Studies on the effects of salinity and heavy metals on the sporophytic & gametophytic generations of Arabidopsis thaliana (L.) heynh

Abstract

The aim of this research was to study the effects of Salinity & Heavy Metals on the Sporophyte & Gametophyte generations of Arabidopsis thaliana. After developing a pollen germinating protocol, some basic experiments were conducted to determine the effects on chemical & physical parameters on pollen germination & pollen tube growth. Additions of heavy metals & high concentrations of buffer (Tris-HCI) in pollen germinating medium (PGM), inhibited pollen germination (PG) & pollen tube length (PTL), while pH range (7.0-9.0) had little effect. The idea of 2-fold effect (nutritional & osmotic) of sucrose was supported in results. In vivo pollen growth was much greater than in vitro & genotypic differences occurred among the mutants for PG & PTL. Comparing the pollen nutritional requirements for PG & PTL between Cicer arietinum & A. thaliana, it was noted that the pollen of the former had a greater requirement for H(_3)BO(_3) & Ca(N0(_3))(_2) than latter. The range of temperatures (0 C-35 C) studied, indicated that the most favourable temperature for PG was 20 C & for PTL 25 C , while extreme temperatures (0 C, 35 C) were harmful to A. thaliana pollen. 20 C was found to be optimum temperature for PG & PTL of C. arietinum. Correlation between pollen tube growth rates (PTGRs) and sporophytic traits of Fls showed a generally strong positive correlation with most of sporophytic traits, but a very loose to negative correlation for earliness parameters. Recording the impacts of salinity, parallel effects were found for the sporophytic & gametophytic generations of the plant, as reported by earlier workers. The results also indicated that it is possible to develop a pollen plant salinity index (PPSI) for plant species under specific plant growing conditions. It was also observed that salinity stress during pollen gametogenesis preconditioned pollen to high salinity levels, resulting in higher PG & PTL than in pollen raised under non-saline conditions. Similarly, the fruit-setting, seed-setting & seed-filling were relatively less affected by saline-stress, in progeny when pollen and pistilate plants were grown under similar saline regimes. Anatomical studies showed that salinity induced changes in the epidermis, cortex, pericycle, xylem, pith & cross sectional diameter of vascular bundle (CSDVB) in stem, leaf & root tissues. Lower reductions for the fruit-setting, seed-setting & seed-filling were recorded in response to the high heavy metal concentrations in progeny when pollen and pistilate plants were raised under similar heavy metal regimes. It was apparent that pollen become conditioned to a stress environment during its development in the anthers of stressed plants, & this preconditioning allowed its pollen tubes to grow more successfully in the styles of the female parents growing in a similarly stressed environment, where the pistil may have accumulated higher levels of heavy metals. Heavy metals induced changes in stem shape, epidermis, cortex, vascular bundles & chloroplasts in stems. The metals induced changes in the leaf thickness, epidermis, palisade cell size, spongy mesophyll area, size of intercellular spaces & chloroplasts in the leaf, causing disruption of cortical layers, lignification of pericycle & phloem, damage to the endodermis and increase in xylem cell size & stele diameter in roots.