Frank W. Telewski

A unified hypothesis of mechanoperception in plants

The perception of mechanical stimuli in the environment is crucial to the survival of all living organisms. Recent advances have led to the proposal of a plant-specific mechanosensory network within plant cells that is similar to the previously described network in animal systems. This sensory network is the basis for a unifying hypothesis, which may account for the perception of numerous mechanical signals including gravitropic, thigmomorphic, thigmotropic, self-loading, growth strains, turgor pressure, xylem pressure potential, and sound. The current state of our knowledge of a mechanosensory network in plants is reviewed, and two mechanoreceptor models are considered: a plasmodesmata-based cytoskeleton-plasma membrane-cell wall (CPMCW) network vs. stretch-activated ion channels. Post-mechanosensory physiological responses to mechanical stresses are also reviewed, and future research directions in the area of mechanoperception and response are recommended.

The 120-yr period for Dr. Beal's seed viability experiment

After 120 yr of burial in moist, well-aerated sand, 23 seeds of Verbascum blattaria and two seeds of a Verbascum sp. germinated and produced normal plants (50% germination for Verbascum). After a 6-wk cold treatment, a single seed of Malva rotundifolia germinated also, producing a normal plant (2% germination). Plants were grown to maturity in a greenhouse, and flowering was induced by exposure to a 6-wk cold treatment. Flowers were artificially pollinated to produce seed of both Verbascum blattaria and Malva rotundifolia. The Verbascum sp. failed to set seed. Collected seeds were subsequently germinated, producing normal plants. F(1) seeds of V. blattaria had a germination of 64%. Seeds (6%) of M. rotundifolia germinated after a cold treatment.

Biomechanics and transgenic wood

Wood, or secondary xylem, is composed mostly of three components-cellulose, hemicelluloses, and lignin. Yet this apparent simplicity is deceiving because the sophisticated arrangement of the components on various structural levels, ranging from intricate molecular architecture to defined cellular arrangements to tissue morphology, makes wood a challenging and interesting subject of biomechanical investigation. Recent advances in genetic transformation, providing easier access to wood of specifically altered composition or structure, have opened new opportunities for research on the intricate relation between material structure and composition and mechanical properties. At the same time, investigations into the mechanical properties have provided new information regarding the structural configuration of wood. The present paper reviews the work conducted in this field and outlines future perspectives and prospects for research.

Hydraulic, biomechanical, and anatomical interactions of xylem from five species of Acer (Aceraceae)

Possible trade-offs between hydraulic conductivity and mechanical properties of woody stems from five species were assessed. Acer negundo is a ruderal tree, A. saccharinum, and A. rubrum are fast-growing and shade-intolerant soft maples, whereas A. nigrum and A. saccharum are slow-growing and shade-tolerant hard maples. It was hypothesized that the ruderal and soft maples would have lower modulus of elasticity (MOE) and modulus of rupture (MOR), but higher maximum specific conductivity (K(s max)) than hard maples. Many anatomical and general morphological characteristics were measured in an attempt to correlate them to water transport and/or mechanical strength differences between species. No difference was found between species in vessel diameter, fiber wall thickness, initial hydraulic conductivity (K(h initial)), specific conductivity (K(s max)), native percent embolism, or Huber value. Similarly, no trade-off was found between K(s max) and MOE or MOR across the genus. However, fiber lumen diameter was inversely correlated to MOE and MOR. Surprisingly, percentage of ray parenchyma was positively related to MOE. The results suggest transport/mechanical trade-offs do not occur in Acer and differences in mechanical properties may be due to fiber lumen differences that do not influence the efficient transport of water.

Is windswept tree growth negative thigmotropism

The scientific investigation of the influence of wind on tree growth has been conducted for over 200 years. One influence of wind on trees is the formation of an asymmetric crown, usually characterized as being windswept under moderate windy conditions. As wind exposure increases, the terms applied to this growth form include flag-tree, banner-tree, and krummholz. The modification in crown morphology has been widely recognized and studied, especial in the area of wind prospecting or as a bioindicator of wind speed in environments lacking monitoring stations. However, the causes and physiology underlying this response is little understood. The windswept morphology is consistent with the morphologies associated with other tropisms (i.e. phototropism and gravitropism). Tropisms are defined as a growth response towards (positive) or away (negative) from an environmental stimulus. The asymmetric growth form of windswept trees appears to be a negative thigmotropic growth response to wind. In this review, evidence will be presented to support or reject two hypotheses; H₁ the windswept growth form is the result of a negative thigmotropic growth response or H₂ the windswept growth form is determined by the biophysical properties of wood. It is argued that the windswept growth form is more likely the product of biomechanical properties (accept H₂) than of a physiological thigmotropic growth response (reject H₁). However, proper testing of both hypotheses is still required before a final confirmation can be established.

Climate-Growth Relationships for Native and Nonnative Pinaceae in Northern Michigan's Pine Barrens

Secondary growth responses of native and nonnative trees exposed to the same climatic conditions can elucidate sensitivities and thus adaptability to a particular region. A long-term mixed-species planting in the pine barrens of northern lower Michigan presented an opportunity to discriminate responses from species commonly planted in this region. Mean ring-width chronologies from living native Pinus resinosa Ait. and P. strobus L. and nonnative P. sylvestris L. and Picea abies (L.) Karst. at this plantation were generated, standardized, and analyzed by correlation analysis against mean monthly climatic variables. The native pine chronologies had the highest mean ring widths and signal-to-noise ratios (SNR), were highly correlated to each other, and exhibited positive responses to years with above-normal April temperatures but no significant relationships to variations in precipitation. The P. sylvestris chronology was highly correlated to the other two pine chronologies and responded similarly to April temperatures but exhibited negative correlations to January and April precipitation and positive correlations to September precipitation. The P. abies chronology had the highest mean sensitivity and was correlated with the P. strobus chronology but only responded positively to precipitation from the previous December. The low SNR (P. sylvestris, P. abies), high mean sensitivity (P. abies), and larger number of significant correlations to variations in monthly climatic variables (P. sylvestris) suggest that these nonnative species are more sensitive to this local climate. These results provide insights to the adaptability, establishment, and geographic distribution of the nonnative Pinaceae.

Ethylene production by different age class ponderosa and Jeffery pine needles as related to ozone exposure and visible injury

SummaryEthylene produced by different needle age classes representing natural populations of two ponderosa pine varieties [Pinus ponderosa var. arizonica (Engelm.) Shaw and var. ponderosa Dougl. ex Laws.] and Jeffery pine (Pinus jeffreyi Grev. and Balf.) was characterized using mercuric perchlorate traps. All populations contained individual trees which were either symptomatic or asymptomatic with respect to visible ozone injury. Ethylene production by different needle age classes was also characterized in P. ponderosa var. ponderosa seedlings grown in open top ozone fumigation chambers. Older age class needles produce significantly (P>0.05) more ethylene than younger age class needles. Needles of both P. ponderosa var. ponderosa and P. jeffreyi exhibiting ozone injury in the field produced significantly (P>0.05) higher levels of ethylene than asymptomatic conspecific trees. Seedlings exposed to the highest level of ozone in the fumigation study produced the highest levels of ethylene, followed by fumigation with medium and low ozone concentrations and carbon filtered air. These data indicate that the measurement of ethylene in conifer needles, as a measure of stress, needs to be calibrated for needle age class. It also suggests that the sensitivity of a tree to ozone injury may be regulated by the inherent ability of the individual to produce ethylene.

Acclimation of mechanical and hydraulic functions in trees: impact of the thigmomorphogenetic process.

The secondary xylem (wood) of trees mediates several functions including water transport and storage, mechanical support and storage of photosynthates. The optimal structures for each of these functions will most likely differ. The complex structure and function of xylem could lead to trade-offs between conductive efficiency, resistance to embolism, and mechanical strength needed to count for mechanical loading due to gravity and wind. This has been referred to as the trade-off triangle, with the different optimal solutions to the structure/function problems depending on the environmental constraints as well as taxonomic histories. Thus, the optimisation of each function will lead to drastically different anatomical structures. Trees are able to acclimate the internal structure of their trunk and branches according to the stress they experience. These acclimations lead to specific structures that favor the efficiency or the safety of one function but can be antagonistic with other functions. Currently, there are no means to predict the way a tree will acclimate or optimize its internal structure in support of its various functions under differing environmental conditions. In this review, we will focus on the acclimation of xylem anatomy and its resulting mechanical and hydraulic functions to recurrent mechanical strain that usually result from wind-induced thigmomorphogenesis with a special focus on the construction cost and the possible trade-off between wood functions.

Clinal differentiation and putative hybridization in a contact zone of Pinus ponderosa and P. arizonica (Pinaceae)

The widely distributed Pinus subsection Ponderosae is a species complex that has a transition zone among taxa in the southwestern United States. In southern Arizona and New Mexico at least two recognized taxa, Pinus ponderosa var. scopulorum and Pinus arizonica or P. ponderosa var. arizonica, are known to coexist in close proximity. In this study, we report the existence of populations where the taxa are sympatric. One of the key characteristics distinguishing taxa is the number of needles per fascicle; P. ponderosa typically has three, P. arizonica has five. We examined the spatial distribution of needle-number types in a belt transect that covers a transition zone from nearly pure three-needle types at the top of Mount Lemmon to five-needle types downslope, in the Santa Catalina Mountains, Arizona. The spatial distribution is inconsistent with there being both free interbreeding among types and selective neutrality of types. Trees with intermediate types, having combinations of three, four, and five needles and/or mean numbers of needles between 3.0 and 5.0, are spatially concentrated in the middle of the transition zone. The spatial distribution supports the occurrence of hybridization and introgression, and this is consistent with reported crossabilities of the types. The results suggest that selection is acting, either on needle number per se or on other traits of the ecotype with which it may be in linkage disequilibrium, to maintain the observed steep clinal differentiation.

Spatial pattern of allozyme variation in a contact zone of Pinus Ponderosa and P. arizonica (Pinaceae)

The spatial distribution of genotypes for nine polymorphic allozyme loci was examined in a contact zone between Pinus ponderosa var. scopulorum and another tree regarded as either a separate species, Pinus arizonica, or variety, Pinus ponderosa var. arizonica, in southern Arizona. Previous work had identified a steep elevational cline for a key taxonomic trait, number of leaf-needles per fascicle, on the south slope of Mt. Lemmon. The present results indicate that the taxa are not fully interbreeding in this contact zone, because allozyme genotypes are considerably more spatially structured than expected for the dispersal characteristics of pines. The amount of spatial differentiation is also much less than that observed for needle number. It appears that this is due to the lack of differentiation for allozyme gene frequencies for the two types of trees, which is further evidenced by analysis of samples from two other populations away from the contact zone. It is likely that if the two taxa were isolated in the past, it was not for long enough nor complete enough to allow mutation-drift to create substantial differentiation between them. Another possible explanation is that introgression after recontact is so advanced that any differences have been erased throughout the Santa Catalina mountain range.