Dennis W. Gray

Variation in isoprene emission from Quercus rubra: Sources, causes, and consequences for estimating fluxes

[1] Isoprene is the dominant volatile organic compound produced in many forest systems. Uncertainty in estimates of leaf level isoprene emission rate stems from an insufficient understanding of the patterns and processes controlling isoprene emission capacity in plant leaves. Previous studies suggest that variation in isoprene emission capacity is substantial; however, it is not known at what scale emission capacity is the most variable. Identifying the sources of variation in emission capacity has implications for conducting measurements and for model development, which will ultimately improve emission estimates and models of tropospheric chemistry. In addition, understanding the sources of variation will help to develop a comprehensive understanding of the physiological controls over isoprene emission. This study applied a variance partitioning approach to identify the major sources of variation in isoprene emission capacity from two populations of northern red oak (Quercus rubra) over three growing seasons. Specifically, we evaluated variation due to climate, populations, trees, branches, leaves, seasons, and years. Overall, the dominant source of variation was the effect of a moderate drought event. In the years without drought events, variation among individual trees (intraspecific) explained approximately 60% of the total variance. Within the midseason, isoprene emission capacity of sun leaves varied by a factor of 2 among trees. During the third year a moderate 20-day drought event caused isoprene emission capacity to decrease fourfold, and the relative importance of intraspecific variation was reduced to 24% of total variance. Overall, ambient temperature, light, and a drought index were poor predictors of isoprene emission capacity over a 0 to 14-day period across growing seasons. The drought event captured in this study emphasizes the need to incorporate environmental influences into leaf level emission models.

The green algal underground : evolutionary secrets of desert cells

ABSTRACT Microscopic, unicellular, free-living green algae are found in desert microbiotic crusts worldwide. Although morphologically simple, green algae in desert crusts have recently been found to be extraordinarily diverse, with membership spanning five green algal classes and encompassing many taxa new to science. This overview explores this remarkable diversity and its potential to lead to new perspectives on the diversity and evolution of green plants. Molecular systematic and physiological data gathered from desert taxa demonstrate that these algae are long-term members of desert communities, not transient visitors from aquatic habitats. Variations in desiccation tolerance and photophysiology among these algae include diverse evolutionary innovations that developed under selective pressures in the desert. Combined with the single embryophyte lineage to which more familiar terrestrial green plants belong, multiple desert green algal lineages provide independent evolutionary units that may enhance understanding of the evolution and ecology of eukaryotic photosynthetic life on land.

INFLUENCES OF TEMPERATURE HISTORY, WATER STRESS, AND NEEDLE AGE ON METHYLBUTENOL EMISSIONS

Volatile organic compounds emitted by plants have long been recognized as having important influences on tropospheric chemistry, most notably in contributing to the production of tropospheric ozone and aerosols. One such compound, methylbutenol (MBO), was recently identified as a major component of the volatiles emitted by ponderosa pine and several other species of pine in western North America. On short time scales, MBO emissions increase with light intensity in parallel with photosynthetic responses, but emissions and photosynthesis show opposite responses to increases in temperature. We investigate the response of MBO emission to water stress, ambient temperature, and the aging of needles in field grown Pinus ponderosa (ponderosa pine). Photosynthetic rates and MBO production capacities of P. ponderosa were measured over the course of a season on saplings at a site that experienced summer drought and at a site that received supplementary water. In addition, a water-stress alleviation experiment was performed in which drought-stressed P. ponderosa seedlings were re-watered at the end of the season and pre-/post-alleviation photosynthetic rates and MBO production capacities were compared. Over the season photosynthesis declined while MBO production capacity tracked changes in ambient temperature linearly both over the entire season and on a day-to-day basis. Although severe water stress reduced photosynthetic rates, there was no difference in the response of MBO production capacity to ambient temperature under either drought-stressed or well-watered conditions, and there was no change in MBO production capacity following water stress alleviation. MBO emission declined with needle age. The correlation between MBO production capacity and ambient temperature is consistent with the view that MBO may provide a protection against high temperature stress similar to that suggested for isoprene. Corresponding Editor: K. F. Raffa

Turgor, solute import and growth in maize roots treated with galactose

It has been observed that extension growth in maize roots is almost stopped by exposure to 5 mm d-galactose in the root medium, while the import of recent photoassimilate into the entire root system is temporarily promoted by the same treatment. The aim of this study was to reconcile these two apparently incompatible observations. We examined events near the root tip before and after galactose treatment since the tip region is the site of elongation and of high carbon deposition in the root. The treatment rapidly decreased root extension along the whole growing zone. In contrast, turgor pressure, measured directly with the pressure probe in the cortical cells of the growing zone, rapidly increased by 0.15 MPa within the first hour following treatment, and the increase was maintained over the following 24 h. Both tensiometric measurements and a comparison of turgor pressure with local growth rate demonstrated that a rapid tightening of the cell wall caused the reduction in growth. Single cell sampling showed cell osmotic pressure increased by 0.3 MPa owing to accumulation of both organic and inorganic solutes. The corresponding change in cell water potential was a rise from -0.18 MPa to approximately zero. More mature cells at 14 mm from the root tip (just outside the growing region) showed a qualitatively similar response.

Field response of Ips paraconfusus, Dendroctonus brevicomis, and their predators to 2-methyl-3-buten-2-ol, a novel alcohol emitted by ponderosa pine.

Methylbutenol (MBO) is a major component of the aggregation pheromone of the European spruce beetle Ips typographus and also has been found to be emitted in large amounts by several species of pine native to western North America. This study investigates the influence this signal may have on the behavior of North American bark beetles and examines whether MBO functions as a defensive compound for emitting pines. The response of two North American bark beetles (Ips paraconfusus and Dendroctonus brevicomis) and their predaceous beetles (Trogositidae and Cleridae) to MBO, pheromone, and monoterpenes in varying release rates was investigated in the field using Lindgren funnel traps. MBO exhibited no repellent properties when tested alone, nor did MBO appear to have any effect on the aggregation response of these bark beetles and their predators to their pheromones. These results provide no support for a defensive function of MBO.