Effects of genetic and environmental factors on variations of seed heteromorphism in Suaeda aralocaspica

Seed heteromorphism is an adaptive strategy towards adversity in many halophytes. However, the underlying mechanisms and ecological significance of seed heteromorphism have not been deeply explored. Using Suaeda aralocaspica, a typical C4 annual halophyte without Kranz anatomy, we studied seed morphology, differentiation of morphs and fruit-setting patterns, and correlated these traits with germination responses, seed characteristics and heteromorphic seed ratio. To elucidate the genetic basis of seed heteromorphism, we analysed correlated patterns of gene expression for seed development-related genes as well. We observed that S. aralocaspica produced three types of seed morph: brown, large black and small black with differences in colour, size, mass and germination behaviour; the latter two were further distinguished by their origin in female or bisexual flowers, respectively. Further analysis revealed that seed heteromorphism was associated with genetic aspects including seed positioning, seed coat differentiation and seed developmental gene expression, while variations in seed heteromorphism may be associated with environmental conditions, e.g. annual precipitation, temperature, daylight and their monthly distribution in different calendar years. Seed heteromorphism and its variations in S. aralocaspica show multilevel regulation of the bet-hedging strategy that influences phenotypic plasticity, which is a consequence of internal genetic and external environmental factor interaction. Our findings contribute to the understanding of seed heteromorphism as a potential adaptive trait of desert plant species.

Heteromorphic seed germination and seedling emergence in the legume Teramnus labialis (L.f.) Spreng (Fabacaeae)

Seed heteromorphism can influence germination and ultimately seedling establishment, particularly in disturbed habitats. This study compared seed and seedling traits across three distinctly colored seed morphs (viz. light-brown, brown, and dark-brown) of the forage legume, Teramnus labialis (L.f.) Spreng. The best quality seeds (i.e., un-parasitized, filled and un-cracked) were brown: 389.3 quality seeds per 1000 units compared with <270/1000 units for the other two morphs. Length, width, volume, and water content were lowest in the light-brown and highest in the dark-brown seeds. Seed thickness and mass were lower in the light-brown seeds. Dark-brown seeds imbibed fastest from 2 h onwards. Germination was comparable across the morphs after 7 days but was lowest in the light-brown (17% at 21 days) and highest in the dark-brown seeds (36% at 21 days) at 14 and 21 days. At 7 days, seedling emergence in the dark-brown seeds (15.0%) was higher than in the other two morphs (4%–6%); this remained so at 14 and 21 days. Seedling growth (number of leaves, stem height and diameter, and root length) was superior in the dark-brown seeds. Seed heteromorphism in T. labialis may allow its persistence in disturbed habitats, and the dark-brown seeds are best suited for seeding in revegetation projects, given their superior germination capacity and seedling vigor.

Maternal effects on seed heteromorphism: a dual dynamic bet hedging strategy

Maternal effects on offspring seeds are mainly caused by seed position on, and the abiotic environment of, the mother plant. Seed heteromorphism, a special form of position effect, is the production by an individual plant of morphologically distinct seed types, usually with different ecological behaviours. Seed heteromorphism is assumed to be a form of bet hedging and provides an ideal biological model to test theoretical predictions. Most studies of maternal effects on seeds have focused on abiotic environmental factors and changes in mean seed traits of offspring. Suaeda salsa is an annual halophyte that produces dimorphic seeds within the same inflorescence. We tested the hypothesis that plants grown from brown seeds of S. salsa have a higher offspring brown seed:black seed morph ratio and variance in seed size than plants from black seeds. Results from a pot experiment showed that plants grown from brown seeds had a higher brown seed:black seed ratio than plants grown from black seeds. This is the first layer of dynamic bet hedging. Brown seeds had higher size variation than black seeds, and seeds produced by plants from brown seeds also had higher seed size variation than plants grown from black seeds. This is the second layer of dynamic bet hedging. Thus, the maternal effect of seed heteromorphism is dual dynamic bet hedging. Furthermore, for seed traits we verified for the first time the theoretical prediction that an increase in offspring size variability induces an increase in the mean size of offspring.

Heteromorphism in seeds ofLeptocereus scopulophilus(Cactaceae) from Pan de Matanzas, Cuba

Seed heteromorphism is the formation of different seed morphs from the same individual. Two seed morphs have been preliminarily observed inLeptocereus scopulophilus. One morph shows an apparent natural scarification of its coat. Herein we describe the seeds, taking into account shape, coat integrity, surface, dimensions, mass and the position of germination cracks. We defined two seed morphs using the integrity of the spermoderma: fragmented seed coats (FSC) and complete seed coats (CSC). We also evaluated minimum germination time, germination rate and germinability. The seed morphs did not differ significantly in traits; however, regular striations along the cuticle of the periclinal walls were more visible in the FSC compared with the CSC. Both seed morphs displayed anticlinal cell boundaries in the border region that are channelled and straight in the dorsal-ventral region but difficult to define in the lateral region. We found four morphological variations in different positions where the radicle or cotyledons emerge and variations in cuticle thickness in different regions of the seed that could determine the formation of cracks during germination. All germination variants occurred in both seed morphs, albeit in different proportions. Germination was higher and faster for the FSC compared with the CSC. These germination differences could be related to a thinner cuticle in the FSC and the punctual release of its spermoderma, which facilitates a quick imbibition of the embryo and the breaking of the seed coat. Our results indicate that differences in germination parameters between the two seed morphs relate to differences in the percentage of dormant seeds, which favour the temporal expansion of germination and reduce competition between siblings. To propagate the species for conservation purposes, we recommend using FSC, while CSC may be used to establish a seed collectionex situ.