Effect of a strong cold storage induced desiccation on metabolic solutes,stock quality and regrowth,in seedlings of two oak species

Bare-root seedlings of pedunculate oak (Quercus robur L.) and northern red oak (Quercus rubra L.) were lifted in January and stored at 1.8°C, at 82% relative humidity, until their fresh weight declined by 33%. Root growth potential (RGP), fine root electrolyte leakage (REL), fine root water content (RWC), shoot tip water content (SWC), starch and metabolic solute contents in root and shoot, were measured just after lifting and after treatment. Survival of treated seedlings was also assessed in a field trial. RWC, SWC, REL, RGP were dramatically affected by desiccation during cold storage. In both species, root soluble carbohydrate level, inositol level and isocitrate level increased, whereas root starch level and shoot soluble carbohydrate level decreased. In northern red oak, treated seedlings had higher root contents of soluble carbohydrates, inositol and proline than in pedunculate oak. Moreover, treatment induced proline accumulation only in northern red oak roots. These differences could explain why field survival of treated seedlings was significantly better in northern red oak than in pedunculate oak.     

Physiological and growth responses of Cedrus libani seedlings to cold storage

The effects of cold storage duration on the physiological characteristics and growth of two-year-old Taurus cedar (Cedrus libani A. Rich) seedlings were studied. Taurus cedar seedlings were lifted in December, January and February and stored at +4 °C (cold storage) for 0, 2, 3 and 4 months. Xylem water potential (Ψ), shoot (SMC) and root moisture (RMC) contents, root growth potential (RGP), root electrolyte leakage (REL) and total carbohydrate contents were determined before and after the cold storage. The survival and growth were also evaluated at the end of the first growing season. Ψ, SMC and RMC, RGP and total carbohydrate contents were dramatically affected by the storage duration. The decrease in total carbohydrate contents during the storage showed a parallelism with RGP and survival. It was also found that storage duration had important effects on survival and growth. While survival was above 85 % even after storage of 4 months in seedlings that were lifted in December and January, this rate was reduced to 30 % after storage of 4 months in seedlings that were lifted in February. Total carbohydrate content and RGP can be used as an indicator of survival after cold storage.     

Storage and mobilization of nonstructural carbohydrates and biomass development of beech seedlings (Fagus sylvatica L .) under different light regimes

 Beech seedlings were grown under 8%, 13%, 23% and 100% relative light intensity for 2 years after germination. Starch, sucrose and monosaccharides from the bark and wood parenchyma of shoots and roots were analyzed during the course of the second year. The annual allocation pattern of starch revealed five successive phases: starch disintegration in November (1) was paralleled by high monosaccharide concentrations in the shoot cortex (≤ 33.4 mg/g DW). Seedlings of all light variants reached maximum sucrose concentrations (≤ 82.8 mg/g DW) during starch disintegration in January (2) that coincided with decreased monosaccharide contents. Up to mid-April, resynthesis of starch (3) occurred in most shaded and unshaded seedlings. In May, starch was converted into monosaccharides in all storage tissues (4). Seedlings grown under 13% light intensity showed de novo synthesis of starch (5) 4 weeks after bud burst. These seedlings reached 98% maximum starch storage capacity of the shoot and 89% of the root in July. In mid-October, the maximum starch concentration of the roots increased with light intensity, and this corresponded with an increase of lateral root growth. The variation of shoot and root dry weight was closely related to the content of nonstructural carbohydrates during the second year. The shift of shoot growth to the first half of the growing season and the suppression of lateral root growth during the second half is assumed to be a strategy of young beech to survive under light limiting conditions. Received: 22 May 1997 / Accepted: 10 October 1997     

Seasonal allocation of photoassimilated carbon in douglas fir seedlings

The uptake of CO₂ by Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) seedlings and the allocation of photoassimilated carbon among five vegetative tissues were closely related to seedling phenology. In May, newly flushing needles required 5.2% day⁻¹ of photoassimilated carbon relative to needle tissue carbon. As these needles matured, this carbon requirement declined to 1.95% day⁻¹ in August, to 0.94% day⁻¹ in November, and to 0.76% day⁻¹ in January. Other tissues of Douglas fir seedlings required different amounts of photoassimilated carbon for growth and metabolism. These data provide a strong link between daily CO₂ uptake and the regulation of carbon allocation by seasonal phenology.     

Morphology and Stomatal Function of Douglas Fir Needles Exposed to Climate Change: Elevated CO2 and Temperature

Climate change may have an impact on the productivity of conifer trees by influencing the morphology (size and surface characteristics) and function (capacity for gas exchange) of conifer needles. In order to test the responses of needles to climatic variables, Douglas fir (Pseudotsuga menziesii [Mirb.] Franco), saplings were grown in sunlit controlled environment chambers at ambient or elevated (+200 parts per million above ambient) CO2 and at ambient or elevated temperature (+4 degrees C above ambient). Needle characteristics, including length, width, area, stomatal density (stomata per mm2), percentage of stomatal occlusion, and the morphology of epicuticular wax, were evaluated. Needle function was evaluated as stomatal conductance to water vapor and transpiration. Needle length increased significantly with elevated temperature but not with elevated CO2. Neither elevated CO2 nor elevated temperature affected stomatal density or stomatal number in these hypostomatous needles. Epicuticular wax was less finely granular at elevated than at ambient temperature and was similar in appearance at elevated and ambient CO2. Stomatal conductance and transpiration increased with elevated temperature and associated increased vapor pressure deficit; however, neither conductance nor transpiration was affected by elevated CO2. These results indicate that simulated climate change influences Douglas fir needle structure and function.     

Phenylpropanoids in mycorrhizas of the Pinaceae

Tissue-specific accumulation of phenylpropanoids was studied in mycorrhizas of the conifers, silver fir (Abies alba Mill.), Norway spruce [Picea abies (L.) Karst.], white pine (Pinus strobus L.), Scots pine (Pinus silvestris L.), and Douglas fir [Pseudotsuga menziesii (Mirbel) Franco], using high-performance liquid chromatography and histochemical methods. The compounds identified were soluble flavanols (catechin and epicatechin), proanthocyanidins (mainly dimeric catechins and/or epicatechins), stilbene glucosides (astringin and isorhapontin), one dihydroflavonol glucoside (taxifolin 3′-O-glucopyranoside), and a hydroxycinnamate derivative (unknown ferulate conjugate). In addition, a cell wall-bound hydroxycinnamate (ferulate) and a hydroxybenzaldehyde (vanillin) were analysed. Colonisation of the root by the fungal symbiont correlated with the distribution pattern of the above phenylpropanoids in mycorrhizas suggesting that these compounds play an essential role in restricting fungal growth. The levels of flavanols and cell wall-bound ferulate within the cortex were high in the apical part and decreased to the proximal side of the mycorrhizas. In both Douglas fir and silver fir, which allowed separation of inner and outer parts of the cortical tissues, a characteristic transversal distribution of these compounds was found: high levels in the inner non-colonised part of the cortex and low levels in the outer part where the Hartig net is formed. Restriction of fungal growth to the outer cortex may also be achieved by characteristic cell wall thickening of the inner cortex which exhibited flavanolic wall infusions in Douglas fir mycorrhizas. Long and short roots of conifers from natural stands showed similar distribution patterns of phenylpropanoids and cell wall thickening compared to the respective mycorrhizas. These results are discussed with respect to co-evolutionary adaptation of both symbiotic partners regarding root structure (anatomy) and root chemistry. Received: 25 May 1998 / Accepted: 6 November 1998     

Uptake and translocation of manganese in seedlings of two varieties of Douglas fir (Pseudotsuga menziesii var. viridis and glauca)

Douglas fir (Pseudotsuga menziesii) variety glauca (DFG) but not the variety viridis (DFV) showed symptoms of manganese (Mn) toxicity in some field sites. We hypothesized that these two varieties differed in Mn metabolism. To test this hypothesis, biomass partitioning, Mn concentrations, subcellular localization and 54Mn-transport were investigated. Total Mn uptake was three-times higher in DFG than in DFV. DFV retained > 90% of 54Mn in roots, whereas > 60% was transported to the shoot in DFG. The epidermis was probably the most efficient Mn barrier since DFV contained lower Mn concentrations in cortical cells and vacuoles of roots than DFG. In both varieties, xylem loading was restricted and phloem transport was low. However, sieve cells still contained high Mn concentrations. DFV displayed higher biomass production and higher shoot : root ratios than DFG. Our results clearly show that both varieties of Douglas fir differ significantly in Mn-uptake and allocation patterns rendering DFG more vulnerable to Mn toxicity.     

Establishment of

We compared establishment of Douglas fir (Pseudotsuga menziesii) and Corsican pine (Pinus nigra) seedlings in kanuka (Kunzea ericoides) and manuka (Leptospermum scoparium) shrubland to test the hypothesis that Douglas fir, because of its greater shade tolerance, is better able to establish in woody communities than pine species. Seed of the conifer species was sown under a range of canopy covers at six sites, the cover being low-statured vegetation in openings between stands, stand edges, and moderate and dense canopies. After three growing seasons, survival of Corsican pine seedlings was greatest in the open and declined progressively as canopy cover increased. This contrasted with Douglas fir, where survival was greatest at the canopy edge. Survival of Douglas fir seedlings significantly exceeded that of Corscican pine seedlings under dense canopy positions. Seedling numbers of both species declined significantly with increasing leaf area index of manuka, but not kanuka stands, where seedling numbers were lower. Leaf area index of manuka stands accounted for substantially greater variation in number and survival of Corsican pine than Douglas fir seedlings. It is concluded that Douglas fir is better able to establish in shaded environments in woody communities than Corsican pine; however, further monitoring is required to confirm the long-term survival of both species under the moderate and dense canopy positions in this trial.     

Characterization of phytohormonal and postharvest senescence responses of balsam fir (Abies balsamea (L.) Mill.) exposed to short-term low temperature

To fulfill the US Thanksgiving and Christmas tree markets, balsam fir (Abies balsamea (L.) Mill.) is generally harvested before the cold season, anecdotally leading to premature needle senescence. Accordingly, we tested the hypothesis that LT exposure before harvest induces specific hormonal changes and delays postharvest senescence and/or abscission in balsam fir. Two hundred and six seedlings exposed to two temperature treatments for 48?h, LT at 5?°C and controls at 22?°C were severed off roots and monitored for their postharvest needle senescence. Root and shoot (needles and buds) tissues were examined for major endogenous hormone metabolites. LT increased shoot ABA (2,007?ng?g^?1 DW) by 2.5× and decreased GA₄₄ (9.84?ng?g^?1 DW) by 3.5× over those in roots. LT did not alter cytokinins, auxins or any root hormonal concentration. With auxins, only IAA, IAA-Asp, IAA-Leu and IAA-Glu were detected and the concentrations of IAA and IAA-Asp in shoots were lower than those found in roots. Among cytokinins, shoot c-ZR (58.95?ng?g^?1 DW) and t-ZR (4.17?ng?g^?1 DW) were 3× higher than those in roots. Apart from GA₄₄, GA₉ (136.76?ng?g^?1 DW) was abundant in shoots. The PBL and PNL were 46 and 1.2?%, irrespective of treatments. LT seedlings held needles 11?days longer than the controls (122?days). In balsam fir, short-term LT exposure augmented ABA and decreased GA₄₄ levels in shoots and delayed postharvest needle senescence.     

Winter hardening in first-year black spruce (Picea mariana) seedlings

Winter hardening of first-year black spruce [ Picea mariana (Mill.) B.S.P.] seedlings was studied by assessing a number of morphological and physiological changes under three hardening regimes: 1) early removal (ER), in which seedlings were exposed to natural daylengths and low ambient temperatures outside. 2) extended greenhouse culture (EG), in which seedlings were exposed to natural daylengths and warm temperatures, and 3) short day (SD), in which seedlings were exposed to short daylengths and low ambient temperatures outside. Measurements included needle primordia initiation, embryonic shoot volume, terminal bud mitotic index, embryonic shoot average cell volume, and shoot tip frost hardiness. EG seedlings formed buds containing 4 times as many needle primordia as ER stock. Embryonic shoot volume increased with number of needle primordia initiated, until late in the hardening period, when significant reductions in meristem volumes of SD and EG stock were observed. Frost hardiness increased sooner in seedlings which set bud in response to short days, but SD treatment did not result in significantly greater frost hardiness at the end of the trial. Frost hardiness was correlated with mitotic index of the embryonic shoot. Cell size in the embryonic shoot declined in seedlings of all treatments during hardening, however, EG seedlings had significantly lower cell volumes by the end of the trial in comparison to ER and SD seedlings.     

Development of Sink Capacity of the "Storage Root" in a Radish Variety with a Low Ratio of "Storage Root" to Shoot

Controling mechanisms of sink capacity are poorly understood.Previously we suggested that sucrose synthase (SuSy), but notinvertase, plays an important role for sink capacity of theradish "storage root" in a variety, Raphanus sativus L. (cv.White Cherish) [plant Cell Physiol. (1999) 40: 369]. With thisvariety about 50% of the total dry weight (DW) was in the "storageroot" at 21 d after sowing (DAS). We investigated the sink capacityof another radish variety, R. sativus L. (cv. Kosena) with alow ratio of "storage root" to shoot. With the latter varietyonly 3% of the total DW was in the "storage root" at 21 DAS.Sink activity (increase in DW of the "storage root" per unitof DW present per unit of time) of the "storage root" in Kosenaas well as White Cherish was strongly related to the level andactivity of SuSy but not to the activity of invertase. Theseresults confirmed that SuSy rather than invertase may be criticalfor the development of the sink activity of the radish "storageroot" and that the reaction products of UDP-glucose and fructoseare utilized for sink growth including biosynthesis of the cellwall. In Kosena photosynthates seemed to be partitioned mainlyinto developing leaves and fibrous roots. Differences in partitioningof photosynthates among various sinks with these two varietiesare discussed including anatomical considerations. (Received July 19, 1999; Accepted September 30, 1999)     

Hydraulic control of stomatal conductance in Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] and alder [Alnus rubra (Bong)] seedlings

Experiments were conducted on 1-year-old Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] and 2- to 3-month-old alder [Alnus rubra (Bong)] seedlings growing in drying soils to determine the relative influence of root and leaf water status on stomatal conductance (g_c). The water status of shoots was manipulated independently of that of the roots using a pressure chamber that enclosed the root system. Pressurizing the chamber increases the turgor of cells in the shoot but not in the roots. Seedling shoots were enclosed in a whole-plant cuvette and transpiration and net photosynthesis rates measured continuously. In both species, stomatal closure in response to soil drying was progressively reversed with increasing pressurization. Responses occurred within minutes of pressurization and measurements almost immediately returned to pre-pressurization levels when the pressure was released. Even in wet soils there was a significant increase in g_c with pressurization. In Douglas fir, the stomatal response to pressurization was the same for seedlings grown in dry soils for up to 120 d as for those subjected to drought stress over 40 to 60 d. The stomatal conductance of both Douglas fir and alder seedlings was less sensitive to root chamber pressure at higher vapour pressure deficits (D), and stomatal closure in response to increasing D from 1.04 to 2.06 kPa was only partially reversed by pressurization. Our results are in contrast to those of other studies on herbaceous species, even though we followed the same experimental approach. They suggest that it is not always appropriate to invoke a ‘feedforward’ model of short-term stomatal response to soil drying, whereby chemical messengers from the roots bring about stomatal closure.     

Effects of carbon dioxide enrichment on response of mycorrhizal pitch pine (Pinus rigida) to aluminum: growth and mineral nutrition

 Carbon dioxide enrichment may increase the Al tolerance of trees by increasing root growth, root exudation and/or mycorrhizal colonization. The effect of elevated CO₂ on the response of mycorrhizal pitch pine (Pinus rigida Mill.) seedlings to Al was determined in two experiments with different levels of nutrients, 0.1- or 0.2-strength Clark solution. During each experiment, seedlings inoculated with the ectomycorrhizal fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown 13 weeks in sand irrigated with nutrient solution (pH 3.8) containing 0, 6.25, 12.5, or 25 mg/l Al (0, 232, 463, or 927 μM Al) in growth chambers fumigated with 350 (ambient) or 700 (elevated) μl/l CO₂. At ambient CO₂, in the absence of Al, mean total dry weights (DW) of seedlings at the high nutrient level were 164% higher than those at the low level. Total DW at elevated CO₂, in the absence of Al, was significantly greater than that in ambient CO₂ at the low (+34%) and high (+16%) nutrient levels. Root and shoot DW at both nutrient levels decreased with increasing Al concentrations with Al reducing root growth more than shoot growth. Although visible symptoms of Al toxicity in roots and needles were reduced by CO₂ enrichment, there were no significant CO₂ × Al interactions for shoot or root DW. The percentage of seedling roots that became mycorrhizal was negatively related to nutrient level and was greater at elevated than at ambient CO₂ levels. Generally, elevated CO₂ had little effect on concentration of mineral nutrients in roots and needles. Aluminum reduced concentrations of most nutrients by inhibiting uptake. Received: 18 June 1997 / Accepted: 8 December 1997     

Root architecture,early development and basal sweep in containerized and bare-rooted Douglas fir (Pseudotsuga menziesii)

Seedlings grown in containers often have deformed, spiralling, kinking root systems (reported especially in pine growing in colder climates like Fenno-Scandinavia). The current study examines the effect of containers on the root systems of Douglas fir planted in Ireland. A sample of sixteen 10-year-old Douglas fir trees, planted as either bare-rooted transplants (2/1) or containerized seedlings (Paperpot 610 (2/0)) on an acid brown earth site in Ireland, were excavated for root achitectural studies. In addition to thorough above-ground measurements, an assessment of basal sweep was also carried out. Root systems were systematically described and horizontal angles, cross-sectional areas (CSA), and maximum depths of all roots>5 mm in diameter were recorded. Various biomass ratios and estimates for dominant roots were also calculated. Symmetry of the root systems, max. and min. numbers, and CSA of roots for sectors of various sizes were compared for the two plant types. The initially (when planted) bigger bare-rooted transplants were still larger than the container-planted trees after ten years. Of the total above-ground fresh weight, the container stock had allocated more to the crown and less to the stem compared to bare-roots. The difference in root weights between stock types was small. Basal sweep occurred on average in 50% of the bare-roots and 35% of the container trees. The direction of the basal sweep leaning was concentrated to the NE, which coincided with the direction of the slope and the prevailing wind direction. No effect of planting position in relation to the direction of site preparation was found for basal sweep. The size of the root system, fresh weight and total cross-sectional area was on average for all trees correlated to both DBH and shoot fresh weights. For containers separately, however, only root area was correlated to DBH and stem fresh weight. In spite of the difference in the above-ground size, there were no significant differences in root numbers and root area (CSA) between the two plant types. No difference in rooting depth between plant types was found. When splitting the root system into 120° horizontal sectors (1/3 of the root system) the highest numbers of roots were concentrated in the NW direction. The highest amount of root area tended to be concentrated along a NW-SE diagonal, with a dominance for the latter (SE). Sectors without dominant roots (expected to be the future main structural, stabilizing roots), varied in size (94–178°) but were on average wider in bare-roots and on average concentrated in the NE direction and the downhill side of the slope. This revised version was published online in June 2006 with corrections to the Cover Date.     

Seasonal changes in C, N and major organic compounds and their significance to morpho-functional processes in the endemic Antarctic brown alga Ascoseira mirabilis

The seasonal and intra-thallus variations in the contents of C, N, proteins and amino acids, as well as in the storage carbohydrates mannitol and laminaran, were measured in the endemic Antarctic brown alga Ascoseira mirabilis between September and February and related to seasonal changes in dark respiration and photosynthesis. Carbon contents between 31 and 37% DW were relatively constant throughout these months and no variations were detected among thallus regions. Nitrogen contents, by contrast, were higher in September/October (3.1% DW) and decreased in January and February (1.8% DW). In general, the basal regions had the lower N contents. Proteins reached maximum values of 13% DW in November and February and were inversely correlated to photosynthesis (net P_max). The amino acid content was also higher in October and November (maxima close to 10% DW), but low between December and February (close to 5% DW), probably related to a seasonal pattern of N allocation in the alga. The storage carbohydrates mannitol and laminaran exhibited inverse seasonal changes: low mannitol values close to 5% DW in September were coupled with high laminaran contents varying between 7 and 15% DW. The existence of high laminaran contents in the distal blade region during September and February suggests that this compound was effectively accumulated in this region. The low P/R ratios in spring and the existence of a significant relationship between mannitol content and seasonal photosynthetic activity in the basal region appear to support the hypothesis of a possible utilization of carbohydrates to power growth in A. mirabilis. Received: 7 November 1996 / Accepted: 28 August 1997     

Seasonal patterns of photosynthesis in Douglas fir seedlings during the third and fourth year of exposure to elevated CO2 and temperature

The interactive effects of elevated atmospheric CO₂ and temperature on seasonal patterns of photosynthesis in Douglas fir (Psuedotsuga menziesii (Mirb.) Franco) seedlings were examined. Seedlings were grown in sunlit chambers controlled to track either ambient (~400 p.p.m.) CO₂ or ambient +200 p.p.m. CO₂, and either ambient temperature or ambient +4 °C. Light‐saturated net photosynthetic rates were measured approximately monthly over a 21 month period. Elevated CO₂ increased net photosynthetic rates by an average of 21% across temperature treatments during both the 1996 hydrologic year, the third year of exposure, and the 1997 hydrologic year. Elevated mean annual temperature increased net photosynthetic rates by an average of 33% across CO₂ treatments during both years. Seasonal temperature changes also affected net photosynthetic rates. Across treatments, net photosynthetic rates were highest in the spring and autumn, and lowest in July, August and December–January. Seasonal increases in temperature were not correlated with increases in the relative photosynthetic response to elevated CO₂. Seasonal shifts in the photosynthetic temperature optimum reduced temperature effects on the relative response to elevated CO₂. These results suggest that the effects of elevated CO₂ on net photosynthetic rates in Douglas fir are largely independent of temperature.     

Parasitism of seed of Douglas fir (<Emphasis Type="Italic">Pseudotsuga menziesii</Emphasis>) by the seed chalcid, <Emphasis Type="Italic">Megastigmus spermotrophus</Emphasis>, and its influence on seed hormone physiology

Parasitism of Douglas fir (Pseudotsuga menziesii) megagametophytes by the seed chalcid, Megastigmus spermotrophus Wachtl, occurs naturally after pollination but before fertilization. In the absence of fertilization, the presence of insect larvae within the megagametophyte prevents abortion and the storage tissue continues to develop as if the seed had been fertilized. We investigated the effect of parasitism on the metabolism of abscisic acid (ABA), auxins, cytokinins, and gibberellins during early development of Douglas fir seeds. Hormones and hormone metabolites of infested and uninfested megagametophytes with or without pollination were analyzed by HPLC–ESI/MS/MS. At 1 week after Megastigmus introduction, the insect’s presence stimulated ABA accumulation in unpollinated megagametophytes compared to unpollinated, unparasitized megagametophytes. In pollinated material, parasitism did not stimulate ABA accumulation compared to levels present in unparasitized megagametophytes. In all four treatments, the metabolism of ABA occurred primarily through conjugation to the ABA glucose ester (ABAGE), while the 7′-, 8′- and, 9′-hydroxylation pathways were only minor. ABAGE levels declined with time in all treatments and this occurred to a greater extent in pollinated, parasitized megagametophytes, suggesting that the insect’s presence induced the dramatic decrease in ABAGE. Although there were temporal variations in the auxin, cytokinin, and gibberellin profiles of parasitized megagametophytes, the profiles were generally similar to those of unparasitized megagametophytes. Our results suggest that failure of parasitized megagametophytes to abort may be due to the insect inducing similar hormone profiles to those present during normal development of Douglas fir seed.     

Physiological responses of black willow (Salix nigra) cuttings to a range of soil moisture regimes

We quantified the physiological responses of black willow to four soil moisture regimes: no flooding (control, C), continuous flooding (CF), periodic flooding (PF), and periodic drought (PD). Stomatal limitation was one of the factors that led to the reduced photosynthetic capacity in CF cuttings. Under PD, stomatal closure, decreased leaf chlorophyll content, and increased dark fluorescence yield contributed to photosynthetic decline. CF cuttings accumulated the lowest shoot biomass while the final height and root growth were most adversely affected by PD. PF cuttings tended to allocate more photoassimilates to root growth than to shoots.This revised version was published online in March 2005 with corrections to the page numbers.     

Seedling development of the miombo woodland tree Julbernardia globiflora

Abstract. The development of seedlings of the miombo tree (Julbernardia globiflora) was studied for 28 months (December 1987 - April 1990) at a Zambian grassland site from which fire has been excluded for many years. Germination occurred 10–25 days after seed planting and germination rates under laboratory and field conditions did not differ significantly. Seed germinability after one year of storage declined to 38 %. Leaf production occurred between December and April with peaks in January each year. Leaf fall occurred steadily throughout the dry season but leafless seedlings were first observed in August of each year. All seedlings had shed all their leaves by the end of November during the first year while 10% of the seedlings remained leafy throughout the second year. Annual seedling mortality was about 60 % during the first year and declined to 3 6 % during the second year. Survivorship appeared to stabilise at 3 8 % when seedlings were 5 5 - 5 9 weeks old. Every seedling experienced shoot die-back during the first hot dry season (September - November) when open-pan evaporation rates averaged 5–10 mm / day. However, shoot mortality did not necessarily result in plant mortality. Because of recurrent shoot die-back there was little increase in shoot biomass and height at the end of the second and third growing seasons. Seedlings that survived shoot die-back sprouted from roots which showed a steady annual accumulation of biomass. Early shoot die-back in seedlings of Julbernardia globiflora as documented in this study was not causedby fire but most probably by drought and / or nutritional stress.     

Contribution of vegetative storage proteins to seasonal nitrogen variations in the young shoots of peach trees (Prunus persica L. Batsch)

Qualitative and quantitative variations in the level of two low molecular weight vegetative storage proteins (VSP 19 kDa and 16.5 kDa) in peach shoots were compared with annual variations in total nitrogen and total soluble proteins. Protein patterns were obtained by SDS-PAGE and silver staining on each of the 12 kinetic samples collected between October 1995 and November 1996. VSP 16.5 kDa and 19 kDa exhibited typical annual VSP variations in both parenchyma and phloem. In wood, VSP 16.5 kDa was only present in November. All N compounds tested were stored in the autumn and their levels fell in the spring. Parenchyma was the principal stem storage tissue for all N compounds tested, even if proteins were more often highly concentrated in phloem and even if wood was the major shoot constituent. In winter, the two VSP accounted for 13% of bark proteins and 11% of wood proteins. Their storage yield, given by the winter/summer (W/S) ratio was higher (18.5) than that of total proteins (4). Between August to March, i.e. during the storage phase, N fractions obtained from VSP (N3) and total soluble proteins minus VSP (N2) accounted, respectively, for only 3% and 21% of total N accumulation in the bark, the remainder being due to the fraction not extracted (N1). A marked drop in all N compound levels characterized the mobilization phase (March to April), particularly for N3 (-84% between March and April) which were mobilized slightly before other N compounds. Although N3 exhibited the best mobilization yield, it represented only 5% of the total N mobilized. So, in spite of a similarity between VSP and N annual variation patterns, there was no tight correlation between their contents in bark. N2 supplied a high proportion of the N used for spring regrowth (40%), but the larger share (55%) came from N1 which was probably made up of free amino acids. Very tight positive correlations have been observed between these two N fractions and the N status. The lower bark total N content measured in August (6.4 mg N g(-1 )DW) during the assimilation phase (April to August) was equal to the unavailable N fraction, and the bark N mobilization potential (between March and August) was estimated at 6.35 mg N g(-1) DW. VSP did not quantitatively represent the main stored N pool. But, because of their high W/S ratio and their early remobilization, they seemed to play an important role in spring regrowth initiation.