous dicot perennials, and graminoid perennials. Summer midday leaf temperatures for species with foliage held close to ground surface were up to 20°C higher than air temperatures, whereas on upright species, leaves away from the ground surface closely matched ambient temperatures. For the eight species, peak values of mean maximum photosynthetic rates ranged from 11.5–25.5 μmol CO2 m−2 s−1, typical of published values, although chamaephytes in the study showed higher rates comparable to herbaceous perennials. Water-use efficiency, as estimated by a ratio of internal to ambient CO2, was relatively high (c i:c a ratios of 0.43–0.59) compared to published data. During the stressful end of the growing season, neither predawn nor midday shoot water potentials ever reached low levels, presenting conflicting evidence for the role of soil moisture as a limiting factor. Overall, the data on plant functional attributes showed no strong patterns of differences between categories of life forms in the fellfield community, suggesting that classical life forms in this habitat do not represent plant functional groups.

Effect of drought stress on chlorophyll a fluorescence and electrical admittance of shoots in Norway spruce seedlings

Effects of mild and severe soil drought on the water status of needles, chlorophyll a fluorescence, shoot electrical admittance, and concentrations of photosynthetic pigments in needles of seedlings of Picea abies (L.) Karst. were examined under controlled greenhouse conditions. Drought stress reduced shoot admittance linearly with a decrease in shoot water potential (Ψw) and increase in water deficit (WD) and led to a decrease in concentrations of chlorophyll a, b and carotenoids. Severe water stress (shoot Ψw=−2.4 MPa) had a negative effect on chlorophyll a fluorescence parameters including PSII activity (Fv/Fm), and the vitality index (Rfd). Variations in these parameters suggest an inhibition of the photosynthetic electron transport in spruce needles. Water stress led to a decrease in the mobility of electrolytes in tissues, which was reflected by decreased shoot electrical admittance. After re-watering for 21 days the WD in needles decreased and the shoot water potential increased. In the re-watered plants, the chloroplast function was restored and chlorophyll a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in the seedlings triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. We conclude that the shoot electrical admittance and photosynthetic electron transport in leaves are closely linked to changes in water status and their decrease is among the initial responses of seedlings to water stress.

Phenological comparison between two co-occurring Mediterranean woody species differing in growth form

Halimium atriplicifolium and Thymus vulgaris are two Mediterranean woody species, which differ in growth form and may co-occur under the same climatic constraints. Both possess distinct short (SS) and long shoots (LS). The aims of this work were: (1) to compare their phenological patterns, (2) to relate plant phenology with root depth and summer water potential, and (3) to compare the structure and phenology of SS and LS. Pre-dawn and mid-day shoot water potentials ( Ψ pd and Ψ md ) were assessed at the beginning and at the end of the driest period. SS and LS growth, flowering and fruit setting was followed every month throughout the annual cycle. Leaf shedding was followed with litter traps and leaf demography were monitored separately in SS and LS. Ψ pd and Ψ md exhibited a sharper drop in T. vulgaris than in H. atriplicifolium along the summer. Root depth of H. atriplicifolium was more than twice T. vulgaris. Phenophases of H. atriplicifolium occurred between spring and summer, while those of T. vulgaris concentrated in spring. The latter species shed many current-year leaves in September, probably in response to water shortage. In both species, LS growth occurred during a more rainy period than SS growth and during a warmer period in H. atriplicifolium. Leaf area and leaf mass per area were smaller for SS leaves than LS, probably due to water and carbon shortage at the time of SS growth. In conclusion, T. vulgaris suffered from more severe water stress than H. atriplicifilium due to its shallower root system and arrested phenological activity earlier in the summer. The different morphological and phenological traits of LS and SS suggest a specialisation in carbon gain along different time periods of the year.

Preplanting indicators of survival and growth of desiccated Abies procera bareroot planting stock

Plant moisture loss during preplanting handling is considered one of the major elements in transplanting shock. The aim of the present study was to evaluate test parameters that may be suitable for assessment of the physiological condition of noble fir (Abies procera Rehd.) planting stock that has been subjected to desiccation stress. The transplanting-sensitive species was subjected to full exposure (roots and shoots) for 0, 0.5, 1.5, 3.0 and 6 h, or partial exposure (shoots) for 0, 3, 10, and 24 h before planting. The effects of desiccation on moisture content (MC), shoot water potential (ψ), root growth potential (RGP), root electrolyte leakage (REL), survival and height growth were evaluated. Desiccation effects on plant parameters and performance were generally significant after 1.5 h of full or 10 h of partial exposure to treatment. REL explained the largest percentage of performance variation in fully exposed plants, followed by MC and ψ. RGP tended to underestimate performance potential. Factors affecting the reliability of the test parameters, e.g. moisture loss rate, root rewetting and site conditions, are discussed.

Effect of water deficit on oxidative stress and degradation of cell membranes in needles of Norway spruce (Picea abies)

We studied the impact of mild and severe drought stresses for 42 days and rehydration for 21 days on 4-year-old seedlings of Norway spruce. Water relations in spruce tissues were determined on the basis relative water content of needles and shoot water potential (Ψshoot). During the stress, we measured the level of: reactive oxygen species (ROS), antioxidants, and degradation of cell membranes. In the seedlings subjected to severe stress, Ψshoot decreased to −2.4 MPa, while in those subjected to mild stress, to −0.8 MPa. After rehydration, shoot water potential increased, but did not reach the control level. Water deficit caused oxidative stress, reflected in an increased production of ROS: superoxide anion radical (\(\left( {\mathop O\nolimits_{2^{ \bullet ^{\text{ - }} } } } \right)\)) and hydrogen peroxide (H2O2). Their concentrations in needles were the highest in seedlings subjected to severe stress, where they exceeded the control level by 116% and 30%, respectively. During rehydration, the differences in ROS levels between treated and control seedlings diminished. Oxidative stress causing degradation of cell membranes included: de-esterification of phospholipids, oxidation of fatty acids, and increase in concentration of malondialdehyde, as their permeability to ions increased by 125%. In the defence against the oxidative stress in needles, an important role was played by low-molecule antioxidants such as glutathione, ascorbic acid, flavonoids, α-tocopherol and antioxidant enzymes. An increase in intensity of water deficit caused a significant reductio in the level of low-molecular antioxidants, which attests to their utilization during the process of scavenging for free radicals. Water deficit at Ψshoot=−1.7 MPa caused a decline in ascorbic acid level by 37% in needle cells. An effective defensive mechanism removing the excess of ROS was also reflected in the activity of the main enzymes of oxidative stress: superoxide dismutase (SOD) and guaiacol peroxidase (PO). As a result of water deficit, SOD activity increased by 80 %, while PO activity decreased by 82 %.