Stomatal response to vapour pressure deficit and the effect of plant water stress

Abstract The dynamic response of stomata to changes in atmospheric humidity was investigated in Fragaria × ananassa Duch., Picea engelmannii Parry, and Pseudotsuga menziesii (Mirb.) Franco; and the effect of water stress on this response was determined in Pseudotsuga menziesii. The plants were rotated through three regimes of ambient temperature and vapour pressure deficit: 35°C–3. 5kPa, 35°C–0. 5 kPa, and 20°C–1. 5kPa. Branch and leaflet conductance were measured with a steady-state porometer, first at ambient vapour pressure deficit and then at one of four treatment conditions achieved by increasing or decreasing vapour pressure within the porometer cuvette. All three species showed similar stomatal response: enhanced conductance at low vapour pressure deficit and depressed conductance at high vapour pressure deficit. Engelmann spruce was more sensitive than Douglas fir and strawberry. Plant water status significantly altered stomatal response to vapour pressure deficit. The relationship of conductance of xylem water potential was linear under ambient conditions but became curvilinear when conductance was measured above and below ambient vapour pressure deficit. Between ?0. 5 MPa and ?2. 0 MPa xylem water potential, the stomata were sensitive to vapour pressure deficit, but below ? 2. 0 MPa, the sensitivity decreased.     

Temperature sensitivity and substrate quality in soil organic matter decomposition: results of an incubation study with three substrates

Kinetic theory suggests that the temperature sensitivity of decomposition of soil organic matter should increase with increasing recalcitrance. This ‘temperature–quality hypothesis’ was tested in a laboratory experiment. Microcosms with wheat straw, spruce needle litter and mor humus were initially placed at 5, 15 and 25 °C until the same cumulative amount of CO₂ had been respired. Thereafter, microcosms from each single temperature were moved to a final set of incubation temperatures of 5, 15 and 25 °C. Straw decomposed faster than needle litter at 25 and 15 °C, but slower than needle litter at 5 °C, and showed a higher temperature sensitivity (expressed as Q₁₀) than needle litter at low temperatures. When moved to the same temperature, needle litter initially incubated at 5 and 15 °C had significantly higher respiration rates in the final incubation than litters initially placed at 25 °C. Mor humus placed at equal temperatures during the initial and final incubations had higher cumulative respiration during the final incubation than humus experiencing a shift in temperature, both up‐ and downwards. These results indicate that other factors than substrate quality are needed to fully explain the temperature dependence. In agreement with the hypothesis, Q₁₀ was always higher for the temperature step between 5 and 15 °C than between 15 and 25 °C. Also in agreement with the temperature–quality hypothesis, Q₁₀ significantly increased with increasing degree of decomposition in five out of the six constant temperature treatments with needle litter and mor humus. Q₁₀s for substrates moved between temperatures tended to be higher than for substrates remaining at the initial temperature and an upward shift in temperature increased Q₁₀ more than a downward shift. This study largely supports the temperature–quality hypothesis. However, other factors like acclimation and synthesis of recalcitrant compounds can modify the temperature response.     

A study of the surface proteins of Entomophthora egressa protoplasts and of larval spruce budworm hemocytes

Entomophthora egressa protoplasts either exposed to or not exposed to trypsin were not attacked by either trypsinized or non-trypsinized larval spruce budworm granulocytes. Granulocytes adhered to protoplasts exposed to papain, and this adhesion could be prevented by papainizing the hemocytes. Differences were observed in the responses of two E. egressa isolates when exposed to papain or to the papain-control solutions. Exposure of hemocytes to trypsin did not reduce either the number of Absidia repens sporangiospores per granulocyte or the percentage of granulocytes with spores, whereas, exposure to papain did. The role of surface proteins, particularly glycoproteins, in hemocyte-fungal cell interactions is briefly discussed.     

The role of stem recharge in reducing the winter desiccation of Picea engelmannii (Pinaceae) needles at alpine timberline

The winter desiccation of needles is thought to limit tree growth and survival within alpine timberline ecotones of the southern Rocky Mountains, USA. To better understand the factors contributing to this desiccation damage, the extent to which stem water was available to needles of Picea engelmannii undergoing desiccation at timberline near Monarch Pass, Colorado, was monitored throughout the winter. Severed shoots experienced significantly greater desiccation than did intact shoots, indicating the availability of stem water to needles despite presumably frozen soil, roots, and stems. A model of water relations during winter predicted more extreme desiccation of severed shoots than observed. This suggests that one or more of the common assumptions concerning the winter water relations of timberline trees is in error. The influence of cold, dry conditions on the cuticular conductance of Picea engelmannii needles is not known and therefore not accounted for in current models of winter water relations. The assumption that cuticular conductance is not influenced by temperature or humidity is a likely source of error in such models.     

Structure of lignins in developing xylem of Norway spruce

The developing xylem in a Norway spruce (Picea abies) clone was investigated during a growth season and compared to lignin from sapwood of the same tree clone. Klason and acid-soluble lignin contents were determined as well as the carbohydrate monomer distribution and protein content. By analyzing lignin thioacidolysis products, it was shown that only guaiacyl units could be detected in the materials, and the relative amount of beta-O-4' bonds was assessed. Monomeric and selected dimeric lignin products were identified by mass spectrometry. The specimens were embedded and thin sections examined by microscopy to determine the state of cell differentiation in the samples. In the spring and early summer, growth was very rapid and the intention was to collect tissue in which exclusively the middle lamella/primary cell wall had begun to lignify. Combining data regarding Klason lignin, protein content and carbohydrate monomer distribution with microscopy, it was found that the developing xylem sample from mid-June contained lignin from exclusively middle lamella/primary wall. The Klason lignin content in the developing xylem during the growth season was 20%, 5% and 10% in April, June and August, respectively. Thioacidolysis showed that the lignin had more condensed structures than lignin from the reference Norway spruce clone wood. Mass spectrometry showed that the developing xylem specimens from June and August contained more lignin structures with end-groups than the reference sample. These results suggest that lignification in the cambial layer and early developing xylem may take place more in a bulk fashion during the summer.     

A set of microsatellite markers for use in Sitka spruce (Picea sitchensis) developed from Picea glauca ESTs

Few microsatellite markers have been specifically developed for Picea sitchensis. In January 2004 the appearance of over 10 000 sequences of expressed regions of DNA from P. glauca in GenBank presented an opportunity for the development of additional microsatellite markers in Sitka spruce. Mono‐ and dinucleotide repeat sequences were located in these sequences and primers were designed around these regions. Amplification was attempted in Sitka material from a broad geographical range and the level of polymorphism was assessed. Primers were also tested in progeny of a controlled cross. Nine polymorphic loci that demonstrated Mendelian inheritance in Sitka were discovered in this study.     

Seasonal rate of nitrate reduction in two temperate forest soils

Rainfall and throughfall chemistry beneath Sitka spruce of four ages were determined for a one year period. Throughfall beneath the older stands was consistently much more acid than rainfall. The H-ion flux was more than twice that in rainfall. Throughfall beneath the younger stands was only slightly more acid than rainfall, but the throughfall H-ion flux was considerbly less than in rainfall because the canopy intercepted a large proportion of rainfall. Concentrations and fluxes of other major cations and anions were greater in throughfall than precipitation for all crop ages.     

Sphaeronema sasseri n. sp. (Tylenchulidae), a Nematode Parasitic on Fraser Fir and Red Spruce

Sphaeronema sasseri n. sp. is described from Fraser fir and red spruce on Mount Mitchell in North Carolina. Females are distinguished from other species in the genus by body shape, occurrence of body annulations, stylet morphology, head shape, and by several morphometric characters. The nematodes occur in colonies surrounding the bases of lateral and feeder roots, and the infected tissues show a general breakdown of the cortex and bark. The roots appear to be severely damaged by high populations of nematodes. This parasite may be important in the etiology of the slow decline of spruce and fir that has occurred in recent years in the southern Appalachian Mountains.