Physiological, morphological and anatomical responses of Fraxinus mandshurica seedlings to flooding

Two-year-old Fraxinus mandshurica Rupr. var. japonica Maxim. seedlings were flooded to 8 cm above soil level for 70 days. The flooding treatment altered the growth, morphology, stem anatomy and ethylene production of the seedlings. Although flooding did not affect height growth, it stimulated diameter growth of the submerged stems by increasing both the number and size of wood fibers produced; however, the thickness of the cell walls of the wood fibers was reduced by flooding. In response to the flooding treatment, the seedlings formed abundant hyperhydric tissues, originating from the vicinity of lenticels on the surface of the flooded stems, and adventitious roots, which grew through the hyperhydric tissues. Aerenchyma tissues were observed in the bark of the adventitious roots. The flooding treatment did not affect dry weight increment of leaves and stems, but it reduced the total dry weight increment of the root system even though it promoted adventitious root formation. Flooding also enhanced ethylene production in the submerged portions of stems. The potential roles of flood-induced ethylene in cambial growth and adventitious root formation in flooded plants are discussed.     

Effects of flooding depth on growth,morphology and photosynthesis in <Emphasis Type="Italic">Alnus japonica</Emphasis> species

The present study deals with effects of flooding depth on growth, morphology and photosynthesis in Alnus japonica species thorough one field study and two controlled experiments. In the field study performed in Kushiro Mire, Hokkaido Island, Japan, tree heights and stem diameters decreased with an increase in water depth accompanied with the reduction of soil redox potential. In contrast, the rate of multiple stems per individual tree increased. In the controlled experiments for seedlings flooding suppressed the shoot elongation and biomass increment in roots. However, diameter increment around water levels, epicormic shoot development and adventitious root formation were enhanced in flooded seedlings. The photosynthetic rate and stomatal conductance of flooded seedlings also were lowered with an increase in flooding depth. The recovery of the reduced photosynthetic rate and stomatal conductance occurred simultaneously with the advancement of adventitious root formation in the flooded seedlings. These results indicate the importance of a series of morphological changes occurring on stems around water levels in flood tolerance in A. japonica species.     

Impact of soil pressure and compaction on tracheids in Norway spruce seedlings

We tested the effect of soil compaction on Norway spruce seedlings in terms of the size and theoretical volume flow rate of the tracheids. The results show that soil pressure limits growth in the diameter of the lumens of tracheids in all parts of seedlings studied. The tracheids of the roots with primary xylem had larger lumens than those of the roots and shoots with secondary xylem in both unloaded and loaded seedlings. This corresponds to the higher cumulative theoretical volume flow rate of the tracheids from roots with primary xylem than those from roots and shoots with secondary xylem. Although the volume flow rate of tracheids, according to the Hagen-Poiseuille law, was directly proportional to the quadratic power of the capillary diameter (tracheid lumen), the cumulative curve of the theoretical hydraulic volume flow rate was higher or relatively comparable in loaded seedlings. An explanation for these findings is that there were higher gradients of water potential values in roots and leaves in loaded seedlings because the lengths of the conductive pathways were 27% shorter than in unloaded seedlings. We hypothesise that trees have adapted to different stresses by shortening their conductive pathways to maintain a transpiration rate similar to that of non-stressed trees. These results concerning the impact of soil compaction on tracheid diameter and volume flow rate improve our understanding of the growth and functioning of different conifer organs and the mechanisms underlying the efficiency of water transport through the root xylem to the shoot.     

Effects of various stress treatments on growth and ethylene evolution in seedlings and sprouts of betula pendula Roth and B. pubescens Ehrh.

Experiments were conducted to determine growth and ethylene evolution in seedlings and coppiced plants of Betula pendula Roth and B. pubescens Ehrh. when subjected to stresses relevant to the maintenance of natural forests, and especially in fuelwood plantations, i.e. cutting of the stem, thinning, bending, flooding and various combinations of these. Most of the experiments were carried out in the laboratory using 1‐year‐old seedlings or 1‐month‐old sprouts. Height and diameter growth, biomass production, morphology and ethylene evolution were studied for 1–2 months. Material for comparison was obtained from young sprouts on the stumps of fully developed trees growing under natural conditions and natural seedlings of a comparable age. Exposure of seedlings and coppiced plants to stress factors usually altered growth and increased ethylene evolution. Cutting of the stem and thinning had similar effects in that they stimulated transient ethylene evolution by both roots and stems. Bending retarded the height growth to some extent, while increasing the ethylene content especially on the lower side of the horizontal stem. Flooding arrested height growth and increased ethylene evolution in the roots. The two species reacted basically in similar ways, but B. pendula produced more ethylene. though at a slower rate, and also showed a more pronounced retardation of growth. The sprouts had a higher ethylene content than the seedlings and also increased their ethylene evolution more rapidly. The changes in ethylene evolution and growth are discussed from the point of view of the resilience of the plants and as an indication of stress factors in experimental plots.     

Xylem cavitation in roots and stems of Douglas-fir and white fir

Roots of hardwoods have been shown to be more vulnerable to xylem cavitation than stems. This study examined whether this pattern is also observed in a conifer species. Vulnerability to cavitation was determined from the pressure required to inject air into the vascular system of hydrated roots and stems, and reduce hydraulic conductance of the xylem. According to the air-seeding hypothesis for the cavitation mechanism, these air pressures predict the negative xylem pressure causing cavitation in dehydrating stems. This was evaluated for stems of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and white fir (Abies concolor (Gord. & Glend.) Lindl.). The air-injection method was applied to roots and stems of different sizes and positions in Douglas-fir trees. Roots, especially smaller roots with a xylem diameter < 5 mm, were more vulnerable to cavitation than stems. Mean cavitation pressure for smaller roots was -2.09 +/- 0.42 versus -3.80 +/- 0.19 MPa for larger roots (> 8 mm diameter). Within the shoot system, smaller stems (< 5 mm diameter) were most vulnerable to cavitation, having a mean cavitation pressure of -4.23 +/- 0.565 versus -5.27 +/- 0.513 MPa for large stems (> 8 mm diameter). There was no correlation between tracheid diameter and mean cavitation pressure within root or stem systems, despite larger tracheid diameters in roots (23.3 +/- 3.9 micro m) than in stems (9.2 +/- 1.6 micro m). Smaller safety margins from cavitation in roots may be beneficial in limiting water use during mild drought, and in protecting the stem from low xylem pressures during extreme drought.     

Effects of flooding on leaf dynamics and other seedling responses in flood-tolerant Alnus japonica and flood-intolerant Betula platyphylla var. japonica

Two-year-old seedlings of Alnus japonica Steud. and Betula platyphylla var. japonica Hara were flooded from mid-June to early November, to study the effects of flooding on seedling survival and growth, morphological changes in stems and roots, leaf emergence, leaf fall, and leaf longevity. In A. japonica, growth was not affected by flooding, except for a slight decrease in height growth, but some morphological changes of stems and roots were observed, i.e., stem base hypertrophy, hypertrophied lenticels, formation of adventitious roots and development of new roots. In B. platyphylla var. japonica, growth was severely reduced by flooding and all seedlings died by the 20th week of flooding, without showing any adaptive morphological changes in stems or roots. Flooding induced rapid depression of leaf emergence, promoted leaf abscission, and reduced leaf longevity in B. platyphylla var. japonica. In contrast, in A. japonica, basal leaf senescence was delayed in flooded seedlings, thereby extending leaf longevity compared with unflooded seedlings.     

Growth and annual ring structure of Larix sibirica grown at different carbon dioxide concentrations and nutrient supply rates

We compared effects of ambient (360 vpm) and elevated (720 vpm) carbon dioxide concentration ([CO2]) and high and low nutrient supply rates on stem growth, annual ring structure and tracheid anatomy of Siberian larch (Larix sibirica Ledeb.) seedlings over two growing seasons. Elevated [CO2] had no significant effect on either stem height or diameter growth; however, both stem height and diameter growth were enhanced by the high nutrient supply rate, and these increases were stimulated by elevated [CO2]. Elevated [CO2] tended to increase the width of the annual xylem ring, the number of cells in a radial file spanning the ring, and tracheid lumen diameter, whereas it tended to reduce cell wall thickness, although there were no statistically significant CO2 effects on tracheid anatomy. Changes in tracheid cell morphology seemed to be dependent on changes in shoot elongation rates.     

Comparison of the responses to flooding of seedlings and cuttings of Gmelina

The effects of flooding on Gmelina arborea Roxb. seedlings and cuttings of the same parent stock were compared to determine their suitability as transplanting stock. Flooding caused reductions in stomatal conductance, xylem pressure potential and dry matter accumulation in both groups of plants. In seedlings, flooding induced formation of hypertrophied lenticels, stem hypertrophy and production of short, thick, adventitious roots in seedlings, whereas in cuttings, only thin roots and numerous smaller lenticels were induced. For 8 days after the flooding treatment ended, the flooded seedlings grew faster than control seedlings, whereas in cuttings, post-flooding growth was similar to that of control plants. It is suggested that seedlings may perform better than cuttings in very wet or saturated soil.     

Growth and nutrient use efficiency of water tupelo seedlings in flooded and well-drained soil

Growth characteristics and nutrient utilization rate of Nyssa aquatica L. seedlings grown in pots containing flooded or well-drained soil were compared in a greenhouse study. For most of the growing season, relative height and diameter growth rates, and biomass accumulation rates were greater for seedlings in flooded soil than for seedlings in well-drained soil. The concentration of Fe in the roots of seedlings in flooded soil was almost tenfold greater than that of seedlings in well-drained soil. However, flooding had no effect on foliar Fe concentrations. The flooding treatment resulted in decreased concentrations of N in all component parts and increased concentrations of P in the roots and stem, but it had no effect on foliar P concentrations. In response to flooding, foliar K concentrations decreased, whereas the concentration of K in the roots increased. Flooding had no effect on the K concentration of the stem. Seedlings in flooded soil produced more total biomass per milligram of nutrient absorbed than seedlings in well-drained soil, suggesting that N. aquatica seedlings are more efficient at producing biomass and height growth under hydric conditions than under mesic conditions.     

Effects of applied gibberellins and uniconazole-P on gravitropism and xylem formation in horizontally positionedFraxinus mandshurica seedlings

The present study deals with the effects of gibberellins (GA₃ GA₄) and uniconazole-P, an inhibitor of gibberellin biosynthesis, on negative gravitropism and xylem formation in the stems of horizontally positioned, 2-year-oldFraxinus mandshurica Rupr. var.japonica Maxim. seedlings. Each growth regulator (100 g) dissolved in 5 l acetone (50%) was applied to the basal node of the current shoot on May 24, 1995. The same treatment was repeated five times weekly until June 28. Five seedlings were used for each treatment. The seedlings were positioned horizontally 24h after the first application on May 25. Within 5 weeks the horizontal stem of control and GA-treated seedlings exhibited negative gravitropism. In contrast, the application of uniconazole-P inhibited negative gravitropic stem bending. The application of GAs increased the number of gelatinous fibers having thickened cell walls on the upper side of stems. The uniconazole-P application decreased xylem cell formation but did not inhibit the formation of gelatinous fibers. These results indicate that not only the differentiation of gelatinous fibers but also xylem increment is important in the negative gravitropism of horizontally positionedF. mandshurica seedlings. These results also suggest that GAs may be involved in xylem cell :formation rather than the differentiation of gelatinous fibers in this species.Part of this research was reported at the 23rd annual meeting of the Plant Growth Regulation Society of America, Calgary, Canada, July 1996     

Seasonal changes in stem radius and production of new tracheids in Norway spruce

The progress of xylem formation in Norway spruce (Picea abies (L.) Karst.) was measured during one growing season in southern Finland. Stem radius was monitored continuously with band dendrometers, and the formation of new tracheids was determined by examination of small increment cores taken twice weekly. Tracheid production started in June and ceased in August. Xylem formation was fastest in early July, when 0.75-1.25 new tracheids were formed per day. The rate of xylem formation was significantly correlated with mean daily temperature. Synchronous fluctuations in tracheid and lumen diameters were observed at the same relative positions within each annual ring, but no relationship existed between the diameters and weather variables. The timing of changes in stem radius differed from the timing of actual xylem formation. Stem radius increased in April and May, and the fastest daily increments were recorded in June. Increases in stem radius slowed in July, but small increases were measured more than a month after xylem formation had ceased. Daily changes in stem radius were correlated with daily precipitation, reflecting changes in stem water content. Therefore, dendrometers are of dubious value for measuring the timing of actual xylem formation. Small increment cores proved to be useful in assessing actual xylem formation, but the method is laborious.     

The nutrient translocation of seedlings with chemical analyses of the xylem saps ofCryptomeria japonica D. Don andPinus densiflora Sieb. et Zucc.

The chemical compositions of xylem saps ofCryptomeria japonica D. Don andPinus densiflora Sieb. et Zucc. were studied. One-year-old seedlings of these species were transplanted into hydroponics in the middle of June. Some seedlings were retained in the nursery. Two nitrogen sources, ammonium and nitrate nitrogen, were applied separately. The samplings of xylem saps were made at the end of July. Saps were collected from both roots and tops of the seedlings using a pressure chamber. InC. japonica, the fact that the translocation form of nitrogen from the roots to shoots mainly was citrulline was confirmed. The result ofP. densiflora in this study showed that glutamine was the predominant compound of nitrogen translocation. Total nitrogen ofP. densiflora had a large value in the series of ammonium. Glutamine increased in the ammonium series of both species. Nitrate nitrogen was found in whole samples ofC. japonica, and was not found inP. densiflora. As the result of cations in the saps, monovalent cations were rich inP. densiflora, and divalent cation concentrations were greater inC. japonica. The idea that the great oxygen requirement ofP. densiflora produces the negative charge gives an explanation of the great retention of divalent cations in the roots. The idea that the mineral distribution indicates root activity related to its negative charge production also is considered a possibility.     

How timing of stem girdling affects needle xylem structure in Scots pine

While needles represent a proportionally large fraction of whole-plant hydraulic resistance, no studies to date have investigated how source–sink disturbances affect needle xylem structure. In this study, we evaluated structural changes in xylem in current-year needles of Scots pine 227 and 411 days after stem girdling (hereafter referred to as DAG). Maximum and minimum tracheid lumen diameters and therefore also the size of tracheid lumen areas increased in needles 227 DAG compared to control needles. In contrast, tracheid dimensions were similar in needles 411 DAG as in the control needles, but smaller xylem area and lower number of tracheids resulted in the lower theoretical needle hydraulic conductivity of those needles. Several needle xylem parameters were intercorrelated in both control and girdled trees. These observed changes provide a new understanding of the processes that occur following a source–sink disturbance. Considering anatomical parameters such as the number of tracheids, tracheid dimension, or needle xylem area, which are rarely described in physiological studies, could be helpful, for example, in understanding to tree hydraulic systems or for modeling gas exchange. Finally, empirical equations were developed to calculate needle theoretical hydraulic conductivity and the number of tracheids in needles using an easily measurable parameter of needle xylem area.     

Tracheid length in relation to seedling height in conifers

Summary Changes in tracheid length during the development of primary and secondary xylem are analysed in relation to the division and elongation of cambial initials, in first year seedlings ofPinus sylvestris andPicea sitchensis. During primary and early secondary growth tracheid length varies with leaf internode length, but this relationship soon becomes obscured during later secondary growth. Correlations between seedling height and tracheid length thus depend on variation in tracheid length with radial xylem increment before and after terminal bud production. Since the relation between shoot and tracheid length is a complex one, the clonseness of the correlation varies with environmental treatment, but overall, a doubling of shoot length increased tracheid length by about 10%.I am grateful to ProfessorS. D. Richardson andDr. G. K. Elliott for many helpful suggestions. This work was financed by a grant from the Natural Environment Research Council. Plants were grown from seed kindly supplied by the Forestry Commission Research Division (Scotland and North England).     

人工兴安落叶松次生木质部的解剖学研究

运用木材解剖图像分析系统和显微照相的方法对人工兴安落叶松次生木质部的解剖结构进行研究,结果表明:落叶松具正常树脂道和受伤树脂道两种类型,前者常见于晚材。落叶松生长轮内的早晚材在干和枝内急变,在根内缓变。早材管胞呈六边形至多边形,胞壁常见单列具缘纹孔,偶见对列具缘纹孔;晚材管胞多呈矩形,胞壁鲜见具缘纹孔,通常为单列具缘纹孔。落叶松木射线同时具有单列木射线和纺锤形木射线两种类型,纺锤形木射线中仅含一枚纵行树脂道。纵行管胞与木射线交叉形成的纹孔场为云杉型。从根到干再到枝,管胞逐渐细化,管胞长度逐渐减小,木射线分布由密到疏。     

Responses of water-stressedPinus thunbergii to inoculation with avirulent pine wood nematode (Bursaphelenchus xylophilus): Water relations and xylem histology

The effect of water-stress conditioning on water relations and histological features ofPinus thunbergii Parl. inoculated with avirulent isolate ofBursaphelenchus xylophilus (Steiner and Buhrer) Nickle, pine wood nematode, were investigated. Pines were kept under 8 days cycle of severe water stress. One-half of the water-stressed pines died as a result of infection by avirulent pine wood nematode and water stress tended to induce increased susceptibility and/or decreased resistance of pines to avirulent pine wood nematode. In dead pines, the water conducting function of xylem was lost, and all of the parenchyma cells died. In surviving pines, the xylem hydraulic conductivity and the xylem water content were significantly reduced (12 to 23% and 77 to 83%, respectively) compared to controls. Safranin dye perfusion of excised axis stem segments indicated that the water conductance was limited to the very narrow peripheral area of xylem. Embolism caused by cavitation in the tracheids occurred in the central part of xylem and in that dysfunctional region of the xylem the axial parenchyma cells surrounding the epithelial cells, and ray parenchyma cells partly degenerated but the epithelial cells survived. The disruption of tracheid shape observed in surviving pines indicates that avirulent pine wood nematode temporarily disturbed cell division of the cambium. Considering the differences in responses between dead pines and surviving pines after inoculation with avirulent pine wood nematode, the death of water-stressed pines apparently resulted from death of cells, in particular the vascular cambium and the loss of xylem hydraulic function by cavitation.     

Ethylene production during symptom development of pine-wilt disease

Ethylene release from Pinus thunbergii Parl. seedlings was examined after inoculation with virulent and avirulent isolates of Bursaphelenchus xylophilus. Enhanced ethylene release was observed only in seedlings inoculated with the virulent isolate. The avirulent isolate induced necrosis of xylem ray cells and embolism in some tracheids, however, the cambium and outermost xylem was intact where water conduction was maintained. The ethylene increase coincided with cambial death in seedlings inoculated with the virulent isolate.     

Potassium distribution in black heartwood of sugi (Cryptomeria japonica) I: Localization in axial parenchyma cells

This is the first report to ascertain potassium location in black heartwood of sugi (Cryptomeria japonica). The objective of this study was to understand the distribution of potassium in sugi black heartwood in connection with anatomical morphology. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDXA) was used to investigate the distribution of inorganic elements, especially potassium. In black heartwood, potassium was detected in all tissues (tracheids, ray parenchyma, and axial parenchyma), but its concentrations were different in each xylem tissue. Potassium was particularly accumulated in the droplets in axial parenchyma cells. In addition, calcium was also detected in all tissues. Phosphorus sometimes existed in the granules in ray parenchyma cells. From optical microscopy, the axial parenchyma cells were observed to be arranged in a tangential direction on transverse section and the droplets in these cells were distributed in areas of high ash and green moisture content. For red heartwood, potassium detected by SEM-EDXA was far lower in all tissues than in the black heartwood, while the trend for calcium was the converse. Potassium in axial parenchyma cells was more abundant in black heartwood than in typical pale red heartwood.     

Growth,morphology and photosynthetic activity in flooded <Emphasis Type="Italic">Alnus japonica</Emphasis> seedlings

This study was conducted on Alnus japonica seedlings subjected to flooding for 2, 4, and 6 weeks to examine responses in growth, morphology, and photosynthesis to different periods of flooding. Seedlings subjected to flooding for 2 and 4 weeks were drained after flooding then watered daily. Increases in biomass of leaves, roots, and whole plants were less for 6-week-flooded seedlings. Rate of photosynthesis and stomatal conductance of flooded seedlings decreased within 2 weeks. For 2-week-flooded seedlings recovery from reduced stomatal conductance and recovery of photosynthetic activity occurred after drainage. For the 6-week-flooded seedlings stomatal conductance recovered by the end of the experiment. Adventitious root formation by the 4 and 6-week-flooded seedlings was observed from the third week of flooding. These results suggest that recovery of reduced function in leaves may progress with development of adventitious roots during the period of flooding.