Ecological Adaptive Strategies Of Heteromorphic Fruit And Seed In Ephemeral Diptychocarpus Strictus

Diptychocarpus strictus (Brassicaceae) is the only species in the genus. In China, this species is found only in the north of Xinjiang province and it is common species in Junggar Desert. The species has two flower-colour morphs (purple and white), and two morphologically distinct types of fruits (upper and lower) and seeds are produced by each of them. Morphology and dispersal of siliques were measured, germination and responses of plants from heteromorphic seeds on heteromorphic siliques and seeds production to environmental variation were studied using standard procedures. Survival strategies of the species in the harsh environment of Junggar Desert were explored. Since the results of the two flower-colour morphs were essentially identical, only those for the purple-flower morph will be presented in this paper. The main results were as follows.(1) Plants produce dehiscent upper siliques being elongated and compressed and with a thin pericarp and their seeds are the dispersal units; indehiscent lower siliques with a thick pericarp are approximately columnar and highly lignified and intact siliques are the dispersal units. Seeds from upper silique have wide wings, while seeds from lower silique are nearly wingless. Thus, the dispersal ability of seeds from upper siliques was much greater than that of intact lower siliques. Seeds from upper silique have a thick mucilage layer, while seeds from lower silique are with thin mucilage layer. Mucilage increased the amount of water absorbed by seeds and decreased the rate of dehydration. Seeds with a thick mucilage layer adhered to soil particles much better than those with a thin mucilage layer or those from which mucilage had been removed. Since soil particles tenaciously adhere to mucilaginous seeds from upper siliques, thereby increasing seed mass and the great increase in mass of these seeds may sink below the soil surface and prevent them from being further dispersed by ants from favorable microhabitats.(2) Freshly-matured seeds from both silique morphs are dormant. Dormant seeds from the two silique types gradually afterripened during storage in the laboratory and field conditions. Afterripened seeds can germinate to high percentages in both light/dark and dark. After 3- to 4-months of afterripening under laboratory and field conditions during summer, the seeds germinate to high percentages at cool but not at warm temperatures. After 12 months of afterripening under laboratory conditions, seeds had high germination over the range of cool to warm temperature regimes. Seeds stored in the field germinated to high percentage at cool temperatures, but to low percentage at warm temperatures when tested germination of seeds stored in the field until April (10-months old). Dormancy break occurs in summer, but seeds are not induced back into dormancy (secondary dormancy) during winter. Seeds have nondeep physiological dormancy. Germination of the excised seeds from the upper and lower siliques was not inhibited by mucilage. The pericarp physically restricts/prevents germination of seeds within the lower siliques.(3) Soil moisture influence germination of seeds from upper and lower siliques in desert. If the soil is sufficiently moist during summer and autumn, they germinate in autumn. If the nondormant seeds fail to germinate in autumn, they germinate in next spring, when soil moisture (precipitation plus snow melt) were suitable for germination. Seeds within these indehiscent siliques are prevented from germinating during the first spring germination season by the mechanical restraint of the thick, rigid pericarp. Thus, the lower siliques can form a persistent seed bank. For a given germination timing plants from the two seed morphs did not differ in life history traits and a high percentage of plants survived and reproduced. However, there were great differences in all life history traits between autumn- and spring-germinating plants from both of seeds. There were significant differences in life span and plant size of those that germinated in autumn vs. spring. Autumn-germinating plants had a longer life span and larger plant size than did spring-germinating plants. Number and length of branches and reproduction of siliques and seeds was positively correlated with plant size. Allocation to roots and stems was higher in autumn-germinating plants than in spring-germinating plants, with the opposite result for dry mass allocation to leaf and reproductive organs. Our results indicated that there differed in soil seed bank between heteromorphic siliques and seeds, and plasticity in life history traits was related to season of germination.(4) There did not any differ in responses to environmental variation, i.e. soil moisture, nutrients, density/competition and herbivory, between plants from upper and lower seeds. With soil moisture increasing, nutrient supply raising, plant density decreasing or intensities of herbivory reducing, the number and mass of upper and lower siliques and seed production of plants from the both seeds were significantly increased; on the contrary, the number and mass of upper and lower siliques and seed production of plants from the both seeds were significantly decreased. In despite of conditions, the number and mass of upper siliques and their seed production exhibited greater variation than that of lower siliques and seed. In favourable environment conditions, plants of D. strictus from the two seed morphs produced a greater number and mass of upper silique morphs, which producted a large number of seeds with high dispersibility and a low level of dormancy in order to exploit new habitats and enlarge their population. In stress conditions, plants of D. strictus from the two seed morphs insured the number and mass of lower siliques and their seed production, which can form a persistent seed bank via stay-at-home dispersal and delayed germination to maintain their population.The existence of both dimorphic siliques and dimorphic seeds in a raceme of D. strictus confers differences in the capability for dispersal and germination of seeds and siliques and seed production in responses to environmental variation, which may be viewed as adaptive bet-hedging in an unpredictable environment. High dispersal ability and reduced dormancy of seeds (from upper siliques) provide D. strictus with the chance of rapidly colonizing new sites and enlarging its population. Thus, upper siliques present a phenotypy of"high-risk"strategy. Furthermore, lack of seed dispersal and delay of germination of seeds from lower siliques give the species a better chance of retaining the mother site and of colonizing suitable local sites, thus increasing the probability of persistence of D. strictus populations. Therefore, lower siliques present a phenotypy of"low-risk"strategy.