Tansley Review No. 59 Leaf boundary layers

Studies of heat and mass exchange between leaves and their local environment are central to our understanding of plant-atmosphere interactions. The transfer across aerodynamic leaf boundary layers is generally described by non-dimensional expressions which reflect largely empirical adaptations of engineering models derived for flat plates. This paper reviews studies on leaves, and leaf models with varying degrees of abstraction, in free and forced convection. It discusses implecations of finding for leaf morphology as it affects – and is affected by – the local microclimate. Predictions of transfer from many leaves in plant communities are complicated by physical and physiological feedback mechanisms between leaves and their environment. Some common approaches, and the current challenge of integrating leaf-atmosphere interactions into models of global relevance, are also briefly addressed.     

Comparative physiology and demography of three Neotropical forest shrubs: alternative shade-adaptive character syndromes

A suite of functionally-related characters and demography of three species of Neotropical shadeadapted understory shrubs (Psychotria, Rubiaceae) were studied in the field over five years. Plants were growing in large-scale irrigated and control treatments in gaps and shade in old-growth moist forest at Barro Colorado Island, Panama. Irrigation demonstrated that dry-season drought limited stomatal conductance, light saturated photosynthesis, and leaf longevity in all three species. Drought increased mortality of P. furcata. In contrast, irrigation did not affect measures of photosynthetic capacity determined with an oxygen electrode or from photosynthesis-CO₂ response curves in the field. Drought stress limited field photosynthesis and leaf and plant survivorship without affecting photosynthetic capacity during late dry season. Leaves grown in high light in naturally occurring treefall gaps had higher photosynthetic capacity, dark respiration and mass per unit area than leaves grown in the shaded understory. P. furcata had the lowest acclimation to high light for all of these characters, and plant mortality was greater in gaps than in shaded understory for this species. The higher photosynthetic capacity of gap-grown leaves was also apparent when photosynthetic capacity was calculated on a leaf mass basis. Acclimation to high light involved repackaging (higher mass per unit leaf area) as well as higher photosynthetic capacity per unit leaf mass in these species. The three species showed two distinct syndromes of functionally-related adaptations to low light. P. limonensis and P. marginata had high leaf longevity (3 years), high plant survivorship, low leaf nitrogen content, and high leaf mass per unit area. In contrast, P. furcata had low leaf survivorship (1 year), high plant mortality (77–96% in 39 months), low leaf mass per unit area, high leaf nitrogen content, and the highest leaf area to total plant mass; the lowest levels of shelf shading, dark respiration and light compensation; and the highest stem diameter growth rates. This suite of characters may permit higher whole-plant carbon gain and high leaf and population turnover in P. furcata. Growth in deep shade can be accomplished through alternative character syndromes, and leaf longevity may not be correlated with photosynthetic capacity in shade adapted plants.     

Brief communication: Variation in the frequency and form of the lower permanent molar middle trigonid crest

The frequency and form of the middle trigonid crest (MTC) in lower permanent molars is reported for 1,131 dental casts of Bushman (San), Bantu, Solomons, Hawaiians, Pima, Eskimo, Navajo, Chinese, and American whites. The MTC occurs most often on the first molar. We found very little intra-trait variation, so observations were scored on a present-absent basis. The MTC is most frequent in the African samples and rare in those of the other populations. Two reference plaques can be obtained to add to the existing series in the ASU dental anthropology system. © 1993 Wiley-Liss, Inc.     

Presence of anisotropic (birefringent) crystalline structures in embryonic and juvenile monogonont rotifers

A systematic survey for the presence of birefringent (anisotropic) structures in rotifers was undertaken. Several common features of rotifers exhibit anisotropism (e.g. trophi & muscles). However, unusual anisotropic crystalline structures (ACS) were found in late stage embryos (i.e. possessing eyespots and trophi, and showing movement). ACS were found in 18 of 26 species of monogonont rotifers (comprising 11 genera of 5 families). In Sinantherina socialis, ACS were present in the lower gut as compact, spherical masses of minute crystals that slowly broke apart and disappeared within 20 hours of hatching. Although several authors have described the existence of refractive bodies in rotifers, to my knowledge this is the first report of their birefringent properties.     

Leaf nutrients in relation to stature and life form in tropical rain forest

Abstract. To document the relationship between a plant's position in the canopy and its leaf nutrient content, leaf nitrogen and phosphorus were determined for 30 species growing in mature evergreen lowland rain forest at La Selva Biological Station, Costa Rica. Species that grow either in the understory, midstory, or the canopy were selected. Species were further separated into three life forms: self-supporting monocots, self-supporting dicots, and climbers. Mass-based nutrient concentrations were expected to decrease with stature, as has been reported in studies of other forests. In fact, mass-based nitrogen and phosphorus did not vary significantly among the three adult-stature classes, although area-based values differed greatly: canopy plants averaged 60 % more nitrogen and 90 % more phosphorus per unit leaf area than understory plants. Differences in leaf characteristics were evident among the three life forms. Most notably, area-based phosphorus and leaf specific mass were lowest in climbers, intermediate in self-supporting dicots, and highest in self-supporting monocots. These results support the characterization of climbers as investing in inexpensive structures, perhaps in order to gain competitive advantage in light capture by allocating resources to maximize elongation rates.     

Lawns have vertical stratification

Abstract. Vertical stratification was examined in a closely-mown lawn, at both the beginning and end of the mowing/regrowth cycle. Two treatments were examined: Undisturbed, which had been maintained as lawn for 30 yr, and Mechanically Perturbed, on which succession was occurring, after complete removal of vegetation 9.5 months previously. There was significant vertical stratification at both stages of the mowing/regrowth cycle, i.e. even immediately after mowing to a height of 3 cm, and in both treatments. There was general consistency of the vertical role of species during the cycle and between treatments. Relative vertical positions were not closely correlated with absolute leaf heights. However, in no case was the vertical position of a pair of species significantly reversed during the cycle or between treatments. No mutually cyclic vertical relations between species were seen. It is concluded that vertical stratification occurs in even the shortest communities, it starts to develop quite early in succession, and it is dependent largely on the intrinsic properties of the species, their height and (at least in lawns) their recovery from defoliation.     

Differentiation and structural decline in the leaves and bark of birch (Betula pendula) under low ozone concentrations

Summary Leaf and bark structure of a birch clone (Betula pendula Roth) continuously exposed to charcoal-filtered air or charcoal-filtered air plus ozone (0.05, 0.075, 0.1 l 1^-1) was investigated throughout one growing season. Increasing ozone dose influenced leaf differentiation by reducing leaf area and increasing inner leaf air space, density of cells developing into stomata, scales and hairs. When approximately the same ozone dose had been reached, macroscopical and microscopical symptoms appeared irrespective of the ozone concentration used during treatment. Structural decline began in mesophyll cells around stomatal cavities (droplet-like exudates on the cell walls), continued with disintegration of the cytoplasma and ended in cell collapse. Epidermal cells showed shrinkage of the mucilaginous layer (related to water loss). Their collapse marked the final stage of leaf decline. When subsidiary cells collapsed, guard cells passively opened for a transitory period before collapsing and closing. With increasing ozone dose starch remained accumulated along the small leaf veins and in guard cells. IIK-positive grains were formed in the epidermal cells. This contrasted with the senescent leaves, where starch was entirely retranslocated. Injury symptoms in stem and petiole proceeded from the epidermis to the cambium. Reduced tissue area indicated reduced cambial activity. In plants grown in filtered air and transferred into ozone on 20 August, injury symptoms developed faster than in leaves formed in the presence of ozone. Results are discussed with regard to O₃-caused acclimation and injury mechanisms.     

Growth and nutrition of birch seedlings in relation to potassium supply rate

Summary Growth of hydroponically cultivated birch seedlings (Betula pendula Roth.) at sub- and supra-optimum potassium supply rates was investigated. Potassium was supplied either as a relative addition rate (r _k = 5, 10, 15 and 20% increase day^-1) or as fixed concentrations (0.2, 3, 6, 12 and 15 mM) in the culture solution. After an acclimation period the growth rate of the seedlings in the suboptimum treatments reached values close to the treatment variable, the relative rate of K-addition. Deficiency symptoms, in the form of chlorosis and necroses along the leaf margins, developed initially in all suboptimum treatments, but very few new symptoms appeared once the seedlings had reached the phase of steady-state nutrition and growth. At supra-optimum K-supply levels, i.e. at 0.2–15 mM K in the culture solution, no symptoms of deficiency or toxicity developed, and the relative growth rate of the seedlings remained maximum. The relative growth rate of the seedlings was linearly related to the plant K-status for K contents ranging from 0.2 to 1.0% of dry weight (DW). At higher internal K-concentrations, 1.0–3.0% DW, no further increase in relative growth rate was achieved. A shortage of K resulted in a decrease in the net assimilation rate. This effect was counterbalanced by the absence of shift in he leaf weight ratio as well as by the production of relatively thin leaves. The fraction of dry matter allocated to roots decreased in K-limited plants, as did the leaf contents of soluble carbohydrates and starch.