Stomatal and nonstomatal limitations to net photosynthesis in seedlings of woody angiosperms

Comparative responses of net photosynthesis (A) to water stress in woody species from a variety of habitats were studied to assess the relationship between photosynthetic attributes and drought tolerance. Stomatal and nonstomatal limitations to A were compared in three-month-old white oak (Quercus alba L.), post oak (Quercus stellata Wangenh.), sugar maple (Acer saccharum Marsh.), and black walnut (Juglans nigra L.) seedlings during a drying cycle. Relative stomatal limitation of photosynthesis (I) was less than 50% in all species except for Q. stellata seedlings subjected to severe water stress. No significant changes in I were observed in Q. alba and J. nigra before, during, and after drought. In A. saccharum, I was generally low and decreased significantly under water stress. Under well-watered conditions, A was highest in Q. stellata, intermediate in Q. alba, and lower in A. saccharum and J. nigra. High A in well-watered Q. stellata was associated with high stomatal conductance and carboxylation efficiency, whereas low A was associated with low stomatal conductance and carboxylation efficiency in A. saccharum and low stomatal conductance, low carboxylation efficiency, and high CO₂ compensation point in J. nigra. Under severe water stress, A, carboxylation efficiency, and stomatal conductance decreased substantially in all species; however, Q. stellata had the highest carboxylation efficiency and lowest CO₂ compensation point under these conditions. After 5 days at high soil moisture after drought, stomatal and mesophyll components of A in A. saccharum and J. nigra had not recovered to predrought levels, whereas they had completely recovered in Q. stellata and Q. alba. The photosynthetic apparatus, especially mesophyll components, of drought-tolerant Quercus species showed either less inhibition under water stress, superior recovery to predrought capacity, or both. Exposure of the leaves to ¹⁴CO₂ indicated apparent asymmetric stomatal closure for mildly water-stressed seedlings, but not for leaves of well-watered, severely stressed, or rehydrated plants. These results suggest that patchy stomatal closure under mild water stress might be important for water stress-induced inhibition of photosynthesis, but not under the more severe water stress imposed in this study.     

Canopy disturbance patterns and regeneration of Quercus species in two Ohio old-growth forests

This study was conducted to determine the abundance of Quercus species, the spatial pattern of Quercus regeneration, the current canopy disturbance pattern, and their interrelationship in two old-growth deciduous forests in Ohio (Goll Woods and Sears-Carmean Woods). Acer saccharum and Fagus grandifolia had the greatest density and basal area in both forests, yet the largest trees (by basal area) present at each site were Quercus spp. Quercus spp. appeared to be decreasing in abundance in both sites. Though Quercus seedlings were common, few Quercus saplings or subcanopy trees were present. The current disturbance regimes were dominated by small canopy gaps created by death of 1–2 trees; canopy gaps 100 m² in size were rare and only 2.5–2.8% of the forest area was covered by recognizable canopy gaps. No significant differences in the density of Quercus seedlings or saplings were found between gaps and non-gap areas at either site. Though no significant barrier to seedling establishment appeared to exist, the present disturbance regimes are not well suited for the growth of Quercus into the subcanopy size class or the recruitment of Quercus into the canopy. The most frequent gapmakers in Goll Woods were Tilia americana and Acer saccharum, and those in Sears-Carmean Woods were A. saccharum and F. grandifolia. The species most frequent as gap fillers were A. saccharum (in both sites) and F. grandifolia (in Goll Woods). These results suggest that A. saccharum will continue to increase in abundance, and Quercus decrease in abundance, in these two old-growth stands.     

Seasonal water potential changes and proline accumulation in mediterranean shrubland species

We studied the water relations of 6 shrub and 3 tree species typical of the mediterranean climate region of central Spain to identify differential responses to water stress between and within species, and to determine if free proline concentration in leaves could be used as a water stress indicator. Predawn and midday water potentials (w) on a seasonal basis, relative water content (RWC), leaf mass per area, foliar nitrogen and free proline concentrations were measured. The lowest water potentials were observed at the end of the summer, with recovery to higher water potentials in the fall and winter seasons. Species differed regarding the annual w fluctuation. Thymus zygis, Halimium viscosum, Genista hirsuta and Juniperus oxycedrus exhibited the most negative midday and predawn w (both less than -6 MPa) with a large magnitude of response to changing conditions in soil moisture of the upper horizon of the soil. Lavandula pedunculata and Cistus ladanifer showed a moderate response. Quercus rotundifolia, Quercus faginea and Retama sphaerocarpa showed a modest response. The w of different size individuals of Quercus rotundifolia and Cistus ladanifer were compared. The annual w fluctuation was greater in small individuals as compared to large individuals. In every species, there was an increase in proline concentration of bulk leaf tissues when predawn w dropped below -5 MPa. Small plants of Cistus ladanifer reached lower water potentials and also higher concentration of proline than bigger plants. Proline could possibly be used as a drought stress indicator in every species except Q. rotundifolia. It is suggested that in addition to water stress avoidance due to deep root systems, some mechanisms of water stress tolerance may operate among shrub and tree species of central Spain.     

Sex identification by male-specific growth hormone pseudogene (GH-Ψ) in Oncorhynchus masou complex and a related hybrid

It is often difficult to identify sexes of many fish species by conventional cytological method because of the lack of heteromorphic sex chromosomes. Isolation of sex-specific molecular markers is thus important for sexing and for understanding sex chromosome evolution in these species. We have identified genetic sexes by PCR-based male-specificity of a growth hormone pseudogene (GH-) in masu and Biwa salmon, two subspecies of the Oncorhynchus masou complex, and their hybrid Honmasu. PCRs with primers designed from sequences of chinook salmon GH genes amplified GH-I and GH-II fragments in both sexes, but a third GH- fragment was detected in predominant proportion of males and very few phenotypic females. The consistency of phenotypic sex with genetic sex identified by GH- for masu salmon, Biwa salmon and Honmasu was 93.1, 96.7 and 94%, respectively. The remaining individuals showed inconsistency or deviation from sex-specificity: a few phenotypic males lacked the GH-, whereas a few phenotypic females possessed the GH-. Sequence of the putative GH- fragment from such females was identical to that from genetic males, and shared about 95% homology with the corresponding GH- fragment from chinook salmon. This result confirmed that these females were really GH--bearing individuals. PCR analyses with primers designed from masu salmon GH- gave identical results, indicating that the absence of GH- in a few males was not resulted from primer mismatching. These GH--bearing females and GH--absent males were more likely to originate from spontaneous sex reversion than from crossing-over between GH- and the sex determination gene/region.     

Successional trends and apparent <Emphasis Type="Italic">Acer saccharum</Emphasis> regeneration failure in an oak-hickory forest in central Missouri,USA

Permanent vegetation plots established in 1968 at the Baskett Research and Education Area (BREA) were remeasured in 2004/2005 to assess trends in canopy composition, vegetation–environment relationships, and regeneration patterns. Quercus spp. [particularly Quercus alba (white oak)] remained dominant in BREA forests. However, Nonmetric Multidimensional Scaling Ordination of a dataset composed of tree importance values from both samples indicated the two most common community types (Dry Ridge and Slope; Mesic Slope) were shifted away from Quercus spp. abundance toward greater importance of Acer saccharum (sugar maple) in ordination space. Plots in glade-like environments shifted toward greater Juniperus virginiana (eastern redcedar) importance. Environmental analysis indicated that soil properties (pH, organic matter, Ca, Mg, and K) and slope position were highly correlated with ordination axes. Saplings of Quercus spp. exhibited greatly reduced abundance in the most recent sample and the reduction in Quercus spp. density was highest in plots with greatest canopy cover. While Acer saccharum saplings were still quite abundant in BREA forests, smaller sapling size classes were becoming notably depleted, reflecting growth of a cohort of plants established before the 1968 sample toward and into tree-sized individuals. Seedlings surveys emphasized that A. saccharum seedlings have become very rare in BREA forests, while Quercus spp. seedling densities are higher, but likely below those needed for eventual adequate recruitment to the canopy. Although a number of mechanisms to explain apparent A. saccharum regeneration failure can be developed, herbivory, recent climate trends (warming late-winter conditions) and increased litter layer depth are the most plausible explanations for the observed scarcity of A. saccharum seedlings. Although increasing importance of A. saccharum in the canopy of BREA forests appears inevitable in coming decades, longer-term trends are unclear given the shortage of seedling regeneration.     

Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher , g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and , but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO₂ concentration relationships were consistent with the explanation that increased allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low- limitations on non-stomatal components of A. Our working hypothesis is that galls increase and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.     

Effects of domestication and agronomic selection on phytoalexin antifungal defense in <Emphasis Type="Italic">Phaseolus</Emphasis> beans

Systems of wild and cultivated relatives are good experimental systems to test chemical defense theory because they provide closely related varieties that differ in discrete traits. To determine the relationship between resistance and chemical defense diversity among wild, landrace and cultivar accessions of Phaseolus vulgaris, we measured resistance to fungal infection in laboratory and field experiments, quantified phytoalexin diversity, and assessed the effectiveness of phytoalexin mixtures extracted from live tissue. Results show a gradient of resistance to fungal infections between wild, landrace and cultivar varieties. In the laboratory, wild seedlings were more resistant (93% non-infected) than landrace seedlings (80% non-infected) and modern cultivar seedlings (68% non-infected). Under field conditions wild seedlings were more resistant (97% non-infected) than cultivar seedlings (71% non-infected). Wild seedlings presented the highest phytoalexin diversity (H=1.11), while those of the landrace presented an intermediate level (H=0.97) and cultivar seedlings presented the lowest diversity (H=0.93). No differences were found in total concentrations. The in vitro inhibitory properties on hyphal growth of the isoflavonoid mixtures produced by individual seedlings showed a similar trend. Our results are consistent with similar gradients in other species of Phaseolus beans and resistance to Colletotrichum sublineolum in sorghum.     

Gas Exchange,Photochemical Efficiency,and Leaf Water Potential in Three Salix Species

Gas exchange, photochemical efficiency, and leaf water potential (_l) of Salix matsudana (non-indigenous species), S. microstachya and S. gordejevii (indigenous species) were studied in Hunshandak Sandland, China. _l of all the three species decreased from 06:00 to 12:00, and increased afterwards. S. matsudana showed higher values of _l than others. Net photosynthetic rate (P _N) and stomatal conductance (g _s) of S. matsudana were the lowest among all, with the maximum P _N at 10:00 being 75% of that of S. gordejevii. Compared with the indigenous species, the non-indigenous S. matsudana had also lower transpiration rate (E) and water use efficiency (WUE). The values of F_v/F_m in all the species were lower from 06:00 to 14:00 than those after 14:00, indicating an obvious depression in photochemical efficiency of photosystem 2 in both non-indigenous and native species. However, it was much more depressed in S. matsudana, the non-indigenous tree. P _N was positively correlated to g _s and negatively related to _l. The relationship between g _s and vapour pressure difference (VPD) was exponential, while negative linear correlation was found between g _s and _l.     

Moisture retention properties of a mycorrhizal soil

The water relations of arbuscular mycorrhizal plants have been compared often, but virtually nothing is known about the comparative water relations of mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis typically affects soil structure, and soil structure affects water retention properties; therefore, it seems likely that mycorrhizal symbiosis may affect soil water relations. We examined the water retention properties of a Sequatchie fine sandy loam subjected to three treatments: seven months of root growth by (1) nonmycorrhizal Vigna unguiculata given low phosphorus fertilization, (2) nonmycorrhizal Vigna unguiculata given high phosphorus fertilization, (3) Vigna unguiculata colonized by Glomus intraradices and given low phosphorus fertilization. Mycorrhization of soil had a slight but significant effect on the soil moisture characteristic curve. Once soil matric potential (_m) began to decline, changes in _m per unit change in soil water content were smaller in mycorrhizal than in the two nonmycorrhizal soils. Within the range of about –1 to –5 MPa, the mycorrhizal soil had to dry more than the nonmycorrhizal soils to reach the same _m. Soil characteristic curves of nonmycorrhizal soils were similar, whether they contained roots of plants fed high or low phosphorus. The mycorrhizal soil had significantly more water stable aggregates and substantially higher extraradical hyphal densities than the nonmycorrhizal soils. Importantly, we were able to factor out the possibly confounding influence of differential root growth among mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis affected the soil moisture characteristic and soil structure, even though root mass, root length, root surface area and root volume densities were similar in mycorrhizal and nonmycorrhizal soils.     

The ability of acidic pH,growth inhibitors,and glucose to increase the proton motive force and energy spilling of amino acid-fermenting <Emphasis Type="Italic">Clostridium sporogenes</Emphasis> MD1 cultures

Clostridium sporogenes MD1 grew rapidly with peptides and amino acids as an energy source at pH 6.7. However, the proton motive force (p) was only –25 mV, and protonophores did not inhibit growth. When extracellular pH was decreased with HCl, the chemical gradient of protons (ZpH) and the electrical membrane potential () increased. The p was –125 mV at pH 4.7, even though growth was not observed. At pH 6.7, glucose addition did not cause an increase in growth rate, but increased to –70 mV. Protein synthesis inhibitors also significantly increased . Non-growing, arginine-energized cells had a of –80 mV at pH 6.7 or pH 4.7, but was not detected if the F₁F₀ ATPase was inhibited. Arginine-energized cells initiated growth if other amino acids were added at pH 6.7, and and ATP declined. At pH 4.7, ATP production remained high. However, growth could not be initiated, and neither nor the intracellular ATP concentration declined. Based on these results, it appears that C. sporogenes MD1 does not need a large p to grow, and p appears to serve as a mechanism of ATP dissipation or energy spilling.Mandatory disclaimer: Proprietary or brand names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product, and exclusion of others that may be suitable.     

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 (_w) and increase in water deficit (WD) and led to a decrease in concentrations of chlorophyll a, b and carotenoids. Severe water stress (shoot _w=–2.4 MPa) had a negative effect on chlorophyll a fluorescence parameters including PSII activity (F_v/F_m), and the vitality index (R_fd). 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.     

Osmotic adjustment and the inhibition of leaf,root, stem and silk growth at low water potentials in maize

The expansion growth of plant organs is inhibited at low water potentials ( _w), but the inhibition has not been compared in different organs of the same plant. Therefore, we determined elongation rates of the roots, stems, leaves, and styles (silks) of maize (Zea mays L.) as soil water was depleted. The _w was measured in the region of cell expansion of each organ. The complicating effects of transpiration were avoided by making measurements at the end of the dark period when the air had been saturated with water vapor for 10 h and transpiration was less than 1% of the rate in the light. Growth was inhibited as the _w in the region of cell expansion decreased in each organ. The _w required to stop growth was-0.50,-0.75, and-1.00 MPa, in this order, in the stem, silks, and leaves. However, the roots grew at these _w and ceased only when _w was lower than-1.4 MPa. The osmotic potential decreased in each region of cell expansion and, in leaves, roots and stems, the decrease was sufficient to maintain turgor fully. In the silks, the decrease was less and turgor fell. In the mature tissue, the _w of the stem, leaves and roots was similar to that of the soil when adequate water was supplied. This indicated that an equilibrium existed between these tissues, the vascular system, and the soil. At the same time, the _w was lower in the expanding regions than in the mature tissues, indicating that there was a _w disequilibrium between the growing tissue and the vascular system. The disequilibrium was interpreted as a _w gradient for supplying water to the enlarging cells. When water was withheld, this gradient disappeared in the leaf because _w decreased more in the xylem than in the soil, indicating that a high flow resistance had developed in the xylem. In the roots, the gradient did not decrease because vascular _w changed about the same amount as the soil _w. Therefore, the gradient in _w favored water uptake by roots but not leaves at low _w. The data show that expansion growth responds to low _w differently in different growing regions of the plant. Because growth depends on the maintenance of turgor for extending the cell walls and the presence of _w gradients for supplying water to the expanding cells, several factors could have been responsible for these differences. The decrease of turgor in the silks and the loss of the _w gradient in the leaves probably contributed to the high sensitivity of these organs. In the leaves, the gradient loss was so complete that it would have prevented growth regardless of other changes. In the roots, the maintenance of turgor and _w gradients probably allowed growth to continue. This difference in turgor and gradient maintenance could contribute to the increase in root/shoot ratios generally observed in water-limited conditions.Symbols _s osmotic potential - _w water potential     

Control of the rate of cell enlargement: Excision,wall relaxation,and growth-induced water potentials

A new guillotine thermocouple psychrometer was used to make continuous measurements of water potential before and after the excision of elongating and mature regions of darkgrown soybean (Glycine max L. Merr.) stems. Transpiration could not occur, but growth took place during the measurement if the tissue was intact. Tests showed that the instrument measured the average water potential of the sampled tissue and responded rapidly to changes in water potential. By measuring tissue osmotic potential ( _s ), turgor pressure ( _p ) could be calculated. In the intact plant, _s and _p were essentially constant for the entire 22 h measurement, but _s was lower and _p higher in the elongating region than in the mature region. This caused the water potential in the elongating region to be lower than in the mature region. The mature tissue equilibrated with the water potential of the xylem. Therefore, the difference in water potential between mature and elongating tissue represented a difference between the xylem and the elongating region, reflecting a water potential gradient from the xylem to the epidermis that was involved in supplying water for elongation. When mature tissue was excised with the guillotine, _s and _p did not change. However, when elongating tissue was excised, water was absorbed from the xylem, whose water potential decreased. This collapsed the gradient and prevented further water uptake. Tissue _p then decreased rapidly (5 min) by about 0.1 MPa in the elongating tissue. The _p decreased because the cell walls relaxed as extension, caused by _p , continued briefly without water uptake. The _p decreased until the minimum for wall extension (Y) was reached, whereupon elongation ceased. This was followed by a slow further decrease in Y but no additional elongation. In elongating tissue excised with mature tissue attached, there was almost no effect on water potential or _p for several hours. Nevertheless, growth was reduced immediately and continued at a decreasing rate. In this case, the mature tissue supplied water to the elongating tissue and the cell walls did not relax. Based on these measurements, a theory is presented for simultaneously evaluating the effects of water supply and water demand associated with growth. Because wall relaxation measured with the psychrometer provided a new method for determining Y and wall extensibility, all the factors required by the theory could be evaluated for the first time in a single sample. The analysis showed that water uptake and wall extension co-limited elongation in soybean stems under our conditions. This co-limitation explains why elongation responded immediately to a decrease in the water potential of the xylem and why excision with attached mature tissue caused an immediate decrease in growth rate without an immediate change in _p Abbreviations and symbols L tissue conductance for water - m wall extensibility - Y average yield threshold (MPa) - _o water potential of the xylem - _p turgor pressure - _s osmotic potential - _w water potential of the elon gating tissue     

Pollen selection for Alternaria resistance in oilseed brassicas: responses of pollen grains and leaves to a toxin of A. brassicae

Summary The effects of destruxin B, a host-specific toxin of Alternaria brassicae that causes black spot disease in oilseed brassicas, were studied on in vitro pollen germination and pollen-tube growth of Brassica campestris var brown sarson, B. juncea, B. napus cvs Westar and Cresor, B. nigra and Sinapis alba. Pollen grains of B. nigra, B. juncea and B. campestris were the most sensitive and those of S. alba the least sensitive to the toxin. Effects of the toxin were also studied on the leaves of these species, and the degree of sensitivity of leaves of different species was comparable to that of their pollen grains. The results on the responses of pollen grains as well as leaves to the toxin are in agreement with the degree of susceptibility/resistance of these species to A. brassicae reported in the literature, indicating that the genes imparting susceptibility/restistance are expressed in the pollen, a prerequisite for pollen selection. Results are also presented which show that the toxin fed to the cut end of isolated inflorescence axis is readily taken up by the developing pollen and results in the inhibition of germination of susceptible pollen. This technique offers a simple and effective method for application of selection pressure to eliminate pollen grains susceptible to the toxin from effecting fertilization.     

Intraspecific variation in the water relations of Salix arctica,an arctic-alpine dwarf willow

Summary The seasonal and diurnal water relations were investigated within arctic and alpine populations of the dwarf willow Salix arctica. Marked differences that were habitat dependent (e.g. xeric vs. mesic) occured both within and between the populations. The environmental variables that most affected plant water balance and the bulk tissue water relations were soil water potential (_soil) and the leafto-air water vapor pressure gradient (w), however, low soil temperature (<4.0° C) also had a marked effect in the wet to mesic habitats. The effects of declining _soil and increased w were most pronounced in the plants growing in xiric habitats in both populations. Stomatal response to increased w was two-fold greater in alpine versus arctic plants and is hypothesized to have arisen in response to more frequent exposure to the higher evaporative conditions of alpine existance. Seasonal fluctuations in the osmotic potential closely followed changes in _soil, suggesting that these were active rather that passive changes. Additionally, plants from xeric habiats had a lower bulk tissue elastic modulus (more elastic tissues) in both arctic and alpine populations. The osmotic and elastic properties enhanced turgor maintenance over a broad range of leaf water potentials and during periods when w was high. Turgor maintenance also correlated to continued transpiration despite fluctuations in soil and atmospherically induced water deficits. Arctic habitats have a shorter growing season, lower soil temperatures due to the presence of permafrost, but higher soil water potentials and lower leaf-to-air vapor pressure gradients than alpine habitats. The observed variation in patterns of stomatal conductance and in values of tissue water relations characteristics between arctic and alpine populations of S. arctica is hypothesized to have arisen in response to these different environmental regimes which represent different selective regimes that occur along the arctic-alpine environmental continuum inhabited by this wide ranging species.     

Comparative Field Summer Stress of Three Tree Species Co-occurring in Mediterranean Coastal Dunes

Chlorophyll a fluorescence, water potential (_s), and root system of Juniperus oxycedrus ssp. macrocarpa, Juniperus phoenicea ssp. turbinata, and Pinus pinea were studied in Mediterranean coastal dunes of SW Spain during summer drought and after fall rains in 1999, the driest year in the 90's. A strong and reversible depression in the photochemical efficiency of photosystem 2 of the three species was recorded, which happened concomitantly with the diurnal increase and decrease in radiation. J. phoenicea, with superficial root system, was the most affected species by summer drought. It showed high rates of down-regulation of photosynthesis by photoinhibition and positive correlation between _s and F_v/F_p, with _s lower than -7 MPa. However, it tolerated this high stress, showing a fast recovery of its physiological state after fall rains. On the other hand, J. oxycedrus and P. pinea, both with deep root systems, kept their _s values up to -3 MPa, showing lower stress during summer drought. On the other hand, J. oxycedrus and J. phoenicea were more sensible to changes in edaphic water content than P. pinea. These specific responses to summer drought would be determined by their root distributions and stomatal control of transpiration, conditioning the efficiency in getting and using the available water resources. Ecophysiological responses indicate that these species are well-adapted to long periods of drought in Mediterranean climate areas, developing different strategies: J. phoenicea tolerates high stress with a fast recovery after fall rains, while J. oxycedrus and P. pinea are less affected by summer drought since their deep root systems would allow them to reach deep water resources.     

Decline of <Emphasis Type="Italic">Cornus florida</Emphasis> and forest succession in a <Emphasis Type="Italic">Quercus–Carya</Emphasis> forest

Cornus florida is a common understory species in many hardwood forests in eastern North America. It plays an important role in nutrient cycling and is an important food resource for many vertebrate species, especially migratory birds. We used data collected over a 16-year period to examine population dynamics of a tagged population of C. florida in a 6.4 ha area in the context of change in the protected Quercus–Carya forest of the Ross Biological Reserve, Indiana. We examined the hypothesis that forest dynamics result from interactions between long-term ecological succession and pathogens. The C. florida population at the Ross Reserve declined by 50% between 1983 and 2000, with a survivorship of 24%. Analysis of 40 years of forest survey data showed that Quercus and Carya populations declined in importance, while Acer saccharum increased dramatically. This change in forest structure is consistent with successional changes occurring throughout the Midwest and can be attributed to suppression of disturbance. Cornus florida declined more sharply where A. saccharum increased. From 1983 to 1999, C. florida were less likely to survive if they were within 5 m of a A. saccharum. Light measurements showed that A. saccharum abundance correlated negatively with light available to C. florida, suggesting that increased shading by A. saccharum contributed to C. florida decline. The fungus, Discula destructiva causes the disease dogwood anthracnose that is associated with widespread decline of C. florida in the eastern United States. Tests for this pathogen in our study area were mostly negative. Other tests revealed that Armillaria root rot infected most C. florida, but this disease seemed to be a secondary effect of shading by A. saccharum. These results suggest that the lack of fire and other anthropogenic disturbances has resulted in an accelerated shift in dominance from Quercus and Carya to A. saccharum in the main canopy, and this shift, in turn, has resulted in increased shading of C. florida and its decline in previously more open Midwestern forests.     

Gas exchange and photosynthesis of Eucalyptus camaldulensis seedlings inoculated with different ectomycorrhizal symbionts

Eucalyptus camaldulensis Dehnh. seedlings inoculated with Pisolithus tinctorius (Pers.) Coker & Couch and Thelephora terrestris Ehrl. per Fr. were grown in well watered soil (_s –0.03 MPa) or subjected to a long-term soil water stress of up to –1.0 MPa over 13-week period in a glasshouse. After 13 weeks, all seedling containers were watered to field capacity and then water was withheld from the E. camaldulensis seedlings to induce a short-term drought. Diurnal measurements of seedling photosynthesis rate (A), leaf stomatal conductance (g) and leaf water potential (_p) were completed before, during, and after the short term drought. Although they were growing in an equal soil volume, photosynthesis rate (A), leaf stomatal conductance and leaf water potential (_p) of larger seedlings with P. tinctorius ectomycorrhizae were similar to those of smaller seedlings colonized with T. terrestris during the short-term drought period. Seedlings inoculated with Pisolithus tinctorius maintained higher photosynthesis rates over the course of the short-term drought. Thus, P. tinctorius ectomycorrhizae appear to be more efficient than those of T. terrestris in assisting seedlings to maintain gas exchange and photosynthesis under limited soil moisture conditions.     

Alternaria incidence in some alloplasmic lines of Indian mustard (Brassica juncea (L.) Coss.)

Summary The cytoplasmic substitution lines of Brassica juncea (L.) Coss were evaluated for their field resistance to Alternaria blight (Alternaria brassicae). The euplasmic B. juncea cv. RLM 198 had a mesothetic reaction while alloplasmic B. juncea lines with cytoplasms of B. campestris, B. chinensis, and B. japonica were highly susceptible. B. nigra cytoplasm did not have any effect on the disease reaction of the B. juncea genome. However, the alloplasmic lines with the cytoplasm of B. napus and B. carinata revealed a comparatively higher degree of resistance. The study underlined the utility of cytoplasmic manipulations in modifying the phenotypic expression of nuclear genes.     

Pollen competition among Populus deltoides Marsh., P. nigra L. and P. maximowiczii Henry in fertilizing P. deltoides ovules and siring its seed crop

Summary Interspecific pollen competition among Populus deltoides, P. nigra and P. maximowiczii in fertilizing P. deltoides ovules was studied by using a pollen mixture technique, allozymes and leaf morphology. The frequencies of F₁ seedlings of different paternities in pollen-mix crosses showed highly significant (P<0.01) departures from the 11 ratio expected if pollen selection was random. P. deltoides pollen was the most competitive. The mean percentages of F₁ seedlings of P. deltoides paternity in crosses with pollen mixes P. deltodes + P. nigra, P. deltoides + P. maximowiczii, and P. deltoides + P. nigra + P. maximowiczii were 95.0, 92.5, and 84.8, respectively. P. maximowiczii pollen was more competitive than P. nigra pollen, which was at a selective disadvantage. An average of 83.6% of F₁ progenies of the eight crosses with P. nigra + P. maximowiczii pollen showed P. maximowiczii paternity. Also, in four crosses with P. deltoides + P. nigra + P. maximowiczii pollen, the relative proportion of P. deltoides × P. maximowiczii seedlings (13.4%) was higher than that of P. deltoides × P. nigra seedlings (1.8%). Pollen proportions in the pollen mixes and pollen size did not significantly affect the competitive ability of the pollen. The relative pollen competitive ability indicated reproductive affinities among the species.