STUDIES ON THE POPULATION BIOLOGY OF THE GENUS VIOLA. II. THE EFFECT OF PLANT SIZE ON FITNESS IN VIOLA SORORIA

Evaluation of secondary sexual dimorphism of the dioecious Amaranthus palmeri under abiotic stress

The evolution of secondary sex-specific traits of dioecious species under abiotic stress conditions has received limited research, especially in the case of Amaranthus palmeri, a fast adapting and highly competing plant. Here, we have examined the interactive effects of abiotic stress on mineral accumulation, chlorophyll a and b content, and the operating capacity of Photosystem II (PSII) in both male and female A. palmeri plants grown under three different intensities of white light, and under N, K or P deficiency. Mineral profiling of the leaves and stems (with inflorescence) highlighted intra- and intersexual differences in their accumulation pattern and mineral associations. Chlorophyll a and chlorophyll b were different between the male and the female plants, being slightly lower in the latter, at high light intensity towards maturity, or under K or P deficiency. Further, slight, although statistically significant differences were recorded in the chlorophyll a/b ratio, which was lower at the higher light intensity in the female, over that in the male, plants towards maturity. Chlorophyll fluorescence parameters, i.e., steady state and maximum fluorescence increased under high light intensity, whereas the PSII operating efficiency decreased in the female plants, indicating reduced PSII capacity. Sex-specific differences in A. palmeri showed a differential response to stressful conditions because of differences in their ontogeny and physiology, and possibly due to the cost of reproduction. We suggest that the breeding system of dioecious species has weaknesses that can be used for the ecological management of dioecious weeds without relying on the use of herbicides.

Climate change is associated with increased allocation to potential outcrossing in a common mixed mating species

PREMISE

Although the balance between cross- and self-fertilization is driven by the environment, no long-term study has documented whether anthropogenic climate change is affecting reproductive strategy allocation in species with mixed mating systems. Here, we test whether the common blue violet (Viola sororia; Violaceae) has altered relative allocation to the production of potentially outcrossing flowers as the climate has changed throughout the 20th century.

METHODS

Using herbarium records spanning from 1875 to 2015 from the central United States, we quantified production of obligately selfing cleistogamous (CL) flowers and potentially outcrossing chasmogamous (CH) flowers by V. sororia, coupled these records with historic temperature and precipitation data, and tested whether changes to the proportion of CL flowers correlate with temporal climate trends.

RESULTS

We find that V. sororia progressively produced lower proportions of CL flowers across the past century and in environments with lower mean annual temperature and higher total annual precipitation. We also find that both CL and CH flower phenology has advanced across this time period.

CONCLUSIONS

Our results suggest that V. sororia has responded to lower temperatures and greater water availability by shifting reproductive strategy allocation away from selfing and toward potential outcrossing. This provides the first long-term study of how climate change may affect relative allocation to potential outcrossing in species with mixed mating systems. By revealing that CL flowering is associated with low water availability and high temperature, our results suggest the production of obligately selfing flowers is favored in water limited environments.

Selective forces on the maintenance of outcrossing in an almost exclusively cleistogamous violet species

PREMISE

Cleistogamous species constitute interesting study systems to resolve the longstanding question of how outcrossing is maintained given that seed production is ensured through selfing. In this work, we investigate the selective forces that allow the persistence of producing self-pollinated cleistogamous (CL) and chasmogamous (CH) flowers in Viola jaubertiana Marès & Vigin.

METHODS

We monitored three populations at different elevation for two years, and studied the flowering phenology and the relative contribution of each flower morph to parental fitness. We tested whether allocation to CH and CL flowers differed across populations and if it covaried with herbivory and water stress conditions. We also performed hand-pollination and bagging experiments in CH flowers to estimate inbreeding depression and heterosis.

RESULTS

The CH flowers open in winter under unfavorable conditions for pollination, show high pollen limitation and no-delayed selfing, and thus produce a low amount of seeds. Conversely, CL flowers appear in early spring, are physiologically cheaper to produce (i.e., dry weight is 3.4 times lower than that of CH flowers), and yield approximately 100 times more seeds than CH flowers. The CH flowers were favored under water stress and low herbivory. Crosses between populations showed up to 25% greater fitness than those within populations.

CONCLUSIONS

Despite the great pollen limitation in CH flowers, we suggest that the interaction among different environmental determinants and heterosis are probably sufficient forces to maintain chasmogamy in this long-lived species, reducing deleterious fixed mutations in the selfed lines.

FITNESS CONSEQUENCES OF MULTIPLE PATERNITY IN WILD RADISH, RAPHANUS SATIVUS

In natural populations, wild radish plants typically mate with 6-8 pollen donors, and seeds of individual fruits are usually sired by 1-4 fathers. Since radish fruits are indehiscent and gravity-dispersed, progeny are most likely to compete with a mixture of full and half siblings. The fitness consequences of single and multiple paternity were investigated in a greenhouse experiment. Seeds of every possible cross in a 5 times 5 reciprocal diallel mating design were assigned to one of three competition regimes (four full siblings, four maternal half siblings, or four unrelated individuals per pot) or were grown as singletons. After 14 weeks, the aboveground biomass of all plants was harvested and oven-dried. The dry weight of singletons was more than three times that of progeny grown in competition, indicating that intraspecific competition had occurred. Full- and half-sib progenies did not differ in mean dry weight. Thus, there was no evidence that multiple paternity enhances this aspect of maternal fitness. However, the competition regime dramatically affected the coefficient of variation in dry weight of progeny within a pot. Weight hierarchies were much more pronounced in pots of half sibs and unrelated neighbors than in pots of full sibs. Also variance in dry weight attributable to sire was greatest in the half-sib and "unrelated neighbors" competition regimes. These results suggest that weight hierarchies reinforce genetic differences among the offspring.

Scale-invariant structure of size fluctuations in plants

A wide range of physical and biological systems exhibit complex behaviours characterised by a scale-invariant structure of the fluctuations in their output signals. In the context of plant populations, scaling relationships are typically allometric. In this study, we analysed spatial variation in the size of maize plants (Zea Mays L.) grown in agricultural plots at constant densities and found evidence of scaling in the size fluctuations of plants. The findings indicate that the scaling of the probability distribution of spatial size fluctuation exhibits non-Gaussian behaviour compatible with a Lévy stable process. The scaling relationships were observed for spatial scales spanning three orders of magnitude. These findings should provide additional information for the selection and development of empirically accurate models of pattern formation in plant populations.

Effects of low atmospheric CO2 and elevated temperature during growth on the gas exchange responses of C3, C3-C4 intermediate, and C4 species from three evolutionary lineages of C4 photosynthesis

This study evaluates acclimation of photosynthesis and stomatal conductance in three evolutionary lineages of C(3), C(3)-C(4) intermediate, and C(4) species grown in the low CO(2) and hot conditions proposed to favo r the evolution of C(4) photosynthesis. Closely related C(3), C(3)-C(4), and C(4) species in the genera Flaveria, Heliotropium, and Alternanthera were grown near 380 and 180 μmol CO(2) mol(-1) air and day/night temperatures of 37/29°C. Growth CO(2) had no effect on photosynthetic capacity or nitrogen allocation to Rubisco and electron transport in any of the species. There was also no effect of growth CO(2) on photosynthetic and stomatal responses to intercellular CO(2) concentration. These results demonstrate little ability to acclimate to low CO(2) growth conditions in closely related C(3) and C(3)-C(4) species, indicating that, during past episodes of low CO(2), individual C(3) plants had little ability to adjust their photosynthetic physiology to compensate for carbon starvation. This deficiency could have favored selection for more efficient modes of carbon assimilation, such as C(3)-C(4) intermediacy. The C(3)-C(4) species had approximately 50% greater rates of net CO(2) assimilation than the C(3) species when measured at the growth conditions of 180 μmol mol(-1) and 37°C, demonstrating the superiority of the C(3)-C(4) pathway in low atmospheric CO(2) and hot climates of recent geological time.

Reproductive yield of individuals competing for light in a dense stand of an annual, Xanthium canadense

In a dense stand, individuals compete with each other for resources, especially for light. Light availability decreases with increasing depth in the canopy, thus light competition becoming stronger with time in the vegetative phase. In the reproductive phase, on the other hand, leaves start senescing, and the light environment, particularly of smaller individuals, will be improved. To study the effect of change in light climate on reproduction of individuals, we established an experimental stand of an annual, Xanthium canadense, and assessed temporal changes in whole plant photosynthesis through the reproductive phase with particular reference to light availability of individuals. At flowering, 83% of individuals were still alive, but only 27% survived to set seeds. Most of the individuals that died in the reproductive phase were smaller than those that produced seeds. Individuals that died at the early stage of the reproductive phase had a lower leaf to stem mass ratio, suggesting that the fate of individuals was determined partly by the pattern of biomass allocation in this period. At the early stage of the reproductive phase, leaf area index (LAI) of the stand was high and larger individuals had higher whole plant photosynthesis than smaller individuals. Although light availability at later stages was improved with reduction in LAI, whole plant photosynthesis was very low in all individuals due to a lower light use efficiency, which was caused by a decrease in photosynthetic N use efficiency. We conclude that light competition was still strong at the early stage of the reproductive phase and that later improvement of light availability did not ameliorate the photosynthesis of smaller individuals.

Phenotypic variation in seedlings of a "keystone" tree species (Quercus douglasii): the interactive effects of acorn source and competitive environment

Blue oak (Quercus douglasii) is a deciduous tree species endemic to California that currently exhibits poor seedling survival to sapling age classes. We used common garden techniques to examine how genetic variation at regional and local scales affected phenotypic expression in traits affecting oak seedling growth and survival. Between-population variation was examined for seedlings grown from acorns collected from a northern, mesic population and a southern, xeric population. Within-population variation was examined by comparing seedlings from different maternal families within the mesic population. Acorns were planted into neighborhoods of an annual dicot (Erodium botrys), an annual grass (Bromus diandrus), and a perennial bunchgrass (Nassella pulchra). By varying the species composition of herbaceous neighborhoods into which acorns were planted, the interactive effects of competition and acorn germplasm source on phenotypic expression could also be examined. Potential maternal effects, expressed as variation in acorn size, were assessed by weighing each acorn before planting. Probability of seedling emergence increased significantly with acorn size in the xeric population but not in the mesic population. Similarly, the effect of acorn size on seedling leaf area, stem weight, and root weight was also population-dependent. At a within-population level, acorn size effects on seedling traits varied significantly among maternal families. In addition to acorn size effects, rates of oak seedling emergence were also dependent on an interaction of population source and competitive environment. Interactions between maternal family and competitive environment in the expression of seedling leaf characters suggest the possibility of genetic variation for plasticity in traits such as specific leaf area. Using carbon isotope discrimination (Δ) as an index of relative water-use efficiency (WUE), higher water use efficiency was indicated for oak seedlings grown in the annual plant neighborhoods compared to seedlings grown in the bunchgrass neighborhood. This trend may represent an adaptive plastic response because, compared to the bunchgrass neighborhood, soil water depletion was more rapid within annual plant neighborhoods.

Size-dependent allocation to sexual and vegetative reproduction in four clonal composites

Populations of Silphium speciosum, Vernonia baldwinii, Solidago canadensis and Pityopsis graminifolia were studied to determine whether biomass allocation to sexual and vegetative reproduction and the balance between them were size-dependent and whether interpopulation differences in allocation patterns could be predicted from differences in population size distributions. All four species showed strong linear relationships between inflorescence mass and vegetative mass with negative y-intercepts. As a result, sexual reproductive effort (SRE) was a monotonically increasing function of ramet size. Genet size was a poor predictor of SRE. In each species, the regression parameters of these relationships differed significantly between burned and unburned habitats indicating size-independent interpopulation differences in patterns of reproductive effort as well as sizedependent effects. Interpopulation variation in vegetative reproductive effort (VRE) was greater than variation in SRE, but neither VRE nor the pattern of partitioning of VRE among daughter rhizomes showed significant relationships to plant size.

Comparison of skewness coefficient, coefficient of variation, and Gini coefficient as inequality measures within populations

The moment skewness coefficient, coefficient of variation and Gini coefficient are contrasted as statistical measures of inequality among members of plant populations. Constructed examples, real data examples, and distributional considerations are used to illustrate pertinent properties of these statistics to assess inequality. All three statistics possess some undesirable properties but these properties are shown to be often unimportant with real data. If the underlying distribution of the variable follows the often assumed two-parameter lognormal model, it is shown that all three statistics are likely to be highly and positively correlated. In contrast, for distributions which are not two-parameter lognormally distributed, and when the distribution is not concentrated near zero, the coefficient of variation and Gini coefficient, which are sensitive to small shifts in the mean, are often of little practical use in ordering the equality of populations. The coefficent of variation is more sensitive to individuals in the right-hand tail of a distribution than is the Gini coefficient. Therefore, the coefficient of variation may often be recommended over the Gini coefficient if a measure of relative precision is selected to assess inequality. The skewness coeficient is suggested when the distribution is either three-parameter lognormally distributed (or close to such), or when a measure of relative precision is not indicated.

Seed size and emergence time within a stand of wild radish (Raphanus raphanistrum L.): the establishment of a fitness hierarchy

To determine the role of seed size in creating adult plants of different reproductive success, individual seedlings were marked and periodically censused in a natural stand of wild radish (Raphanus raphanistrum L.) in Hamden, Connecticut. Maximum cotyledon witdth is a useful field estimator of seed weight in this species, although dramatic variation in the availability of water can modify this relationship. Using maximum cotyledon width as a linear estimate of seed weight showed that an increase in seed weight gives rise to a disproportionately large increase in an individual's total reproductive output. Analysis of covariance demonstrated that reproductive output is influenced more heavily by seed weight (maximum cotyledon width) than by emergence time. Genetic variation for seed size has been maintained in this population, suggesting that selection favoring large seeds during the seedling stage may be counteracted by selection for large numbers of seeds.

The meaning and measurement of size hierarchies in plant populations

The term "size hierarchy" has been used frequently by plant population biologists but it has not been defined. Positive skewness of the size distribution, which has been used to evaluate size hierarchies, is inappropriate. We suggest that size hierarchy is equivalent to size inequality. Methods developed by economists to evaluate inequalities in wealth and income, the Lorenz curve and Gini Coefficient, provide a useful quantification of inequality and allow us to compare populations. A measure of inequality such as the Gini Coefficient will usually be more appropriate than a measure of skewness for addressing questions concerning plant population structure.

Genotype-environment interactions in determining fitness in dense, artificial populations of Phlox drummondii

Survivorship and growth of individuals of the annual plant Phlox drummondii were examined in artificial, density-stressed populations composed of five genetically differentiated varieties sown in monocultures and in five pairwise mixtures. All experiments were replicated under two treatments of nutrient availability.Varieties differed significantly in competitive ability, and relative competitive success was shown to be highly habitat (treatment) dependent. In 19 of 20 mixed populations, the relative rankings of pairs of competitors were consistent with a priori expectations based on the morphological and/or historical differences between seed sources, but were not predictable from relative tolerances to density stress in monoculture.Estimated selection coefficients based on relative competitive abilities ranged from 0.04 to 0.94. Despite these large fitness differences among competing varieties, most of the variance in the absolute fitness of individuals resided within varieties. This variation may be mostly environmental.In the high nutrient treatment, where competition for light was most intense, variation in individual plant size was lower in mixed culture as compared to monocultures. Genetic variation for competitive ability may actually lead to greater size uniformity among survivors by promoting an early initiation of thinning, thereby reducing the density stress on survivors.