Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus)

Whether seed consumers affect plant establishment is an important unresolved question in plant population biology. Seed consumption is ubiquitous; at issue is whether seedling recruitment is limited by safe-sites or seeds. If most seeds inhabit sites unsuitable for germination, post-dispersal seed consumption primarily removes seeds that would otherwise never contribute to the population and granivory has minimal impacts on plant abundance. Alternatively, if most seeds ultimately germinate before they lose viability, there is greater potential for seed consumption to affect plant recruitment. Of the many studies on seed consumption, few ask how seed loss affects seedling recruitment for species with long-lived seed banks. We examined post-dispersal seed predation and seedling emergence in bush lupine (Lupinus arboreus), a woody leguminous shrub of coastal grasslands and dunes in California. We followed the fate of seeds in paired experimental seed plots that were either protected or exposed to rodent granivores in grassland and dune habitats. Significantly more seeds were removed by rodents in dunes than grasslands. In dunes, where rodent granivory was greatest (65% and 86% of seeds removed from plots by rodents in two successive years), there is a sparse seed bank (6.6 seeds m⁻²), and granivory significantly reduced seedling emergence (in the same two years, 18% and 19.4% fewer seedlings emerged from exposed versus protected plots), suggesting seed rather than safe-site limited seedling recruitment. In contrast, rodents removed an average of 6% and 56% of seeds from grassland plots during the same two years, and the grassland seed bank is 43-fold that of the dunes (288 seeds m⁻²). Even high seed consumption in the second year of the study only marginally influenced recruitment because seeds that escaped predation remained dormant. Burial of seeds in both habitats significantly reduced the percentage of seeds removed by rodents. Results suggest that granivores exert strong but habitat-dependent effects on lupine seed survival and seedling emergence. Received: 24 October 1996 / Accepted: 4 February 1997     

Pre-dispersal seed predation and seed limitation in an annual legume

The basic idea of the source simulation technique is to replace the scatterer (or radiator by a system of simple sources located within the envelope of the original body. The extent to which the simulated field reproduces the original field depends on the degree of correspondence between the simulated and the given boundary conditions. Numerical simulations have shown that: (1) the shape of the auxiliary surface, (2) the number of sources, and (3) the way the sources are distributed are the most relevant parameters to ensure an accurate solution for the problem. In the case of the single-layer method, sources should not be positioned close to the center of the body, because the problem becomes ill-conditioned. The auxiliary surface and the scatterer should be as similar as possible in order to minimize the boundary error. With respect to the number of sources (N), there are two opposite effects: (1) if (N) is too small, the sound field is not reproduced accurately; (2) if (N) is too large, computing time increases and solution accuracy decreases. The method beaks down when excitation frequency coincides with the eigenfrequencies — a narrow range of frequencies — of the space formed by the auxiliary surface. As the auxiliary surface is frequently represented by simple surfaces (cylinder, sphere), one can easily calculate the eigenfrequencies and therefore avoid them.

Consumer pressure, seed versus safe-site limitation, and plant population dynamics

Plants often suffer reductions in fecundity due to insect herbivory. Whether this loss of seeds has population-level consequences is much debated and often unknown. For many plants, particularly those with long-lived seedbanks, it is frequently asserted that herbivores have minimal impacts on plant abundance because safe-site availability rather than absolute seed number determines the magnitude of future plant recruitment and hence population abundance. However, empirical tests of this assertion are generally lacking and the interplay between herbivory, spatio-temporal variability in seed- or safe-site-limited recruitment, and seedbank dynamics is likely to be complex. Here we use a stochastic simulation model to explore how changes in the spatial and temporal frequency of seed-limited recruitment, the strength of density-dependent seedling survival, and longevity of seeds in the soil influence the population response to herbivory. Model output reveals several surprising results. First, given a seedbank, herbivores can have substantial effects on mean population abundance even if recruitment is primarily safe-site-limited in either time or space. Second, increasing seedbank longevity increases the population effects of herbivory, because annual reductions in seed input due to herbivory are accumulated in the seedbank. Third, population impacts of herbivory are robust even in the face of moderately strong density-dependent seedling mortality. These results imply that the conditions under which herbivores influence plant population dynamics may be more widespread than heretofore expected. Experiments are now needed to test these predictions. Received: 3 November 1999 / Accepted: 15 February 2000     

Seed mass and seedling dimensions in relation to seedling establishment

Several experiments have shown that seedlings from larger-seeded species are better able to survive various hazards during establishment. Previous work has suggested a general mechanism might underpin this outcome. Larger-seeded species might tend to mobilize their metabolic resources over a longer period into the autotrophically functioning structures of the seedling. Consequently relatively more resources would remain uncommitted at any given time during the early period of the seedling’s growth, and available to support respiration during carbon deficit. An important aspect of this larger-seed-later-commitment mechanism would be that at a given time, larger-seeded species would hold more resources uncommitted not just absolutely, but relative to the functional seedling structures that needed to be supported. Here we quantify, across a wide range of phanerocotylar species, an allometric pattern that supports the generality of a larger-seed-later-commitment mechanism as an explanation for superior performance by larger-seeded species in face of the hazards of seedling establishment. Larger-seeded species allocate relatively less to cotyledon area, reflecting the initial functional size of the seedling, and relatively more to dry mass per unit area of cotyledon, reflecting stored metabolic reserves. The shift in relative allocation is progressive, rather than seedlings falling into discrete morphological types. The allometry is similar whether considered as correlated evolutionary divergences (phylogenetically independent contrasts) or as correlation across present-day species. Received: 7 April 1999 / Accepted: 29 March 2000     

Effects of seed size and emergence time on tree seedling establishment: importance of developmental constraints

Arafura filesnakes (Acrochordus arafurae) are large (up to 2.5 m, 5 kg) aquatic nonvenomous snakes that feed entirely on fishes. A 10-year field study in the Australian wet-dry tropics revealed strong correlations between rainfall patterns, fish abundance, and snake population dynamics. All of these characteristics showed considerable annual variation. High rainfall late in the wet season (February–March) caused prolonged inundation of the floodplain. Following such years, dry-season sampling revealed that fishes were abundant, filesnakes were in good body condition, and a high proportion of adult female filesnakes were reproductive. Annual variation in recruitment to the population (as judged by the relative abundance of yearling snakes) was also correlated with fish abundance and thus, with rainfall patterns in the late-wet season. Our results fit well with those from other studies on a diverse array of aquatic and terrestrial species within the wet-dry tropics. Annual variation in rainfall patterns, via its effects on prey abundance, may drive the population dynamics of many tropical predators. Received: 28 June 1999 / Accepted: 29 February 2000     

Sabal palmetto seed size: causes of variation,choices of predators,and consequences for seedlings

High variation in seed size, as is common among angiosperms, may be maintained in a plant species when several factors select for seed size. Variation may also result from differences among adult plants, such as nutrient and water availability or the amount of photosynthetic tissue. In a study of Sabal palmetto seed ecology I found high seed size variation both within- and among-palms, and investigated possible factors maintaining this variation. Seed size was positively correlated with the number of leaves on parent palms. Larger seeds produced more vigorous seedlings that had greater leaf length, area, and mass, and greater root mass. Caryobruchus gleditsiae (Bruchidae: Coleoptera), whose larvae develop within palm seeds, preferentially oviposited on larger seeds, which in turn produced larger beetle offspring. By choosing the largest seeds available, ovipositing beetles thus affect both the quantity and the quality of seeds available for recruitment. I conclude that because beetle predation selects against large seeds, while larger seeds promote seedling vigor, the maintenance of seed size variation may be an adaptation of S. palmetto promoting both seed escape from predators and seedling vigor.     

A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy

 A hypothesized relationship between seed weight and leaf size was investigated for 58 diverse British (semi-)woody species. Interspecific variation in leaf size of adult plants corresponded allometrically with interspecific variation in the weight of an infructescence (seed-bearing inflorescence). The relationship between seed size and leaf size of adult plants was triangular. The corners of the triangle were interpreted in terms of ecological strategy. Medium-sized infructescences, small seeds and large leaves were seen among medium-sized, fast-growing, earlier-successional, mostly deciduous shrubs and trees; small infructescences, small seeds and small leaves mostly among low, slow-growing evergreens from stress-prone, proclimax habitats; and large infructescences, large seeds and large leaves among slow-growing, later-successional trees of potential competitive vigour. The hypothesis that the combination of large seeds and small leaves is allometrically unlikely was supported by the data. The roles of ontogeny and taxonomic relatedness in the seed size-leaf size relationship were examined by correlative and taxonomic analyses of seed, plant and leaf size during the unfolding of the life history from seed through two seedling phases to adulthood. Deciduous versus evergreen leaf habit was a source of deviation from the otherwise linear allometric relationships during ontogenetic development, none of which were, individually, confounded significantly with taxonomy. Received: 2 March 1998 / Accepted: 15 October 1998     

Priming of field-sown rice seed enhances germination, seedling establishment, allometry and yield

Poor seedling establishment is a major deterrent in adopting direct seeding of rice. Seed priming to obtain better crop stand could be an attractive approach. The objective of this study was to determine the effectiveness of seed priming strategies on the improved agronomic characters of direct-sown rice. Seed priming strategies were: hydropriming for 48 h, osmohardening with KCl or CaCl₂ for 24 h, ascorbate priming for 48 h and seed hardening for 24 h, pre-germination (traditional soaking for nursery raising) and untreated control. Seed priming improved germination and emergence, allometry, kernel yield, and its quality, whilst pre-germination displayed poor and erratic emergence of seedling followed by poor plant performance. Faster and uniform emergence was due to improved α-amylase activity, which increased the level of soluble sugars in the primed kernels. Osmohardening with KCl gave greater kernel and straw yield and harvest index, followed by that of CaCl₂, hardening and ascorbate priming. Improved yield was attributed principally to number of fertile tillers and 1000 kernel weight. A positive correlation between mean emergence time and days to heading, while a negative one between kernel yield and harvest index suggested long-term effects of seed priming on plant growth and development. The results suggest that physiological changes produced by osmohardening enhanced the starch hydrolysis and made more sugars available for embryo growth, vigorous seedling production and, later on, improved allometric, kernel yield and quality attributes.     

Polymorphism, distribution, and segregation of AFLP markers in a doubled haploid rice population

We exploited the newly developed amplified fragment length polymorphism (AFLP) technique to study the polymorphism, distribution and inheritance of AFLP markers with a doubled haploid rice population derived from ‘IR64’/‘Azucena’. Using only 20 pairs of primer combinations, we detected 945 AFLP bands of which 208 were polymorphic. All 208 AFLP markers were mapped and distributed over all 12 chromosomes. When these were compared with RFLP markers already mapped in the population, we found the AFLP markers to be highly polymorphic in rice and to follow Mendelian segregation. As linkage map of rice can be generated rapidly with AFLP markers they will be very useful for marker-assisted backcrossing. Received: 11 April 1996 / Accepted: 14 June 1996     

Plant litter feedback and population dynamics in an annual plant,<Emphasis Type="Italic"> Cardamine pensylvanica</Emphasis>

The presence of litter has the potential to alter the population dynamics of plants. In this paper, we explore the effects of litter on population dynamics using a simple experimental laboratory system with populations of the annual crucifer, Cardamine pensylvanica. Using a factorial experiment with four densities and three litter levels, we determined the effect of litter on biomass and plant fecundity, and the life stages responsible for these changes in yield. Although litter had significant effects on seed germination and on seedling survivorship, we show, using a population dynamics model, that these effects were not demographically significant. Rather, the potential effect of litter on population dynamics resulted almost entirely from its effect on biomass. Persistent litter suppressed plant biomass and apparently removed the direct density effect present in the absence of litter. Thus, litter changed the shape of the recruitment curve from slightly humped to asymptotic. In addition to changing the shape of the recruitment curve, litter reduced the carrying capacity of the populations. Thus, the population dynamics model indicated that not all statistically significant responses were dynamically significant. Given the potential complexity of litter effects, simple population models provide a powerful tool for understanding the potential consequences of short-term responses. Received: 8 September 1999 / Accepted: 5 April 2000     

Comparison of different methods for plant regeneration and transformation of the legume Galega orientalis Lam. (goat's rue)

 For the first time, regeneration and transformation have been achieved from the legume Galega orientalis Lam. (goat's rue). Two different regeneration protocols are described, one based on direct shoot induction from meristems and the other involving callus induction and shoot induction from callus with the plant growth regulator thidiazuron (TDZ). Different media and explants were evaluated. Three different transformation methods were compared: cocultivation with four different Agrobacterium tumefaciens strains, electroporation of embryos and apical meristems and particle bombardment of embryos. TDZ-promoted shoot induction on calli from immature embryos gave the best results. Transformation using this regeneration protocol was most successful with particle bombardment. Stable transformation has yet to be proven. Received: 11August 1997 / Revision received: 6 April 1998 / Accepted: 1 March 1999     

Inbreeding and true seed in tetrasomic potato. III. Early selection for seedling vigor in open-pollinated populations

Tuber yield in open-pollinated (OP) offspring of potato cultivars appears to be correlated with the proportion of hybrids within each population. OP true potato seed (TPS) from ten selected clones was tested in an experiment at two Peruvian locations, San Ramón (mid-altitude warm tropics) and Huancayo (highland cool tropics), to investigate the early selection effect on OP populations. Another experiment in La Molina (dry coast) was included to confirm the results from these other two locations. The results clearly showed that plant survival, tuber yield and berry number can be increased by a strong early selection. Likewise, the results suggested that the most-promising OP offspring for TPS production could be derived from tetraploid clones with a high rate of outcrossing. This experiment demonstrates that high tuber yield may be obtained in OP TPS cultivars with little effort, using early selection for seedling vigor in the nursery. Genetic interpretation of this response to early selection indicates that both the intensity of selection and non-additive genetic variation for tuber yield account for these observed gains. Received: 15 December 2000 / Accepted: 4 April 2001     

The diffusion model for migration and selection in a plant population

 The diffusion approximation is derived for migration and selection at a multiallelic locus in a partially selfing plant population subdivided into a lattice of colonies. Generations are discrete and nonoverlapping; both pollen and seeds disperse. In the diffusion limit, the genotypic frequencies at each point are those determined at equilibrium by the local rate of selfing and allelic frequencies. If the drift and diffusion coefficients are taken as the appropriate linear combination of the corresponding coefficients for pollen and seeds, then the migration terms in the partial differential equation for the allelic frequencies have the standard form for a monoecious animal population. The selection term describes selection on the local genotypic frequencies. The boundary conditions and the unidimensional transition conditions for a geographical barrier and for coincident discontinuities in the carrying capacity and migration rate have the standard form. In the diallelic case, reparametrization renders the entire theory of clines and of the wave of advance of favorable alleles directly applicable to plant populations. Received 30 August 1995; received in revised form 23 February 1996     

Genetic variation in stomatal and biochemical limitations to photosynthesis in the annual plant, Polygonum arenastrum

 Terrestrial plant photosynthesis may be limited both by stomatal behavior and leaf biochemical capacity. While inferences have been made about the importance of stomatal and biochemical limitations to photosynthesis in a variety of species in a range of environments, genetic variation in these limitations has never been documented in wild plant populations. Genetic variation provides the raw material for adaptive evolution in rates of carbon assimilation. We examined genetic variation in gas exchange physiology and in stomatal and biochemical traits in 16 genetic lines of the annual plant, Polygonum arenastrum. The photosynthesis against leaf internal CO₂ (A−ci) response curve was measured on three greenhouse-grown individuals per line. We measured the photosynthetic rate (A) and stomatal conductance (g), and calculated the internal CO₂ concentration (ci) at ambient CO₂ levels. In addition, the following stomatal and biochemical characteristics were obtained from the A−ci curve on each individual: the degree of stomatal limitation to photosynthesis (L_s), the maximum ribulose 1,5-biphosphate carboxylase-oxygenase (Rubisco) activity (Vc_max) and electron transport capacity (J_max). All physiological traits were genetically variable, with broad sense heritabilities ranging from 0.66 for L_s to 0.94 for J_max. Strong positive genetic correlations were found between Vc_max and J_max, and between g and biochemical capacity. Path analyses revealed strong causal influences of stomatal conductance and leaf biochemistry on A and ci. Path analysis also indicated that L_s confounds both stomatal and biochemical effects, and is an appropriate measure of stomatal influences on photosynthesis, only when biochemical variation is accounted for. In total, our results indicate that differences among lines in photosynthesis and ci result from simultaneous changes in biochemical and stomatal characteristics and are consistent with theoretical predictions that there should be co-limitation of photosynthesis by ribulose-1,5-biphosphate (RuBP) utilization and regeneration, and by stomatal conductance and leaf biochemistry. Gas exchange characteristics of genetic lines in the present study were generally consistent with measurements of the same lines in a previous field study. Our new results indicate that the mechanisms underlying variation in gas exchange include variation in both stomatal conductance and biochemical capacity. In addition, A, g, and ci in the present study tended also to be positively correlated with carbon isotope discrimination (Δ), and negatively correlated with time to flowering, life span, and leaf size based on earlier work. The pattern of correlation between physiology and life span among genetic lines of P. arenastrum parallels interspecific patterns of character correlations. We suggest that the range of trait constellations among lines in P. arenastrum represents a continuum between stress avoidance (rapid development, high gas exchange metabolism) and stress tolerance (slow development, low gas exchange metabolism), and that genetic variation in these character combinations may be maintained by environmental variation in stress levels in the species’ ruderal habitat. Received: 28 March 1996 / Accepted: 13 August 1996     

Maternal and direct effects of elevated CO2 on seed provisioning, germination and seedling growth in Bromus erectus

Elevated CO₂ can affect plant fitness not only through its effects on seed production but also by altering the quality of seeds and therefore germination and seedling performance. We collected seeds from mother plants of Bromus erectus grown in field plots at ambient and elevated CO₂ (m-CO₂, maternal CO₂) and germinated them in the greenhouse in a reciprocal design under ambient and elevated CO₂ (o-CO₂, offspring CO₂). This design allowed us to examine both the direct effects of elevated CO₂ on germination and seedling growth and the indirect (maternal) effects via altered seed quality. Elevated m-CO₂ significantly increased seed mass and increased the C:N ratio of seeds from field-grown plants. Percentage and rate of germination were not affected by the m-CO₂ or o-CO₂ treatments. Similarly, elevated m-CO₂ had no significant effect on seedling size as estimated by the total leaf length. When differences in seed mass were adjusted by using seed mass as a covariate in ANOVA, a negative effect of m-CO₂ on seedling size appeared which increased with increasing seed mass (significant covariate×m-CO₂ interaction). This may indicate that the advantage of increased seed mass at elevated m-CO₂ was offset by the reduced concentration of nitrogen (and possibly other nutrients) in these seeds. In contrast to m-CO₂, elevated o-CO₂ greatly increased seedling size, and this stimulatory effect of elevated o-CO₂ was found to increase with increasing seed mass (significant covariate×o-CO₂ interaction). Taken together, these results suggest that in B. erectus transgenerational effects of elevated CO₂ are relatively small. However, other factors (genetic and environmental) that contribute to variation in seed provisioning can critically influence the responsiveness of seedlings to elevated CO₂. Received: 10 May 1999 / Accepted: 6 January 2000     

Effects of experimental manipulation of inflorescence size on pollination and pre-dispersal seed predation in the hummingbird-pollinated plant Ipomopsis aggregata

Large floral displays should theoretically provide advantages to plants through increased pollinator visitation and resulting fruit and seed set. However empirical tests of the response of pollinators to floral display size have been limited by a lack of direct experimentation, and the results of such studies have been equivocal. In addition, other selective agents such as pre-dispersal seed predators might modulate effects of floral display on pollination. By artificially altering flower number, we examined the direct effects of floral display in the monocarpic herb, Ipomopsis aggregata (Polemoniaceae), on visitation rates by broad-tailed and rufous hummingbird pollinators, as well destruction of fruits by a pre-dispersal seed predator (Hylemya: Anthomyiidae). In addition, we quantified the ultimate effects of flower number on female reproductive success. Plants with larger floral displays were most likely to be visited first in any given foraging bout (P < 0.01). As expected, plants with more flowers received more total flower visits. However, we found no gain in the proportion of flowers visited for many- versus few-flowered plants, or the total number of approaches/hour. In fact, a significantly greater percentage of flowers were visited on few-flowered plants. Plants did not compensate for our reduction in flowers by increasing investment in the number or proportion of flowers that set fruit, the number of seeds/fruit, or seed weight. Pre-dispersal seed predation was greater for many- than for few-flowered plants (P < 0.001), but this did not offset the potential fitness gains of producing large displays. Our data support the hypothesis that large floral displays function primarily in long-distance attraction of pollinators, and enhance maternal success. Received: 21 March 1996 / Accepted: 24 October 1996     

Novel mutations affecting leaf stearate content and plant size in Arabidopsis

 The fab2-1 mutant of Arabidopsis is an extreme dwarf as a direct result of an increase in the levels of stearate (18 : 0) in membrane lipids. We isolated a series of lines in which second-site suppressor mutations partly alleviate the dwarf phenotype. In all four of the suppressor lines examined, restoration of more normal morphology is accompanied by decreases in leaf 18 : 0 content. Three of the isolated suppressors suppress the high stearate phenotype in both leaves and seeds. The effects of one of the suppressors, TW2-1, is limited to the leaves. A second allele at the fab2 locus, fab2-2, was also identified and plants homozygous for this allele where intermediate in both plant size and 18 : 0 content between wild-type Arabidopsis and fab2-1 mutants. The alleles at fab2 and the suppressor mutations provided a total of nine genotypes which were analyzed to demonstrate a clear-cut relationship between leaf 18 : 0 content (0.7–19.6% of total leaf fatty acids) and reductions in plant size (24–4 mm). These results illustrate the utility of suppressor analysis for addressing problems in biochemistry and plant biology. They also indicate that the genetic control of plant lipid composition is more complex than previously appreciated. Received: 24 January 1997 / Accepted: 14 February 1997     

Outdoor clothing: its relationship to geography, climate, behaviour and cold-related mortality in Europe

It has been suggested, that the inhabitants of northern European regions, who experience little cold-related mortality, protect themselves outdoors by wearing more clothing, at the same temperature, than people living in southern regions where such mortality is high. Outdoor clothing data were collected in eight regions from 6583 people divided by sex and age group (50–59 and 65–74 years). Across Europe, the total clothing worn (as assessed by dry thermal insulation and numbers of items or layers) increased significantly with cold, wind, less physical activity and longer periods outdoors. Men wore 0.14 clo (1 clo=0.115 m² K W^–1) more than women and the older people wore 0.05 clo more than the younger group (both P<0.001). After allowance for these factors, regional differences in insulation and item number were correlated (r=–0.74, P=0.037; r=–0.74, P=0.036 respectively), but not those in clothing layers (r=–0.21; P=0.61), with indices of cold-related mortality. Cold weather most increased the wearing of gloves, scarves and hats. The geographical variation in the wearing of these three together items more closely matched that in cold-related mortality (r=–0.89, P=0.003). A possible explanation for this may be that they protect the head and hands, where stimulation by cold greatly increases peripheral vasoconstriction causing a rise in blood pressure that procedure haemoconcentration and raised cardiovascular risk. Received: 30 September 1999 / Revised: 3 April 2000 / Accepted: 5 June 2000