Thomas L. Rost

Toward a causal explanation of plant invasiveness: Seedling growth and life-history strategies of 29 pine (Pinus) species

We studied 29 pine (Pinus) species to test the hypothesis that invasive species in disturbed habitats have distinct attributes. Seedling relative growth rate (RGR) and measures of invasiveness were positively associated across species as well as within phylogenetically independent contrasts. High RGR, small seed masses, and short generation times characterize pine species that are successful invaders in disturbed habitats. Discriminant analysis and logistic regression revealed that RGR was the most significant factor among these life‐history traits separating invasive and noninvasive species. We also explored the causes of differences in RGR among invasive and noninvasive species. While net assimilation rate, leaf mass ratio, and specific leaf area (SLA) were all found to be contributing positively to RGR, SLA was found to be the main component responsible for differences in RGR between invasive and noninvasive pines. We investigated differences in SLA further by studying leaf anatomy, leaf density, and leaf thickness. We also evaluated relative leaf production rate as an important aspect of SLA. We proposed a hypothetical causal network of all relevant variables.

EVOLUTION OF GENOME SIZE IN PINES (PINUS) AND ITS LIFE‐HISTORY CORRELATES: SUPERTREE ANALYSES

Abstract Genome size has been suggested to be a fundamental biological attribute in determining life‐history traits in many groups of organisms. We examined the relationships between pine genome sizes and pine phylogeny, environmental factors (latitude, elevation, annual rainfall), and biological traits (latitudinal and elevational ranges, seed mass, minimum generation time, interval between large seed crops, seed dispersal mode, relative growth rate, measures of potential and actual invasiveness, and level of rarity). Genome sizes were determined for 60 pine taxa and then combined with published values to make a dataset encompassing 85 species, or 70% of species in the genus. Supertrees were constructed using 20 published source phylogenies. Ancestral genome size was estimated as 32 pg. Genome size has apparently remained stable or increased over evolutionary time in subgenus Strobus, while it has decreased in most subsections in subgenus Pinus. We analyzed relationships between genome size and life‐history variables using cross‐species correlations and phylogenetically independent contrasts derived from supertree constructions. The generally assumed positive relation between genome size and minimum generation time could not be confirmed in phylogenetically controlled analyses. We found that the strongest correlation was between genome size and seed mass. Because the growth quantities specific leaf area and leaf area ratio (and to a lesser extent relative growth rate) are strongly negatively related to seed mass, they were also negatively correlated with genome size. Northern latitudinal limit was negatively correlated with genome size. Invasiveness, particularly of wind‐dispersed species, was negatively associated with both genome size and seed mass. Seed mass and its relationships with seed number, dispersal mode, and growth rate contribute greatly to the differences in life‐history strategies of pines. Many life‐history patterns are therefore indirectly, but consistently, associated with genome size.

Salinity accelerates endodermal development and induces an exodermis in cotton seedling roots

The development of the endodermis was studied in 5- to 48-day-old cotton (Gossypium hirsutum L., cv. Acala SJ-2) seedling roots grown in vermiculite at different NaCl salinity levels. Sensitive fluorochromes (berberine-aniline blue for Casparian bands, and fluorol yellow-aniline blue for suberin lamellae) were used to detect cell wall modifications. Endodermal cells progressed through several developmental stages. In the primary stage, endodermal Casparian bands appeared synchronously in the radial walls. Then, suberin lamellae were laid down asynchronously in all cell walls, but always first in endodermal cells in the phloem sectors. Passage cells without suberin lamellae were usually present in the endodermal layer opposite protoxylem poles. The positions relative to the root tip at which Casparian bands and suberin lamellae developed were dependent on plant age and primary root length for control and salt-stressed seedlings. Salinity induced those structures to mature closer to the root tip for plants up to 10 days old, whereas in older plants no differences between treatments were observed. Exposure to high salinity (200 mM NaCl) induced the formation of an exodermis with Casparian bands and suberin lamellae close to the root base and in the transition zone to the hypocotyl. The exodermis, which never developed in control roots, may play a role in protecting the root from water loss and/or leakage of solutes important for osmotic adjustment. An exodermis was detected in 5- to 28-day-old seedlings. Older plants (48 days), with advanced secondary growth in the root stele, did not differentiate an exodermis.

Root Development and Absorption of Ammonium and Nitrate from the Rhizosphere

Plant roots operate in an environment that is extremely heterogeneous, both spatially and temporally. Nonetheless, under conditions of limited diffusion and against intense competition from soil microorganisms, plant roots locate and acquire vital nitrogen resources. Several factors influence the mechanisms by which roots respond to ammonium and nitrate. Nitrogen that is required for cell division and expansion derives primarily from the apex itself absorbing rhizosphere ammonium and nitrate. Root density and extension are greater in nutrient solutions containing ammonium than in those containing nitrate as the sole nitrogen source. Root nitrogen acquisition alters rhizosphere pH and redox potential, which in turn regulate root cell proliferation and mechanical properties. The net result is that roots proliferate in soil zones rich in nitrogen. Moreover, plants develop thinner and longer roots when ammonium is the primary nitrogen source, an appropriate strategy for a relatively immobile nitrogen form.

Apical organization and maturation of the cortex and vascular cylinder inArabidopsis thaliana (Brassicaceae) roots

Developmental and physiological studies of roots are frequently limited to a post-germination stage. In Arabidopsis, a developmental change in the root meristem architecture during plant ontogenesis has not previously been studied and is addressed presently. Arabidopsis thaliana have closed root apical organization, in which all cell file lineages connect directly to one of three distinct initial tiers. The root meristem organization is dynamic and changes as the root ages from 1 to 4 wk post-germination. During the ontogeny of the root, the number of cells within the root apical meristem (RAM) increases and then decreases due to changes in the number of cortical layers and number of cell files within a central cylinder. The architecture of the initial tiers also changes as the root meristem ages. Included in the RAMs ontogeny is a pattern associated with the periclinal divisions that give rise to the middle cortex and endodermis; the three-dimensional arrangement of periclinally dividing derivative cells resembles one gyre of a helix. Four- or 5-wk-old roots exhibit a disorganized array of vacuolated initial cells that are a manifestation of the determinate nature of the meristem. Vascular cambium is formed via coordinated divisions of vascular parenchyma and pericycle cells. The phellogen is the last meristem to complete its development, and it is derived from pericycle cells that delineate the outer boundary of the root.

The Developmental Anatomy and Ultrastructure of Somatic Embryos from Rice (Oryza sativa L.) Scutellum Epithelial Cells

The ontogeny of somatic embryos from tissue cultures of mature caryopses of rice is described. Somatic embryos arose directly from cells of the scutellum epithelium, preferentially in the basal part of the scutellum. The initial event in somatic embryogenesis was a periclinal division in scutellar epithelial cells between 48 h and 60 h of plating. The resultant basal and terminal cells gave rise to the suspensor and embryo, respectively. The terminal cell underwent a series of random divisions and became a globular embryo. The scutellum was initiated and overtopped the shoot apex, which developed in a lateral notch. The coleoptile arose from a rim of tissue surrounding the shoot apex while the root apex differentiated internal to the coleorhiza. At maturity, a rice somatic embryo closely resembles a zygotic embryo, complete with a scutellum epithelium, which is a source of secondary somatic embryos on further subculturing.

Cell-specific expression of plant histone H2A genes.

Histone H2A is a component of eukaryotic chromatin whose expression has not been studied in plants. We isolated and characterized a tomato and a pea cDNA encoding histone H2A. We found that in tomato H2A is encoded by a small gene family and that both the pea and the tomato mRNAs are polyadenylated. Tomato H2A has 82% amino acid residue identity to pea H2A, 83% to wheat, and 65% to human and yeast H2A. Plant H2As differ from fungal and animal H2As in their amino-terminal and carboxy-terminal regions. Carboxy-terminal plant H2A regions contain the motif SPKK, a peptide implicated in binding of A/T-rich DNA regions. By using RNA gel blot analysis, we determined that the steady-state mRNA level of these genes was abundant in apices and early developing fruit and very low in mature tissues. In situ RNA hybridization showed strong spatial regulation because the mRNA was abundant in some cells and not detectable in others. In tomato shoot tips, H2A-expressing cells were distributed irregularly in or near meristems. In tomato or pea root tips, expressing cells were concentrated near the apex, and their distribution was consistent with that expected of cycling cells. Other H2A transcripts were found in nondividing cortical cells that are known to undergo endoduplication during the late maturation phase of primary development.

Wound‐induced vascular occlusions in Vitis vinifera (Vitaceae): Tyloses in summer and gels in winter1

Vascular occlusion in xylem conduits is a common response to environmental stresses, and plant species are recognized as primarily tylose-forming or gel-forming. These stresses occur throughout the year, but there is little information on the wound responses throughout the year and in growing and dormant tissues. Wound-induced vascular occlusions were evaluated by type (tylose or gel), temporal progress, and spatial distribution for grape stems pruned in four seasons through an entire year. Tyloses were formed predominantly in summer and gels in winter. Cytohistological analyses indicated that wound-induced gels were pectin-rich. Both gel formation and tylose development were complete within 7 d and 10 mm from the cut regardless of the season of the wounding. Most vessels were affected by wounding, but a higher fraction of vessels developed occlusions in summer and autumn (over 80%) than in winter and spring (about 60%). The study is the first to show a single species is capable of producing primarily either tyloses or gels and that the type of wound-induced occlusion is dependent upon the season in which wounding occurs. Winter conditions limit the wound response to reversible gel formation that may contribute to refilling of embolized vessels in the spring.

Ethylene and Not Embolism Is Required for Wound-Induced Tylose Development in Stems of Grapevines

The pruning of actively growing grapevines (Vitis vinifera) resulted in xylem vessel embolisms and a stimulation of tylose formation in the vessels below the pruning wound. Pruning was also followed by a 10-fold increase in the concentration of ethylene at the cut surface. When the pruning cut was made under water and maintained in water, embolisms were prevented, but there was no reduction in the formation of tyloses or the accumulation of ethylene. Treatment of the stems with inhibitors of ethylene biosynthesis (aminoethoxyvinylglycine) and/or action (silver thiosulfate) delayed and greatly reduced the formation of tyloses in xylem tissue and the size and number of those that formed in individual vessels. Our data are consistent with the hypotheses that wound ethylene production is the cause of tylose formation and that embolisms in vessels are not directly required for wound-induced tylosis in pruned grapevines. The possible role of ethylene in the formation of tyloses in response to other stresses and during development, maturation, and senescence is discussed.

The effect of ethylene on adventitious root formation in mung bean (Vigna radiata) cuttings

A promotive effect of ethylene on the formation of adventitious roots by mung bean cuttings was demonstrated using a recirculating solution culture system to apply dissolved ethylene. The number of roots increased in proportion to the length of exposure to the gas. Mean root numbers per cutting for a 4-day exposure to ethylene and an air control were 45 and 19, respectively. The tissue was most sensitive to a 24-h ethylene “pulse” 2–3 days after taking cuttings. Rooting was maximal at a concentration of 13 μl 1−1 ethylene. The ethylene treatment inhibited the growth of roots and terminal buds. Application of Ag+, as silver thiosulfate, reversed the effect of ethylene on the two growth responses but had no effect on root numbers. Norbornadiene, another inhibitor of ethylene action, reversed all three ethylene responses.