Control of Water Loss by Actively Growing Sitka Spruce Seedlings after Transplanting

The roots of actively growing Sitka spruce seedlings were disturbedand damaged to different degrees by transplanting treatmentscarried out in a growthroom. The treatments stopped root extensionand after 2–4 h the transpiration rate of treated plantsdeclined, indicating closure of the stomata, and transpirationcontinued to decrease for a few days. The hydraulic conductivityof the root system was reduced slightly by the treatment butleaf water potential increased or remained unchanged, thereforeno water stress occurred to account for closure of the stomata.Measurements on detached shoots indicated that the water storagecapacity was too small to accommodate changes in the vascularwater supply without reducing leaf water potential. The measuredresponses of the plant were not affected by girdling the baseof the stem prior to root treatment. It is suggested that thetranspiration rate of treated plants was reduced as a resultof a chemical stimulus originating in the root and acting onthe stomata.     

Water Relations of Sitka Spruce Seedlings After Root Damage

Sitka spruce[Picea sitchensis(Bong.) Carr] seedlings were subjectedto varying degrees of root damage in a growth room, rangingfrom careful transplanting to exposure of the root system toair for up to 3 h. After replanting, transpiration (E), leafwater potential (₁) and growth of the shoot and root were measuredand observations made on plant survival. Some plants in the root exposure treatments died 20–85days after planting. In plants which eventually died, E wasdepressed directly after treatment, but ₁ showed a variableresponse. In some plants ₁ decreased from —8·0x 10⁵ to —30 x 10⁵ Pa after only 10 days but in othersthere was little change in ₁ for 50 days. In spite of the maintenanceof a high water potential in some of the latter plants for longperiods, no root or shoot growth occurred. In plants which lived, the root damage reduced root and shootgrowth relative to untreated controls, and most treatments stronglydepressed E but had little or no effect on ₁. The changes of E and ₁ in treated plants suggest that the suppressionof E was often independent of ₁ although water stress eventuallydeveloped in some of the severely treated plants. Sitka spruce, Picea sitchensis (Bong.)Carr, water relations, root damage, transpiration, leaf water potential     

Leaf Conductance as Related to Xylem Water Potential and Carbon Dioxide Concentration in Sitka Spruce

Current year shoots of Sitka spruce [Picea sitchensis Bong. (Carr.)] from the forest canopy were equilibrated in a leaf chamber. The shoots were excised in air, and removed at differing times in order to establish a relationship between stomatal conductance and xylem water potential. The experiment was repeated at five ambient CO₂ concentrations. A second set of excised forest shoots, and shoots excised from 2-year- old nursery seedlings were allowed to evaporate freely in a controlled environment wind tunnel until a constant rate of transpiration was measured, to establish a relationship between cuticular conductance and xylem water potential. Cuticular conductance was estimated to be 0.012 cm s^-1 at high water potential and declined linearly to 0.007 cm s^-1 at ?3.5 MPa. The implication of this decline in the subsequent calculation of stomatal and mesophyll conductance is considered. Stomatal conductance remained constant at water potentials above ?1.4 MPa and was not affected by ambient carbon dioxide concentrations between 20 and 600 cm^-3. At lower water potentials, stomatal conductance declined and approached zero at ?2.5 to ?2.6 MPa. The results suggest that stomatal aperture is not controlled by either ambient or intercellular space carbon dioxide concentration, and that stomatal closure at low water potential is unlikely to be mediated by carbon dioxide.     

Critical Water Potential for Stomatal Closure in Sitka Spruce

Steady state rates of net photosynthesis and stomatal conductance at high water potentials were measured under controlled conditions in a leaf chamber on Sitka spruce [Picea sitchensis (Bong.) Carr.] shoots detached from the forest canopy or on seedlings. The water supply to the seedlings was terminated by excision and the shoot water potential (or critical water potential) and osmotic potential at the onset of stomatal closure measured. The turgor potential was calculated. The initial osmotic potential before insertion of the shoot into the chamber was also measured. Shoot water potential and osmotic potential at stomatal closure, and initial osmotic potential were significantly higher (less negative) in foliage from the lowest level in the canopy compared with foliage in the upper canopy, and higher in shoots of seedlings transferred to low light than in those at high light. Critical water potential also varied with season, being higher in July than in October and November. In all except one instance, turgor potential at the onset of stomatal closure was negative, possibly because of dilution of the cell sap by the extracellular water during the estimate of osmotic potential. Over all the experiments variation in critical water potential was correlated with variation in critical osmotic potential and, to a lesser extent, the initial osmotic potential. However, turgor potential at the critical potential varied from +0.6 to -4.6 bar. This suggests that difference in turgor between the guard cells and subsidiary cells, which controls stomatal aperture, is only loosely coupled with the bulk leaf turgor and hence that bulk leaf turgor is not a good index of the turbor relations of the guard cells.     

Effects of Waterlogging on Water Relations of Actively-growing and Dormant Sitka Spruce Seedlings

Two-year-old Sitka spruce [Picea sitchensis (Bong.) Carr.] seedlings,either actively growing or dormant, were waterlogged in a growthroom at 15 °C. Shoot and root growth, transpiration andleaf water potential were observed. In actively-growing plants shoot extension continued after waterlogging,though at a reduced rate, and shoots of dormant plants brokebud and extended during the waterlogging period. Root growthwas suppressed by waterlogging in both types of plant. The 22day waterlogging treatment eventually killed the actively-growingplants but plants which were dormant at the time of waterloggingwere more tolerant. Changes in plant water relations after waterloggingwere entirely different depending on the condition of the plantswhen the soil was flooded. Dormant plants showed a gradual reductionin transpiration and increased water stress over the waterloggingperiod; after the soil was drained leaf water potential increasedto equal the value of control plants which had been maintainedin a freely drained condition, but transpiration did not increaseuntil root growth began. Actively-growing plants exhibited amore complex behaviour, characterized by a very rapid reductionin transpiration after waterlogging, accompanied by a briefperiod of water stress, followed by a period of increasing transpirationrate in the absence of water stress. Finally a second reductionin transpiration occurred and water stress increased as theseedlings died. The importance of the stage of activity of theroot system to the response of plants to waterlogging is discussed. Picea sitchensis (Bong.) Carr., Sitka spruce, waterlogging, water relations, dormancy, transpiration, water potential     

The Association between Soil and Xylem Water Potential, Leaf Resistance, and Abscisic Acid Content in Droughted Seedlings of Douglas-fir (Pseudotsuga menziesii)

During progressive stages of drought and subsequent recovery through rewatering, foliage of 2-year-old seedlings of Douglas-fir was sampled for water potential, stomatal resistance, and abscisic acid content (ABA). Simultaneous measurements of soil matric potential were also made. There was a rapid rise in stomatal resistance in the range of 1500–2000 ng ABA g^?1 dry weight of foliage corresponding with a plant water potential of –10 to –12 bars. Soil matric potential seemed more closely associated with ABA increase than did plant water potential. After rewatering, seedlings returned to normal ABA levels within 6 days.     

The Effect of Scion Water Potential on Graft Success in Sitka Spruce (Picea sitchensis)

The effect of scion storage on graft success in Sitka sprucehas been examined. Scions kept in cold store for up to 7 d werefound to graft successfully to active rootstocks in about 95%of cases. From 7 d, however, there was a marked decline in successuntil, after 14 d of storage prior to grafting less than 40%of grafts survived. Scion leaf-water potential decreased continuouslyduring storage and fell dramatically during the first 3 d aftergrafting. In successful grafts, scion water potential then recovered,reaching a stable level after 3 weeks. No recovery occurredin grafts, which eventually failed. The relationship betweenscion leaf water potential and graft success rate is discussedwith reference to the role of callus development in graft formation. Picea sitchensis, water potential, graft success     

The Effects of Shoot Water Status on Some Photosynthetic Partial Processes in Sitka Spruce

Clonal cuttings of Picea sitchensis (Bong.) Carr. were grown in a controlled environment and, after completion of shoot extension and maturation, subjected to a drying cycle. Photosynthesis and stomatal conductance were measured in situ using ¹⁴CO₂ and a porometer, respectively. Shoot water potential was measured with the pressure chamber. Photosystem and carboxylase activities of chloroplast preparations were measured in vitro. A considerable fall in photosynthetic rate occurred at low water potential. This was associated with stomatal closure and a decrease in CO₂ transfer or fixation processes in the mesophyll. Little change in activity of photosystem I, photosystem II, and ribulose 1,5-diphosphate carboxylase was detected during the drying cycle. Any decline in activity of the photosynthetic partial processes in vivo under severe water stress (Ψ < – 30 bar) was probably masked in vitro as a result of rehydration prior to assay.     

Annual Variations in Competition Symmetry in Even-aged Sitka Spruce

Understanding of the temporal development of competition symmetry,size hierarchy and the role of episodically supplied resourcesin forest stands is limited. Because of intrinsic differencesin the symmetry of competition for light and below-ground resources,it is hypothesized that the overall expression of competitionvaries as a consequence of variations in water availability.Historical growth and precipitation data from an unthinned standof Sitka spruce (Picea sitchensis(Bong.) Carr.) in Denmark areanalysed with respect to the effects of precipitation on competitionsymmetry. The shape and linearity of the size-increment relationshipare analysed, and the temporal development of diameter distributionand size hierarchy is modelled. The test value of lack of linearfit through the origin for the diameter/increment relationshipplotted against precipitation in the growth period forms a triangularenvelope, indicating a more curvilinear relationship for yearswith high water availability. This indicates that dominant treesapparently benefit more from increased precipitation than suppressedtrees—which is also shown in the analysis of the temporaldevelopment of the diameter distribution—and as a consequencethe size hierarchy increases. Finally, it is proposed that temporaland spatial variations in the availability of water and otherbelow-ground resources may have similar effects on competitionsymmetry. Copyright 2001 Annals of Botany Company Competition symmetry, size hierarchy, precipitation, temporal development, Picea sitchensis     

Leaf Age as a Determinant in Stomatal Control of Water Loss from Cotton during Water Stress

The stomatal resistance of individual leaves of young cotton plants (Gossypium hirsutum L. var. Stoneville 213) was measured during a period of soil moisture stress under conditions of constant evaporative demand. When plants were subjected to increasing soil water stress, increases in stomatal resistance occurred first on the lower leaves and the stomata on the upper surfaces were the most sensitive to decreasing leaf-water potential. Stomatal closure proceeded from the oldest leaves to the youngest as the stress became more severe. This apparent effect of leaf age was not due to radiation differences during the stress period. Radiation adjustments on individual leaves during their development altered the stomatal closure potential for all leaves, but did not change the within-plant pattern. Our data indicate that no single value of leaf water potential will adequately represent a threshold for stomatal closure in cotton. Rather, the stomatal resistance of each leaf is uniquely related to its own water potential as modified by age and radiation regime during development. The effect of age on stress-induced stomatal closure was not associated with a loss of potassium from older leaves. Increases in both the free and bound forms of abscisic acid were observed in water-stressed plants, but the largest accumulations occurred in the youngest leaves. Thus, the pattern of abscisic acid accumulation in response to water stress did not parallel the pattern of stomatal closure induced by water stress.     

Leaf Elongation in Relation to Leaf Water Potential in Soybean

Leaf water potential, turgor pressure, and leaf elongation ratewere measured in soybeans growing in controlled environmentchambers, greenhouses, and outdoors. Plants in chambers hadthe highest water potentials and turgor pressures, and plantsoutdoors the lowest. In all three environments there was a linearrelationship between elongation rate and turgor pressure. Leavesof plants in drier environments required less turgor for elongation,and showed a greater increase in elongation rate per unit increasein turgor. Elongation rates over a 72 h period were equal inthe three environments. Leaves reached the largest final sizein the greenhouse (intermediate in water potential). Epidermalcells were larger in chamber- and greenhouse-grown leaves thanin leaves of plants grown outdoors. The number of epidermalcells per leaf was greater in the greenhouse and outdoors thanin the chamber. Leaf elongation characteristics of greenhouseplants were duplicated by mildly stressing chamber plants, andleaf elongation characteristics of field plants were duplicatedby more severely stressing chamber plants. Leaves of mildlystressed chamber plants also reached a larger final size thanleaves of more severely stressed chamber plants, or leaves ofcontrol plants in the chamber. Water stress in the chamber increasedthe number of epidermal cells per leaf. More severe water stressin the chamber reduced epidermal cell size. Based on the waterstress experiments it is concluded that the differences in plantwater status in the chamber, greenhouse, and field caused differencesin elongation characteristics, and were responsible for thedifferences in leaf size.     

Dissolved inorganic nitrogen concentrations and fluxes in three British Sitka Spruce plantations

A catchment approach was used to estimate mean dissolved inorganic-N concentrations and fluxes through three mature Sitka spruce plantations at Beddgelert (north Wales), Plynlimon (mid-Wales) and Kershope (Cumbria). Dissolved inorganic-N in bulk precipitation, throughfall, stemflow, soil water at 4 or 5 depths, and streamwater was measured every two weeks for periods of 1 or 2 years at all three sites. Bulk precipitation inputs at the three sites varied from 6 to 10.4 kg N ha^-1 a^-1 and stream outputs varied from 6.4 to 13.6 kg N ha^-1 a^-1. Beddgelert Forest had the highest inputs and outputs and is Nitrogen saturated according to certain Scandinavian criteria (Nilsson, 1986). All three sites had much higher outputs than might be expected from the magnitude of the inputs, since conifer forests are normally regarded as being conservative with respect to N. Only at Plynlimon were reductions in dissolved inorganic N flux or concentration observed from bulk precipitation to throughfall and soil waters. At the other two sites, the tree canopies did not assimilate N in incoming rainfall, and active nitrification resulted in high concentrations of nitrate in soil and streamwater. It is proposed that many mature western upland Sitka spruce plantations may behave in a similar manner with respect to dissolved N, in contrast to sites in eastern Scotland where rates of nitrification are slower and nitrogen appears to be less available to plantation trees.     

Matric Water Potential of Leaf Tissue--Measurement and Significance

A thermocouple psychrometer method, previously described foruse in determining tissue and sap water potentials, has beenadapted for determining matric potentials. Matric values ranging from approximately zero to –10 x10⁵ Pa were observed with wilting wheat leaves. Mean valuesof total tissue water potential and of its solute (osmotic),matric, and cell wall pressure components were –16.6,–18.3, –30 and +4.7 Pa x 10⁵, respectively. Matric potentials are often ignored in investigations of plantwater relations. This practice is shown to result in underestimationof wall pressures (in this case by a mean of 64 per cent) andsometimes in spuriously negative values.     

Graft Formation in Sitka Spruce: A Scanning Electron Microscope Study

Low-temperature and conventional scanning electron microscopyhave been used to examine the callus formed at the graft interfacein Picea sitchensis (Bong.) Carr. Callus cells are producedby both cambium and ray parenchyma dedifferentiation and redifferentiationin scion and stock. Adhesion between the cells derived fromscion and rootstock is thought to be my means of pectinaceousbeads at the surface of the callus cells, preceding a more generalfusion of cell walls. The cambia of the two graft componentsare prevented from growing towards each other by the presenceof callus. Instead, the differentiation of new cambium withinthe callus, in the vicinity of the cambia exposed at the preparedsurfaces of the scion and rootstock, links them to form a continuouscambial layer around the combined stem. Callus, cambium, differentiation, grafting, Picea sitchensis     

Effect of Excision on Leaf Water Potential

Improvements of thermocouple hygrometric techniques for in situleaf water potential measurement in the field now allows forcontinuous monitoring of water potential in response to an externalperturbation, such as leaf excision. Using Citrus jambhiri plants,measured leaf water potentials of completely excised leaf portionsimmediately increased when the petiole was excised or incisionswere made either transverse or parallel to the midrib. Incisionsparallel to the midrib were on the side nearest the hygrometeror opposite it if preceded by a petiole excision. Midrib incisionswere 100–150 mm long with the nearest cut edge being 20–50mm from the hygrometer cavity. All excisions were such thatleaf tissue was removed from the leaf with water potential onone of the leaf portions being measured continuously prior toand after excising. The peak increase in measured water potentialof the excised leaf portions ranged between 20 kPa and 80 kPabut averaged 50 kPa. In uncovered leaves, particularly underfield conditions with the associated high evaporative demand,measured leaf water potential declined rapidly after the initialincrease. The increase in measured water potential immediatelyfollowing various types of excision was confirmed for dark andlight conditions (laboratory and field respectively) using bothpsychrometric and dewpoint modes and occurred for secondaryexcisions, but to a lesser extent. Discovery of this phenomenonimplies that water potential measured on detached leaves maynot always represent accurately in situ leaf water potential. Key words: Leaf water potential, Thermocouple hygrometers, Leaf excision effects     

Photoactivation of Oxygen-evolving System in Dark-grown Spruce Seedlings

Plastids prepared from dark-grown spruce seedlings showed a negligible activity of photosystem II, and no fluorescence variation was observed during actinic illumination. The photosystem II reaction centre, however, was present in primary thylakoids. Exposure of such seedlings to continuous light induced the development of photosystem II activity via three stages (rapid, lagged and gradual), and the variable fluorescence appeared. The rapid development of photosystem II may be attributed to the activation of the oxygen-evolving system, possibly the manganese-catalyzing site, and the lagged and gradual developments may be closely related to the formation of thylakoid membranes and their assembly to grana.     

Water Potential and Leaf Elongation in Radiata Pine and Wheat

The rate of leaf elongation in radiata pine (Pinus radiata) and wheat seedlings was closely related to the osmotic potential of the rooting solution. Sudden stress application and removal treatments caused immediate changes in the leaf elongation rate and a new steady-state rate independent of the old was quickly established. The osmotic potential in the active elongation zones of a leaf differed from that of the remainder of the leaf and was considered more appropriate for deriving turgor pressure to be related to leaf elongation. Shifts in elongation rate were associated with corresponding shifts in total leaf water potential in both species and with corresponding shifts in turgor pressure in wheat. There was a linear correlation between the pitch of the helical thickenings in the protoxylem of radiata pine needles and their elongation rate. A record of elongation rate is, therefore, preserved in the needle. Developmental aspects of the deposition of the helix are discussed.     

Diurnal Changes in Citrus Leaf Thickness, Leaf Water Potential and Leaf to Air Temperature Difference

Diurnal changes in leaf water potential and leaf thickness ofwell-watered citrus trees were found to be highly correlated.Midday decreases in leaf thickness of about 30–35 µm reflected midday decreases in leaf water potential of about1.1–1.3 MPa from predawn values. Leaf water potentialwas also correlated with changes in leaf-to-air temperaturedifference and ambient vapour pressure deficit. Leaf thicknessas well as leaf to air temperature difference could possiblybe used to monitor leaf water status continuously as an indicatorof citrus tree water stress.     

Diel Patterns of Water Potential Components for the Crassulacean Acid Metabolism Plant Opuntia ficus-indica when Well-Watered or Droughted

Under well-watered conditions, chlorenchyma acidity in cladodes of Opuntia ficus-indica increased substantially at night, fully accounting for the 0.26-megapascal nocturnal increase in osmotic pressure in the outer 2 millimeters. Osmotic pressure in the inner part of the chlorenchyma and in the water-storage parenchyma did not change significantly over 24-hour periods. Three months of drought decreased nocturnal acid accumulation by 73% and essentially abolished transpiration; also, 27% of the chlorenchyma water and 61% of the parenchyma water was lost during such drought, but the average tissue osmotic pressure was little affected. Turgor pressure was maintained in the chlorenchyma after 3 months of drought, although it decreased sevenfold in the water-storage parenchyma compared with the well-watered condition. Moreover, the nocturnal increases in turgor pressure of about 0.08 megapascal in the outer part of the chlorenchyma was also unchanged by such drought. The water potential magnitudes favored water movement from the parenchyma to the chlorenchyma at the end of the night and in the reverse direction during the late afternoon. Experiments with tritiated water support this pattern of water movement, which is also in agreement with predictions based on electric-circuit analog models for Crassulacean acid metabolism plants.