Sexual and vegetative reproduction in the aboveground part of a dioecious clonal plant, <Emphasis Type="Italic">Dioscorea japonica</Emphasis> (Dioscoreaceae)

In dioecious clonal plants, the reproductive effort required to set seeds will be responsible for the larger investment in sexual reproduction by females. If there will be a trade-off in resource allocation between sexual and clonal reproduction, this differential sexual reproduction will lead to sexual differentiation in the relative amount of clonal reproduction. To test this prediction, we studied differences between the sexes in their phenologies and investments in sexual and vegetative reproduction (clonal reproduction by means of bulbils) with respect to ramet size in a dioecious clonal plant, Dioscorea japonica Thunb. The period of bulbil production overlapped the period during which infructescences developed. Females flowered later, produced heavier inflorescences, and fewer flowers per inflorescence than did males. Regression analysis using the size of the individual plants demonstrated that large females made smaller investments in inflorescences and larger investments in sexual reproduction than did large males. In contrast, females invested fewer resources in vegetative reproduction than did males. However, the total investments in sexual and vegetative reproduction did not differ between the sexes. These results supported our hypothesis on the sexual differentiation in sexual and clonal reproduction.     

Patterns of resource allocation in the dioecious alpine herb Aciphylla simplicifolia (Apiaceae)

Abstract Patterns of reproductive and vegetative biomass allocation were compared in male and female plants of the alpine herb Aciphylla simplicifolia. Male and female plants had similar vegetative biomass but differed in the pattern of resource allocation. Inflorescences of males and females were similar in weight at the time of flowering, but differed in biomass allocation to some structures within the inflorescences, particularly those associated with ovule production and pollinator attraction (number and size of flowers). At the time of fruit production, female inflorescences were 2.6 times heavier than at flowering with developing fruit six times heavier than flowers. In addition to the increase in biomass allocated to structures associated with the provisioning and dissemination of seed, support structures (main and side stalks) were also heavier. As a result of this additional investment of resources at the time of fruit production, the reproductive effort (RE) of female plants was much higher than that of males: 37% of above ground biomass compared with 21% for males. Differences in RE did not change with plant size; however, allocation to reproduction appeared to be a constant proportion of biomass over nearly all plant sizes sampled. These results show that sex‐specific resource allocation can be a complex of temporal and morphological patterns.     

Sexual dimorphism and cost of reproduction in the dioecious shrub Lindera benzoin (Lauraceae)>

We examined sexual dimorphism in reproductive allocation and its effects on growth and subsequent reproduction in a natural population of the dioecious woody shrub Lindera benzoin (L.) Blume. In addition to comparing natural patterns of growth and reproduction in a marked population of 251 females and 87 males, we experimentally examined the effect of reduced reproduction on future growth and reproduction in female plants and examined sexual dimorphism in carbon/nutrient balance. Our results suggest that females of L. benzoin bear greater reproductive costs in terms of both biomass and minerals. These costs were measurable in terms of current biomass and allocation of nitrogen to reproduction, as well as subsequent growth, reproduction, and tissue carbon/nutrient balance. Based upon the results of a fruit-thinning experiment and path analysis, fruit production in 1991 appeared to have direct negative effects on fruit production in 1992, an effect that was not necessarily mediated through effects on plant growth. We discuss our results in the context of other intrinsic and extrinsic factors that can influence growth, reproductive costs, and mortality in this species.     

Do sexual dimorphisms in reproductive allocation and new shoot biomass increase with an increase of altitude? A case of the shrub willow Salix reinii (Salicaceae)

Based on the general tendency for females of dioecious plants to pay higher reproductive cost than males, it has been predicted that females should have much more reduced reproductive outputs and diminished vegetative production than males in energy-limited habitats. Nevertheless, this prediction has rarely been directly investigated. We investigated altitudinal changes in reproductive biomass and shoot production, normalized by plant size, for females and males of a shrub willow, Salix reinii, on Mt. Hakkoda, northeast Japan. Females maintained higher reproductive biomass than males at all altitudes; however, reproductive allocation for both sexes tended to decrease at a similar rate with an increase in altitude. Moreover, females vegetatively produced at the same rate as males at all altitudes. These findings suggest that females have a mechanism to compensate for the extra investment in reproduction irrespective of a changing environment. Shoot production did not change with altitude, suggesting that S. reinii gave priority to vegetative investment at the cost of reproductive output at higher altitudes. Inconsistent with general predictions, females did not respond more sensitively than males to severe environmental conditions in either reproductive allocation or shoot production, despite much higher resource investment in reproduction.     

Sex differences in the effects of juvenile and adult diet on age‐dependent reproductive effort

Sexual selection should cause sex differences in patterns of resource allocation. When current and future reproductive effort trade off, variation in resource acquisition might further cause sex differences in age‐dependent investment, or in sensitivity to changes in resource availability over time. However, the nature and prevalence of sex differences in age‐dependent investment remain unclear. We manipulated resource acquisition at juvenile and adult stages in decorated crickets, Gryllodes sigillatus, and assessed effects on sex‐specific allocation to age‐dependent reproductive effort (calling in males, fecundity in females) and longevity. We predicted that the resource and time demands of egg production would result in relatively consistent female strategies across treatments, whereas male investment should depend sharply on diet. Contrary to expectations, female age‐dependent reproductive effort diverged substantially across treatments, with resource‐limited females showing much lower and later investment in reproduction; the highest fecundity was associated with intermediate lifespans. In contrast, long‐lived males always signalled more than short‐lived males, and male age‐dependent reproductive effort did not depend on diet. We found consistently positive covariance between male reproductive effort and lifespan, whereas diet altered this covariance in females, revealing sex differences in the benefits of allocation to longevity. Our results support sex‐specific selection on allocation patterns, but also suggest a simpler alternative: males may use social feedback to make allocation decisions and preferentially store resources as energetic reserves in its absence. Increased calling effort with age therefore could be caused by gradual resource accumulation, heightened mortality risk over time, and a lack of feedback from available mates.     

Sexual differences in biomass and nutrient allocation of first-year Silene dioica plants

Reproductive and somatic biomass, nitrogen (N), and phosphorus (P) pools were compared between females and males in 1st-year plants of Silene dioica. We estimated irretrievable resources allocated to seeds, pollen, flowers, and unrecovered summer leaf investment by collecting plant parts at abscission throughout the season. At the end of the season, we determined resources lost through senescent stems and autumn leaf turnover and resources stored in perennial roots and overwintering buds. Sexual differences in allocation patterns depended on the resource used for comparison, and whether absolute or proportional resource pools were assessed. Total resource pools in terms of biomass and N were similar for females and males. However, male plants acquired relatively more P. The proportional reproductive investment, i.e., reproductive effort, was similar for males and females in terms of biomass and N. In terms of P, male reproductive effort was higher. There was no difference between sexes in the proportional and relative biomass allocated to perennial roots and overwintering buds. However, in terms of absolute and relative N allocation to below-ground parts, females had larger reserves than males. Females, moreover, had a larger proportion of their P in below-ground parts. However, as male total P pools were larger, absolute P reserves did not differ between sexes. The high reproductive effort and N depletion of below-ground parts in males resulted largely from higher flower production compared to females. In females, seeds were the major component of reproductive effort. These results show that if biomass and nutrient allocation are assessed in parallel for dioecious plants, we obtain a more complete view of their sexual differences. Received: 07 May 1998 / Accepted: 30 October 1998     

SEX-RATIO AND REPRODUCTIVE VARIATION IN THE MISTLETOE PHORADENDRON JUNIPERINUM (VISCACEAE)

Despite the fact that many parasitic and hemiparasitic plant species such as mistletoes are dioecious and occur in both the new and the old world, few data exist on variation in the sex ratio and allocation to reproduction in these taxa. We investigated 1) the sex-ratio of the xylem-tapping mistletoe Phoradendron juniperinum in relation to its age and position within the canopy of its host tree Juniperus osteosperma, and 2) reproductive effort in relation to the gender and age of mistletoe plants. Our surveys showed that P. juniperinum has a male-biased sex ratio. Despite this predominance of male individuals, females lived longer and had a greater reproductive effort than did males. A statistical analysis of the age distribution data indicated that the peak in the frequency of reproductively mature individuals was later in females than in males. These gender-specific distributions may have resulted 1) from sequential hermaphroditism (age-specific sex switching), or 2) because the average age of peak reproduction is later in female individuals. Because sex is genetically determined in a closely related genus of mistletoe and because we have no data to indicate sex switching in this species, we feel that our data support the interpretation that female individuals, on average, show a peak in reproductive vigor at an older age relative to males. While delayed reproduction in females may be favored because reproductive effort and success appear to be age-dependent in females of this species, both sexes can become reproductively mature relatively early in life. Further, because 1) allocation to reproduction as a function of age increases more rapidly for females of this species relative to males, and 2) because there may be a higher resource cost associated with reproduction in females, we hypothesized that female individuals would be more abundant in the best quality locations within the host tree so as to maximize the opportunity to meet those costs. In spite of the association between gender and some host characteristics, there was no indication that female plants were located in sites most favorable to either their carbon or water balance. We discuss reasons why this may be the case.     

Allometric Effects of <Emphasis Type="Italic">Agriophyllum squarrosum</Emphasis> in Response to Soil Nutrients,Water, and Population Density in the Horqin Sandy Land of China

We monitored the allometric effects for greenhouse-grown Agriophyllum squarrosum plants in response to variations in population density and the availability of soil nutrients and water. Biomass allocations were size-dependent. The plasticity of roots, stems, leaves, and reproductive effort was “true” in response to changes in nutrient content. At a low level of soil minerals, plants allocated more resources to the development of roots and reproductive organs than to leaves, but data for stem allocations were consistent for tradeoffs between the effects of nutrients and plant size. The plasticities of leaf allocation and reproductive effort were “true” whereas those of root and stem allocations were “apparent” in response to fluctuations in soil water, being a function of plant size. Decreasing soil water content was associated with higher leaf allocation and lower reproductive effort. Except for this “apparent” plasticity of leaf allocation, none was detected with population density on biomass allocation. Architectural traits were determinants of the latter. For roots, the determining trait was the ratio of plant height to total biomass; for stems and reproduction, plant height; and for leaves, the ratio of branch numbers to plant height.     

DIFFERENCES IN BIOMASS ALLOCATION TO REPRODUCTIVE AND VEGETATIVE STRUCTURES OF MALE AND FEMALE PLANTS OF A DIOECIOUS,PERENNIAL HERB,SILENE ALBA (MILLER) KRAUSE

The pattern of biomass allocation of males and females and the sex ratio and growth characteristics of plants from three seed-size classes in Silene alba were investigated in a greenhouse study. Seed size significantly affected adult plant size and flower production of both male and female plants, but there was no significant difference in the proportion of males and females emerging in three seed-size categories. Male and female plants differed in the proportion of total biomass allocated to vegetative and reproductive structures and these differences were consistent across all seed-size categories. Males allocated a greater proportion of their biomass to flowers than did females. Female reproductive effort was dependent upon the percentage of flowers producing mature capsules. Only females with greater than 20% fruit set have a higher reproductive expenditure than males. Consequently, female expenditure is potentially greater than males, but is spread out over a longer portion of the growing season. This difference in the timing of reproductive expenditures by males and females allows females to allocate more biomass to growth during the early flowering period and may therefore account for the common pattern in herbaceous perennial dioecious species in which adult females are larger than adult males.     

Sexual dimorphism in resource acquisition and deployment: both size and timing matter

Background and Aims

The males and females of many dioecious plant species differ from one another in important life-history traits, such as their size. If male and female reproductive functions draw on different resources, for example, one should expect males and females to display different allocation strategies as they grow. Importantly, these strategies may differ not only between the two sexes, but also between plants of different age and therefore size. Results are presented from an experiment that asks whether males and females of Mercurialis annua, an annual plant with indeterminate growth, differ over time in their allocation of two potentially limiting resources (carbon and nitrogen) to vegetative (below- and above-ground) and reproductive tissues.

Methods

Comparisons were made of the temporal patterns of biomass allocation to shoots, roots and reproduction and the nitrogen content in the leaves between the sexes of M. annua by harvesting plants of each sex after growth over different periods of time.

Key Results and Conclusions

Males and females differed in their temporal patterns of allocation. Males allocated more to reproduction than females at early stages, but this trend was reversed at later stages. Importantly, males allocated proportionally more of their biomass towards roots at later stages, but the roots of females were larger in absolute terms. The study points to the important role played by both the timing of resource deployment and the relative versus absolute sizes of the sinks and sources in sexual dimorphism of an annual plant.     

Reproductive allocation in hermaphrodite and female plants of Sidalcea oregana SSP. spicata (Malvaceae) using four currencies

Reproductive allocation was investigated in female and hermaphrodite plants of gynodioecious Sidalcea oregana ssp. spicata. Total reproductive investment and partitioning of that investment was documented at the level of whole plants in terms of four ecologically relevant currencies: biomass, nitrogen, phosphorus, and potassium. Nutrient augmentations in the field confirmed that nutrients were limiting plant vegetative growth and propensity to flower; thus the use of these nutrients as currency was appropriate. Once the effects of plant size were removed, the sex morphs allocated similar total amounts of biomass, nitrogen, phosphorus, and potassium to reproduction, but partitioned those differentially. For any given individual size, females allocated larger proportions of their reproductive resource budgets to seeds. Hermaphrodites' reproductive investment in pollen and flowers was allocated at the expense of allocation to seeds. These data are relevant to the evolution of gynodioecy from hermaphroditism and support the hypothesis that females reallocate resources not spent on pollen to seeds.     

Gender-specific costs of reproduction on vegetative growth and physiological performance in the dioecious shrub Corema album

Background and Aims

Reproductive costs imply trade-offs in resource distribution at the physiological level, expressed as changes in future growth and/or reproduction. In dioecious species, females generally endure higher reproductive effort, although this is not necessarily expressed through higher somatic costs, as compensatory mechanisms may foster resource uptake during reproduction.

Methods

To assess effects of reproductive allocation on vegetative growth and physiological response in terms of costs and compensation mechanisms, a manipulative experiment of inflorescence bud removal was carried out in the sexually dimorphic species Corema album. Over two consecutive growing seasons, vegetative growth patterns, water status and photochemical efficiency were measured to evaluate gender-related differences.

Key Results

Suppression of reproductive allocation resulted in a direct reduction in somatic costs of reproduction, expressed through changes in growth variables and plant physiological status. Inflorescence bud removal was related to an increase in shoot elongation and water potential in male and female plants. The response to inflorescence bud removal showed gender-related differences that were related to the moment of maximum reproductive effort in each sexual form: flowering in males and fruiting in females. Delayed costs of reproduction were found in both water status and growth variables, showing gender-related differences in resource storage and use.

Conclusions

Results are consistent with the existence of a trade-off between reproductive and vegetative biomass, indicating that reproduction and growth depend on the same resource pool. Gender-related morphological and physiological differences arise as a response to different reproductive resource requirements. Delayed somatic costs provide evidence of gender-related differences in resource allocation and storage. Adaptive differences between genders in C. album may arise through the development of mechanisms which compensate for the cost of reproduction.     

Sexual differences in gas exchange and response to environmental stress in dioecious Silene latifolia (Caryophyllaceae)

Females of dioecious species usually have higher reproductive effort than males because they produce fruits in addition to flowers. Since females have higher reproductive effort, they are expected to be more negatively affected than males by low resource availability. We tested that assumption by growing females and males of Silene latifolia under low levels of light, water, nitrogen, phosphorus, and potassium. Gas exchange of the sexes did not respond differently to low resource availability; higher female reproductive effort relative to males did not differentially affect their ability to assimilate carbon. However, male photosynthesis rates and stomatal conductances were slightly, but consistently, higher than those of females. The intersexual difference in photosynthesis rate may be a proximate result of reproduction if females translocate nutrients, particularly nitrogen, from their leaves to developing fruits. Alternatively (or perhaps additionally), higher male photosynthesis and stomatal conductances relative to females may be the ultimate result of sexual selection. This could be the case if 1) reproductive effort as estimated by biomass allocation is misleading and males actually invest more in reproduction than females, or 2) females experience stronger selection than males to conserve water late in the growing season, when soil moisture is likely to be low but females need to complete fruit maturation. Our results indicated that females had slightly lower leaf nitrogen but higher photosynthetic water-use efficiency than males, so it is possible that both proximate and ultimate factors are operating simultaneously to cause lower female photosynthesis rates.     

Sex Ratios and Resource Allocation among Sexually Reproducing Plants of Rubus chamaemorus

Sex ratios and patterns of size variation and resource allocationwere investigated in the dioecious species Rubus chamaemorus.Sex ratios among flowering ramets varied from 6% to 40% of females.Female ramets were slightly, although not significantly, tallerthan males. It appeared that population effects (including bothgenetic population and environmental site effects) on plantsize and allocation patterns at flowering are considerably greaterthan sex effects. If both flowering and fruit production areconsidered, then female allocation to reproduction clearly exceedsmale allocation. In females, no significant relationship wasdetected between the mass of reproductive and vegetative tissues,while males did exhibit such a relationship. Reproductive effortwas less for tall males than for small males. Despite the occurrence of sexual reproduction, the main modeof reproduction in R. chamaemorus is vegetative propagation,which is the best strategy for reproduction in the unpredictableclimate of high latitudes but which leads to skewed sex ratios.As a consequence of vigorous vegetative reproduction, individualclones can grow to be large. The results of electrophoreticstudies show that the numbers of clones per population are low.Copyright1994, 1999 Academic Press Rubus chamaemorus, cloudberry, sex ratios, resource allocation, clonal structure, electrophoresis     

Size-sex relationship and sexual dimorphism in JapaneseArisaema (Araceae)

The size-sex relationship and sexual differences of sixArisaema species native to Japan were investigated. The size-sex relationship showed almost the same pattern in all species. When the plant was small in size, the sex expression was male, and sex expression changed from male to female as the plant grew larger. Male ratios decreased rapidly around a critical size, but this critical size differed from one species to another. Sexual differences were detected in reproductive structures and behavior, although no difference was detected in vegetative structures. The stoutness, longevity and inner tissue of the scape showed remarkable differences between males and females, and this difference was represented most clearly as the size-weight relationship. Earlier initiation of flowering in males was also observed. No difference was found in resource allocation to reproductive structures between male and female plants at the flowering stage. However, a broad variation in the amount of resource allocation to reproductive structures was found at the fruiting stage in female individuals, which was attributed to differences in the setting rate of mature fruits.     

Differences in response to defoliation between males and females of Silene dioica

Summary The response by male and female plants to herbivory was studied by experimental defoliation of the dioecious perennial herb Silene dioica in a green-house. Male and female plants were defoliated prior to and during the early flowering phase at two intensities (50% and 100% of leaf-area removed) in two consecutive years. Defoliation resulted in a decrease in the number of flowers initiated in both sexes, while a larger delay of peak flowering and a higher mortality was observed in males compared to females. In female plants, severe defoliation resulted in a reduction in seed number per capsule and in seed size compared to control. Females showed a negative correlation between the production of flowers in the first and second season in all treatments, while flowering in males the first season was not correlated with flowering in the second season. Females also showed a lower frequency of flowering than males during the two seasons studied. However, during the flowering period, males allocated significantly more biomass to flowers than did females. This outcome supports the idea that females may have a higher total reproductive expenditure than males, but males have a higher reproductive effort during flowering. Male rosette leaves were significantly preferred by the generalist herbivore Arianta arbustorum in experiments. This preference was most pronounced in trials with leaves from fertilized plants compared to nonfertilized plants. A greater storage of resources in aboveground leaves during winter by males compared to females may explain the higher preference for male leaves and the higher male mortality following early defoliation. Furthermore, males are smaller than females and may have a lower ability than females to replace lost resources needed for reproduction when defoliated early in the season.     

三种同域分布喉毛花的繁殖分配

以青藏高原高寒草甸中三种同域分布的喉毛花为研究对象,通过比较三个种的植株性状和繁殖分配,探讨繁殖分配的种间差异及其与植株个体大小的关系。结果表明:(1)三个种的植株高度、顶花大小和单株花数目、繁殖分配均存在种间差异,这可能与其各自的交配系统和具体的生境以及相应的生活史对策有关;(2)在三种喉毛花中,投入到营养器官和繁殖器官的绝对资源量均呈显著正相关,未检测到植株生长和繁殖间的权衡关系;(3)三个种的个体大小与繁殖器官生物量均呈显著正相关,而与繁殖分配均呈显著负相关,这表明个体越大,繁殖投入越高,而繁殖分配越低,与以往研究结果一致,这可能是由于繁殖分配与个体大小之间存在异速关系。     

Response of the sexes of the subdioecious plant Honckenya peploides to nutrients under different salt spray conditions

Sex dimorphic plants often show sex-specific differences in growth and biomass allocation. These differences have been explained as a consequence of the different reproductive functions performed by the sexes. Such differences may determine the performance of each sex in different habitats and therefore might explain the spatial segregation of the sexes described in many dimorphic plants. We examined the growth, reproductive and physiological responses of the sexes of the subdioecious plant Honckenya peploides to two levels each of salt spray and nutrients, which are assumed to be important selective forces in coastal environments. We found sex-related differences in H. peploides. In particular, females allocated proportionally more dry mass to reproduction and grew less and more slowly than males regardless of salt spray and nutrient conditions, which is interpreted as a trade-off between reproductive and vegetative growth. Regarding physiological response, nutrients significantly increased values of photochemical reflectance index (PRI) in females but not in males, suggesting that photochemical efficiency is more limited by nutrients in females than in males. PRI values also suggest intersexual differences in protection requirements against photoinhibition. The study did not find sex-differential responses to salt spray, which caused a decrease in reproductive effort in both sexes. The sex-related differences in relative growth rates, reproductive allocation and photosynthetic features found here may contribute to explaining habitat-related between-sex differences in performance and, therefore, the spatial segregation of the sexes observed in H. peploides.     

Biomass Allocation in the Dioecious Tropical Palm Chamaedorea tepejilote and Its Life History Consequences

Abstract The patterns of resource allocation are described for a dioecious tropical palm, Chamaedorea tepejilote. Resource allocation was measured by harvesting fifteen plants of C. tepejilote. The relative allocation of biomass in the stem increased with the size of the plant; that in the leaves decreased and that in the other structures remained roughly constant. Female plants showed a greater total reproductive effort, though male plants produced more inflorescences during the flowering season. Both male and female plants allocated more resources to prop root than to hypogeal roots. The annual productivity of reproductive and vegetative parts of C. tepejilote was estimated using allometric relationships for different plant structures and from demographic data obtained from the field. Annually, female plants allocated significantly more resources to leaves than male plants. Yearly productivity of inflorescences was higher for male plants, while female plants had greater total reproductive productivity (inflorescences and fruits). Correlation analysis showed an increase in reproductive effort with plant size, and an inverse relationship between fecundity and probability of survival, fecundity and residual reproductive value, and reproductive effort and life expectancy; these relationships suggested a cost in reproduction. Additionally, mature plants with different growth rates exhibited differences in fecundity: tall plants (>2.5m height) that grew more than 40 cm in height in four years had lower values of fecundity in comparison to plants of slower growth. These data were discussed in the context of the implications in the life history of a dioecious tropical plant.     

Roots, shoots and reproduction: sexual dimorphism in size and costs of reproductive allocation in an annual herb

Females tend to be smaller than males in woody dioecious plant species, but they tend to be larger in herbs. The smaller size of females in woody species has been attributed to higher reproductive costs, yet no satisfactory explanation has been provided for their larger size in herbs. Because herbs have higher nitrogen concentrations in their tissues than woody plants, and because pollen is particularly rich in nitrogen, we predicted that male growth would be more compromised by reproduction than female growth. To test this hypothesis, we conducted three experiments on the annual dioecious herb Mercurialis annua. First, we compared the timing of reproduction between males and females and found that males started flowering earlier than females; early flowering is expected to compromise growth more than later flowering. Second, we compared plants allowed to flower with those prevented from flowering by experimental debudding and found that males incurred a higher reproductive cost than females in terms of both biomass and, particularly, nitrogen. Third, we grew plants under varying levels of nitrogen availability and found that although sexual size dimorphism was unaffected by nitrogen, females, but not males, decreased their relative allocation to both roots and reproduction under high nitrogen availability. We propose that males deal with the high cost of pollen production in terms of nitrogen by allocating biomass to nitrogen-harvesting roots, whereas females pay for carbon-rich seeds and fruits by investing in photosynthetic organs. Sexual dimorphism would thus seem to be the outcome of allocation to above- versus below-ground sinks that supply resources (carbon versus nitrogen) limiting the female and male reproduction differentially.