Biology subjects

Lamont, B. B.

Publications and source records attributed to Lamont, B. B..

3 recordsLinked to original sources

bioRxiv2022

Contrasting leaf thickness & saturated water content explain diverse structural/physiological properties of arid species

Although they account for many thousands of the worlds flowering plants, little is known about the physical/chemical properties of leaf succulents. Eight species in the Namib Desert, South Africa were assessed for leaf area (A) and thickness (z), saturated (Q) and dry mass, relative volume of air (Fa), intrinsic water-us efficiency ({delta}13C), and N, P and (Na+K) contents. As water-storage capacity is a function of Qv and z, this means Q/A (= Qv *z) is an ideal index of succulence compared with specific-leaf-area and other indices that highlight mass rather than volume. Specific gravities have a different relationship with Fa of sclerophyll-mesophylls: rising succulence infers decreasing air content replaced by water rather than dry matter. The trend among succulent species from Argentina/Spain added to our data was characterized by Q/A exceeding 1 mg water/mm2 leaf whose overall slope was ten times that for co-occurring sclerophyll-mesophyll species. (Na+K), N and P concentrations varied on a dry-matter, but not water-volume, basis. Wi relationships were essentially functions of variations in z (transpiration-resistance) and increased metabolic efficiency. We conclude that z and Qv are keys to the special physiological properties of succulent leaves. Including succulents would force many current monotonic relationships to dichotomize. HighlightThe key to understanding leaf succulents is their high volumetric water content and thick leaves. These explain their superior water-use-efficiency and contrasting relationships with other variables compared with temperate species.

plant biology↗

bioRxiv2021

Seed-coat thickness explains contrasting germination responses to smoke and heat by Leucadendron species

Fire stimulates germination of most seeds in fire-prone vegetation. Fruits of Leucadendron (Proteaceae) are winged achenes or nutlets that correlate with their requirements for smoke and/or heat in promoting germination. We describe five possible smoke-heat dormancy-release/germination syndromes among plants, of which Leucadendron displays three (no response, smoke only, smoke and heat). As seed-coat thickness varies with seed-storage location (plant or soil) and morphology (winged or wingless), we tested its possible role in water uptake and germination. Species with winged seeds achieved 100% germination in the absence of smoke/heat, seed coats were an order of magnitude thinner, and their permeability greatly exceeded that of nutlets. As seed-coat thickness increased a) imbibitional water uptake declined at a decreasing rate, and b) the response to smoke, and to a lesser extent heat, increased linearly to reach levels of germination approaching those of winged seeds. For species responsive to smoke and heat, there was no additive effect when applied together, suggesting that they may have promoted the same physiological process. By what mechanisms a) the smoke response is greater the thicker the seed coat, and b) smoke chemicals could increase water permeability to explain the non-additive effect of smoke and heat, warrant further investigation. HighlightWe show Leucadendron seeds are either plant-stored with thin, highly permeable seed-coats that germinate readily; or soil-stored and the thicker their seed-coat, the lower their permeability and greater their need for smoke/heat to promote germination.

plant biology↗

bioRxiv2017

Contribution of transition and stabilization processes to speciation is a function of the ancestral trait state and selective environment in Hakea

Currently the origin and trajectories of novel traits are emphasised in evolutionary studies, the role of stabilization is neglected, and interpretations are often post hoc rather than as hypothesised responses to stated agents of selection. Here we evaluated the impact of changing environmental conditions on trait evolution and stabilization and their relative contribution to diversification in a prominent Australian genus, Hakea (Proteaceae). We assembled a time-based phylogeny for Hakea, reconstructed its ancestral traits for six attributes and determined their evolutionary trajectories in response to the advent or increasing presence of fire, seasonality, aridity, nectar-feeding birds and (in)vertebrate herbivores/granivores. The ancestral Hakea arose 18 million years ago (Ma) and was broad-leaved, non-spinescent, insect-pollinated, had medium-sized, serotinous fruits and resprouted after fire. Of the 190 diversification events that yielded the 82 extant species analysed, 8-50% involved evolution, stabilization or re-evolution (reversal) of individual novel traits. Needle leaves appeared 14 Ma and increased through the Neogene/Quaternary coinciding with intensifying seasonality and aridity. Spinescence arose 12 Ma consistent with the advent of vertebrate herbivores. Bird-pollination appeared 14 Ma in response to advent of the Meliphagidae in the early Miocene. Small and large woody fruits evolved from 12 Ma as alternative defenses against granivory. Fire-caused death evolved 14 Ma, accounting for 50% of subsequent events, as fire became less stochastic. Loss of serotiny began in the late Miocene as non-fireprone habitats became available but only contributed 8% of events. Innovation and subsequent stabilization of functional traits promoted the overall species diversification rate in Hakea by 15 times such that only three species now retain the ancestral phenotype. Our approach holds great promise for understanding the processes responsible for speciation of organisms when the ancestral condition can be identified and the likely selective agents are understood.

ecology↗