Calcium Improves Germination and Growth of Sorghum bicolor Seedlings under Salt Stress

Salinity is a major constraint limiting plant growth and productivity worldwide. Thus, understanding the mechanism underlying plant stress response is of importance to developing new approaches that will increase salt tolerance in crops. This study reports the effects of salt stress on Sorghum bicolor during germination and the role of calcium (Ca2+) to ameliorate some of the effects of salt. To this end, sorghum seeds were germinated in the presence and absence of different NaCl (200 and 300 mM) and Ca2+ (5, 15, or 35 mM) concentrations. Salt stress delayed germination, reduced growth, increased proline, and hydrogen peroxide (H2O2) contents. Salt also induced the expression of key antioxidant (ascorbate peroxidase and catalase) and the Salt Overlay Sensitive1 genes, whereas in the presence of Ca2+ their expression was reduced except for the vacuolar Na+/H+ exchanger antiporter2 gene, which increased by 65-fold compared to the control. Ca2+ reversed the salt-induced delayed germination and promoted seedling growth, which was concomitant with reduced H2O2 and Na+/K+ ratio, indicating a protective effect. Ca2+ also effectively protected the sorghum epidermis and xylem layers from severe damage caused by salt stress. Taken together, our findings suggest that sorghum on its own responds to high salt stress through modulation of osmoprotectants and regulation of stress-responsive genes. Finally, 5 mM exogenously applied Ca2+ was most effective in enhancing salt stress tolerance by counteracting oxidative stress and improving Na+/K+ ratio, which in turn improved germination efficiency and root growth in seedlings stressed by high NaCl.

Impacts of short-term germination delay on fitness of the annual weed Agrostemma githago (L.)

Time of seedling emergence is an important step in the life cycle of annual plants because it may determine subsequent performance and success. Timing of emergence is especially critical to plant performance in habitats like arable fields which are subject to frequent disturbances. Within-season variation in timing of germination in the range of only a few days is typical for many arable weeds. However, since it is unclear whether such small deviations in germination date translate into fitness differences in the course of the life cycle, the aim of this paper was to quantify the effects of short germination delays on plant performance. We conducted two generalized randomized block experiments in an unheated greenhouse to study the impact of delayed germination (1, 2, 3 and 7 d) with and without competition, respectively, on the fitness of the arable weed species Agrostemma githago (L.). We expected that delayed germination significantly reduces fitness in terms of several life-history traits, and that the decrease of fitness is higher in the presence of competition. Under realistic conditions with competition through barley, Agrostemma plants with delayed germination of 7 d produced 54% fewer shoots, 57% less biomass, 52% fewer flowers, 36% lighter seeds and were 23% shorter as compared to control plants without delayed germination. Without additional stress through competition with barley this pattern was less pronounced. Thus, in the situation of interspecific competition, early emerging seedlings have biologically significant fitness advantages over later emerging seedlings of the same species.

Seed extracts impede germination in Brassica rapa plants

Allelopathy is an important mechanism of interference competition in some plants, but little is known about whether compounds exuded from seeds influence the germination of neighbors. We treated seeds of multiple lines of <em>Brassica rapa </em>(field mustard) with aqueous extracts of <em>Brassica rapa</em> Fast Plant seeds and extracts of seeds of the invasive, allelopathic plant <em>Alliaria petiolata</em> (garlic mustard). Germination patterns differed significantly among populations, with Fast Plants germinating earliest. As predicted, <em>Alliaria</em> seed extracts significantly delayed germination compared to controls. Surprisingly, Fast Plant extracts reduced germination probability and delayed germination more than both controls and <em>Alliaria</em> extract-treated seeds. These results suggest that compounds in the seeds of both species appear to be allelopathic, and indicate a potentially important and unrecognized role for allelopathy in intra- and inter- specific competition at the seed stage.