Cotyledon venation patterns in the Leguminosae: Mimosoideae

Cotyledon venation patterns are described for 131 species representing the four main tribes of the Mimosoideae. The range of variation in venation pattern falls within that described previously for the Caesalpinioideae and is consistent with the proposal that all patterns in the Leguminosae have been derived from a prototype with four vascular strands and a protoxylem trace in the petiole and seven primary veins in the lamina. Each tribe is characterized by a particular set of patterns, pattern frequencies and evolutionary trends. In the Mimoseae, correlations between venation pattern and cotyledon size and anatomy match those found in the Caesalpinioideae, but different correlations unique to these tribes occur in the Acacieae and Ingeae.     

Cotyledon anatomy in the Leguminosae

Cotyledon anatomy has been investigated in approximately 900 species representing all the major tribes of the Leguminosae. Four main cotyledon forms are recognized on the basis of morphological, anatomical and functional criteria. The two modal forms are a leaf-like photosynthetic cotyledon, usually borne in an endospermic seed and occurring in about 60% of species; and a fleshy, storage cotyledon, borne in a non-endospermic seed and occurring in about 30% of species. Two intermediate forms combine the structural and functional characteristics of the modal forms in varying degree. The leaf-like cotyledon is considered to be the basic form from which the others have been derived. Cotyledon anatomy shows correlations with seed size and with taxonomic grouping at the tribal and generic levels and thus provides a set of minor characters of potential taxonomic or phylogenetic value or for use in seed and seedling classification and identification.     

Cotyledon architecture and anatomy in the Acacieae (Leguminosae: Mimosoideae)

Cotyledon size, shape, venation pattern and anatomy have been investigated in Faidherbia albida and 152 species of Acacia representing the three subgenera Acacia, Aculeiferum and Heterophyllum. Cell volumes of epidermis, palisade and storage tissue, stomatal frequency, stomatal index and frequency of stomatal types have been determined for F. albida and 12 species from each subgenus. The data obtained support the recognition of the subgenera of Acacia as separate taxa but provide no indication of their status. The evidence from cotyledons also supports the separation of Faidherbia from Acacia , and the amalgamation of the Acacieae and Ingeae.     

The morphology of the wet, non-papillate (WN) stigma form in the tribe Caesalpinieae (Gaesalpinioideae: Leguminosae)

Stigmas in 24 species of 11 genera, classified in three groups, Caesalpinia, Peltophorum and Dimorphandra, of the tribe Caesalpinieae have been examined by light and scanning electron microscopy. All are of the WN (wet, non-papillate) form. The receptive stigmatic surface comprises a crater, generally at the apex of the style, which in fresh flowers is full of clear fluid. The crater rim may be fringed with non-receptive hairs, and its shape and depth are variable between species. Extreme forms are found in Caesalpinia sappan where crater hairs are short and the crater funnel-shaped and very deep, and C. vesicaria and Delonix regia where crater hairs are long and the crater appears very shallow.     

Floral venation patterns in Siphocampylus (Campanulaceae)

Siphocampylus is a neotropical genus that comprises 221 species distributed from Costa Rica to Argentina and in the Greater Antilles. Twenty-eight species have been reported from Brazil, mainly occupying mountainous terrain. The floral venation patterns and the origin of the hypanthium in eight Brazilian species, including three varieties, are described. Eleven ovarian vascular bundles depart from the siphonostele or receptacular stele: five of these bundles result from sepalar and staminal adnation and are alternate to five petalar bundles; the remaining bundle is central carpellary. The staminal bundles diverge from the sepalar bundles at the sinus, while the carpellary bundles form a cross, resulting in four ventral bundles; two of these feed the ovules; the other two feed the style. Apparently, the dorsal carpellary bundles diverge at the same site and then ramify profusely. The venation pattern observed is unprecedented in Siphocampylus and is quite different from other reports on genera of Campanulaceae. Further, these findings suggest that the origin of the hypanthium is appendicular, increasing knowledge of venation in this group, thus providing data for phylogenetic considerations.     

Cotyledon vascular pattern2-mediated inositol (1,4,5) triphosphate signal transduction is essential for closed venation patterns of Arabidopsis foliar organs

Vein patterns in leaves and cotyledons form in a spatially regulated manner through the progressive recruitment of ground cells into vascular cell fate. To gain insight into venation patterning mechanisms, we have characterized the cotyledon vascular pattern2 (cvp2) mutants, which exhibit an increase in free vein endings and a resulting open vein network. We cloned CVP2 by a map-based cloning strategy and found that it encodes an inositol polyphosphate 5' phosphatase (5PTase). 5PTases regulate inositol (1,4,5) triphosphate (IP(3)) signal transduction by hydrolyzing IP(3) and thus terminate IP(3) signaling. CVP2 gene expression is initially broad and then gradually restricted to incipient vascular cells in several developing organs. Consistent with the inferred enzymatic activity of CVP2, IP(3) levels are elevated in cvp2 mutants. In addition, cvp2 mutants exhibit hypersensitivity to the plant hormone abscisic acid. We propose that elevated IP(3) levels in cvp2 mutants reduce ground cell recruitment into vascular cell fate, resulting in premature vein termination and, thus, in an open reticulum.     

Pollen-ovule patterns in tribeTrifolieae (Leguminosae)

Relative pollen and ovule production in the genera of the legume tribeTrifolieae is explored particularly as to howMedicago with its explosive pollination mechanism compares with its allies.Medicago produces much larger, although much fewer pollen per ovule than the other five genera in the tribe; this is interpreted as a consequence of its highly specialized, irreversible pollination mechanism, which allows only one effective exchange of pollen with pollinators.Melilotus andTrifolium produce a comparatively large quantity of pollen; this can be related to their floral characteristics requiring abundant pollen to achieve fertilization. InMedicago andTrigonella, annuals convert a higher proportion of pollen grains and ovules into seeds than perennials.     

Cotyledon damage at the seedling stage affects growth and flowering potential in mature plants

Seedling herbivory is an important selective filter influencing patterns of plant community composition. Nevertheless, while many of the mechanisms governing seedling selection by herbivores are well established, the effects of tissue loss at the seedling stage on subsequent plant development are poorly understood. Here we examined how the removal of 50 or approximately 100% of cotyledon area from 7-d-old chalk grassland seedlings affected subsequent plant growth and flowering over a 100-d period. Cotyledon damage had a significant effect on growth during the establishment phase for six of the nine species. For two species, significant effects on plant growth were manifest in 100-d-old plants. Of the five species that flowered, three developed fewer inflorescences or flowered later as a consequence of cotyledon damage suffered as a seedling. Our results show that, in addition to the direct effect of herbivory on seedling mortality, more subtle sublethal effects may also influence plant establishment. Reduced growth as a result of cotyledon damage may have implications for plant competition during the establishment phase, and on subsequent reproductive success at maturity.     

Order of fertilization within the ovary in Phaseolus coccineus L. (Leguminosae)

Summary Phaseolus coccineus typically has six linearly arranged ovules per ovary. The three ovules near the stylar end of the fruit (positions one, two, and three) are more likely to produce mature seeds, to produce heavier seeds, and to produce more vigorous progeny than the ovules in positions near the peduncular/basal end of the fruit (ovule positions four, five, and six). We conducted a series of field experiments designed to supplement our understanding of the mechanisms determining these position effects. We found that approximately 98% of the ovules in 752 fruits were fertilized — about 0.6% of the stylar ovules were not fertilized, whereas 3.2% of the basal ovules were unfertilized. Moreover, we found that only about 49% of the ovules in these 752 fruits produced mature seeds. Over 60% of the stylar ovules produced mature seeds, whereas only 37% of the basal ovules produced mature seeds. Consequently, the proportion of fertilized ovules cannot explain the differences in seed maturation among the ovule positions. We found that after 6.5 h most of the fertilized ovules were located in the stylar ovule positions, and that there were no fertilized ovules in ovule positions five and six, indicating that the stylar ovules are fertilized first. When only the fastest growing pollen tubes were permitted to enter the ovary (due to exision of the style), only the ovules at the stylar end were fertilized, indicating that the ovule positions that are fertilized first are indeed fertilized by the fastest growing pollen tubes.On leave from the Escuela de Biologia, Universidad de Costa Rica, Cuidad Universitaria Rodrigo Facio, San Jose, Costa Rica, Central America     

Impact of Cotyledon and Leaf Removal on Seedling Survival in Three Tree Species with Contrasting Cotyledon Functions1

The relative importance of cotyledons and leaves for seedling survival was evaluated using a factorial field experiment on three neotropical tree species with contrasting cotyledon functional morphologies (photosynthetic, epigeal reserve vs. hypogeal reserve). In all species, cotyledon and leaf removal shortly after leaf expansion had additive negative effects on seedling survival over 7 weeks. Carbon supplies from cotyledons and other carbohydrate reserves apparently enhanced ability of seedlings to cope with herbivory and disease.     

Influence of heat on seed germination of seven Mediterranean Leguminosae species

The influence of high temperatures (dry heat and hot water) on germination of seven Mediterranean Leguminosae species typical of fire-prone ecosystems in southern Spain is analyzed, in order to know the response of seeds to wildfires and the possible implications in their regeneration after this disturbance. Seeds were heated to a range of temperatures (50 °–150 °C) and exposure times (1–60 min) similar to those registered in the upper soil layers during wildfires. Germination tests were carried out in plastic Petri dishes over 60 days. In general, the degree of seed germination promotion by dry heat treatments showed a wide interspecific variation, although the final germination level was increased in all the studied species except for Scorpiurus muricatus. The thermal pretreatment of 50 °C, however, was not effective for germination in any species, and rising the temperature to 70 °C only slightly enhanced the germination in Cytisus patens. The preheatings of 90 °C (5 and 10 min), 120 °C (5 and 10 min), and 150 °C (1 min) were the most effective in promoting seed germination. Hot water (100 °C) scarification also increased the final germination level in all cases, with the exception of C. patens. The germination rates after preheating were much lower than in mechanically scarified seeds and closely resembled those of the untreated seeds, except for C. reverchonii, whose seed germination rate decreased with heat. The response of species to heat shock had no clear relationship with life trait or with the specific post-fire regeneration strategy (obligate seeder or facultative resprouter). Those species coexisting in the same habitats had different heat optimal requirements for seed germination, an strategy suggested by some authors as minimizing interspecific competition in the secondary succession started after fire.     

Diversification of seed arrangement induced by ovule rotation and septum formation in Leguminosae

The plants of Leguminosae, which comprises 731 genera, bear monocarpellary flowers. The fruit, known as a pod, generally contains a row of seeds, and the internal structure appears to be simple. In Leguminosae, the seeds are generally arranged longitudinally along the suture of the pod but, in a few genera, namely, Cassia, Indigofera, and Senna, they are arranged transversely. Transverse seed arrangement facilitates the accommodation of a considerable number of seeds and has been considered to be established by rotation of seeds during development. We studied the development both these patterns of seed arrangement, i.e., longitudinal and transverse, in Senna and Indigofera in order to elucidate the structural changes occurring in ovaries (or pods) and determine the forces responsible for rotation of ovules (or seeds) by dissecting ovaries and pods at different developmental stages. Our results showed that at an early developmental stage the ovules were obliquely oriented or transversely arranged; later, the direction of their growth was restricted by the ovary walls or neighboring ovules, and therefore the obliquely oriented ovules rotated inward or outward. Thus, it was clarified that the processes involved in the formation of the internal structure of pods are regulated in a complex manner. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Community seed rain patterns and a comparison to adult community structure in a West African tropical forest

We examined the seed rain throughout a twelve month period in a lowlandtropical forest in Cameroon, West Africa, 1996–97. Traps (0.5m², n = 216) were erected throughout a 25km² area in 24 randomly placed clusters of nine trapseach. Fruits and seeds that landed in traps were collected every 7–10daysand classified by species and dispersal type. More than 32,000 seeds fromapproximately 200 species fell into the traps, an average of 297 seedsm^–2 yr^–1 Thirty species represent 82%of the total seed rain while an additional 175 species comprise the remaining18%. When we compared the adult community to the seed rain community within thesame plots, we found no apparent correlation between seed rain patterns andadult community structure for this year of study. Furthermore, only 49% of theadult tree community produced and dispersed seed into traps in this year. Morethan 100 species (52%) found in the seed rain represented long-distanceimmigrant seed rain. Seed rain was highly variable at several scales, bothspatially and seasonally, although seeds arrived in traps during eachcollectionperiod. Cluster analyses showed that traps within plots were seldom moresimilarto one another than traps between plots. While 82% of the tree species in thecommunity are thought to be animal dispersed, only 28% of all seeds that fellinto traps had been obviously handled (bitten, chewed, or passed)by animals. Tests for fruit and seed removal by predators or dispersers found5%or less removal rate from traps.     

Seed rain and advanced regeneration in a tropical rain forest

By comparing seed rain, seedling and sapling community structures we assessed the possible role played by vertebrate seed dispersal as a structuring factor in advanced regeneration of closed-canopied sites in the tropical rain forest of Los Tuxtlas, Mexico. Seed weight, initial morphology of seedlings and species abundance were also analyzed to determine if these traits influenced the probability of establishment in the shade.About half of the seed species falling in five closed forest sites (25×25 m) during one year came from fruiting trees growing within the sites (local seeds) and half from fruiting trees found outside the sites (immigrant seeds). Seeds of liana and upper-canopy species were over-represented among immigrant seeds compared with seeds of understory tree species. This probably reflects the activity of frugivorous arboreal mammals, bats, and birds. Species with immigrant seeds had both a lower abundance and a narrower spatial distribution than locally produced seeds. Therefore, immigrant seeds showed higher diversity values than locally produced seeds.Average seed size and the proportion of epigeous seedlings were similar in local and immigrant species. Under closed-canopied sites, factors affecting community organization seem to operate selectively, favoring the establishment of large-seeded, local abundant species in the advanced regeneration. However, the fact that some saplings of immigrant species were found in the plots suggest that a slow species infiltration may be occurring leading to a slow shift in the advanced regeneration species composition. We propose that the influence of seed dispersal on advanced regeneration structure depends on the disturbance history of the patches where seeds land.     

Dispersal mode,seed shadows,and colonization patterns

This review assesses the state of our knowledge about comparative seed shadows. Using data presently available in the literature, I compare the slopes (on a log-linear scale) of seed shadows for plants with different morphologically characterized modes of dispersal. The seeds of many species have no evident morphological adaptation for dispersal and seem to achieve only short-distance dispersal. Seed shadows for herbaceous species with devices for wind have flatter slopes and more distant modes and maxima than those of ballists, which in turn exceed those with no special devices. Seed shadows for wind-dispersed trees and shrubs had similar or steeper slopes than those for vertebrate-dispersed species in this sample. Species with poor mechanisms for dispersal in space only sometimes had the capacity for better dispersal in time (dormancy). Although some species exhibited seed shadows sufficiently steep to be predicted to colonize new-habitat in a front or phalanx pattern, actual colonization patterns must reflect many other factors.     

Diverse patterns of cell wall mannan/galactomannan occurrence in seeds of the Leguminosae

Endosperms from seeds of different subfamilies of Leguminosae were submitted to sequential aqueous and alkaline aqueous extractions. The extractions from species belonging to the Mimosoideae and Faboideae subfamilies yielded galactomannans with constant Man:Gal ratios, whereas the extractions from Caesalpinioideae seeds gave rise to galactomannans with increasing values of the Man:Gal ratio. The presence of a family of galactomannans within the same species may be a trait found only in Caesalpinioideae subfamily. The final insoluble residues that were obtained after the removal of galactomannans from the Caesalpinioideae and Faboideae subfamilies are composed of pure mannans and do not contain cellulose, while those from the Mimosoideae subfamily are composed of cellulose. A mannan was isolated from the unripe endosperm of Caesalpinia pulcherrima, suggesting no developmental relationship between galactomannan and mannan. These results are consistent with the presence of a distinctive cell wall pattern in the endosperms of Leguminosae species.     

Spatial patterns of the germinable soil seed bank of coexisting perennial-grass species in grazed shrublands of the Patagonian Monte

We compared the size and spatial pattern of the germinable soil seed bank (GSB) of the three dominant perennial-grass species in the arid Patagonian Monte of Argentina. These species differ in plant functional traits, seed morphology, and in their preference by grazers. During three consecutive years, we sampled the soil seed bank at 1 m intervals and the intercepted diameter of perennial-grass species and shrub patches along 50-m linear transects at three sites with different grazing pressure. Soil samples were incubated at field capacity during 12 weeks and emerged seedlings counted. The largest-seeded and most xerophytic Stipa speciosa did not form a GSB. The other two grass species with more mesophytic traits and smaller seeds than S. speciosa formed different GSB. Poa ligularis formed a smaller and more heterogeneous GSB than Stipa tenuis. Grazing had not effect on GSB size of P. ligularis but increased the proportion of seeds under dense shrub canopies. In contrast, grazing reduced the size and also increased the proportion of the GSB of S. tenuis under shrub canopies. We conclude that the size and the spatial pattern of the GSB of the dominant perennial-grass species in the arid Patagonian Monte were related not only to seed and plant traits but also to the spatial distribution of grass plants and their exposure to grazers.