In Amazon secondary forests are dominated by pioneer species that typically produce large amounts of small and dormant seeds that are able to form a persistent soil seed bank. Seed dormancy in this group of species is overcome by environmental conditions found in open areas, such as high irradiation or alternating temperatures. Nevertheless, a variety of germination responses to environmental factors is known among pioneers; some of them may germinate in diffuse light or in darkness condition at constant temperature. Seed mass can be considered as one of the factors that promotes this variety. Regarding species with very small seeds, it seems that the trigger for germination is light and for larger seeds temperature alternation may be a more important stimulus. In this study we established a relationship between seed mass and germination response to light and alternating temperature for a group of seven woody pioneer species from the Amazon forest. We found that an increase in seed mass was followed by a decrease in the need for light and an increase in the tolerance to alternating temperatures. Understanding germination strategies may contribute with the knowledge of species coexistence in high diverse environments and also may assist those involved in forest management and restoration.

The use of biochemical seed viability markers is often compromised by the unknown partitioning of analytes in bulk seed lots consisting of inseparable populations of viable and nonviable seeds. We took advantage of an unusual morphological syndrome found in the recalcitrant, undifferentiated seeds of Eugenia stipitata: one seed can be cut into several parts, each of which can germinate and develop into seedlings. We used four seed parts from one individual seed to analyse seed moisture content (MC), seed viability and the antioxidant glutathione (γ-glutamyl-cysteinyl-glycine; GSH), glutathione disulphide (GSSG) and intermediates of glutathione synthesis and breakdown. Seeds were exposed to different environmental MC to induce various levels of desiccation stress. Upon storage at high seed MC, seed viability was maintained, while GSH concentration increased and the glutathione half-cell reduction potential (E ) was less negative than -215 mV, indicating GSH production and highly reducing conditions. Storage at low seed MC led to loss of GSH, resulting in a shift in E , and seed death. In contrast, the cyst(e)ine half-cell reduction potential (E ) could not distinguish between the viability categories. Previous studies on seed populations revealed that the probability for a seed being alive is 50% at E values between -180 and -160 mV. The single seed approach revealed that the window in which seed viability was lost could be slightly shifted towards more negative values. We discuss the contribution of cellular pH to E and recommend E. stipitata as a recalcitrant seed model to study stress response on a single seed basis.

ABSTRACT. Polyploid induction has been used for plant breeding to produce bigger and more robust plants than diploid types. The present study aimed to develop a methodology for in vitro induction of polyploidy in cassava. Apical and lateral microcuttings from the BRS Formosa variety were treated with six oryzalin concentrations for 24 and 48 h. The same methodology was used for colchicine with different concentrations. After 45 days of cultivation and an additional 45 days of subculture, the viability of the explants was assessed and plant acclimatization was performed. Ploidy was determined using flow cytometry. Oryzalin dose and exposure negatively affected cassava explant growth and development compared to untreated explants. Furthermore, apical and lateral explants responded differently to the treatments, showing a diversity in antimitotic sensitivity and effect that is tissue-type specific. In contrast, the doses of 1.25 to 6.25 mM colchicine resulted in high mortality of cassava explants. Therefore, the type of antimitotic affects the morphophysiological behavior of cassava plants in vitro, although apical explants have higher viability and regenerative capacity compared to lateral explants. In addition, the lateral explants have lower mixoploid rates compared to apical explants. Of the 310 plants generated by oryzalin treatments, 277 were diploid, 31 were mixoploid, and 2 were tetraploid. Exposure to oryzalin led to low rate of tetraploids and colchicine caused phytotoxic reactions and death of the explants. The tetraploids were multiplied in vitro to evaluate their yield in the field as well as their behavior against abiotic and biotic stress.

-The knowledge of the phenotypic variation of cassava (Manihot esculenta Crantz) germplasm allows the estimative of the genetic variability to support the selection of contrasting genitors. Therefore, the aim of this work was to define homogeneous groups of cassava germplasm based on yield traits, disease resistance and root quality using K-means as a nonhierarchical method. Breeding values estimated by Best Linear Unbiased Predictor (BLUP) were used for the cluster analysis. The number of groups was defined according to the stabilization of the smallest within-group sum of squares. Seventeen clusters were defined to represent the diversity of the germplasm, whose number of accessions ranged from 7 (Group 15) to 69 (Group 9). In general, accessions belonging to Groups 1, 4, 7, 12, 15 and 16 showed good agronomic traits, such as high fresh root yield and starch yield (> 60.7 t ha -1 and 18.6 t ha -1 , respectively). In contrast, only Group 15 presented low bacterial blight severity. The groups obtained showed strong differences, as evidenced by the within-groups sums of squares values, which ranged from 215.1 (Group 15) to 2,338.3 (Group 8). The K-means algorithm allowed the formation of consistent groups based on yield traits, disease resistance and root quality. Therefore, the K-means algorithm was efficient in the formation of groups with low within genotypic variation, especially concerning large amounts of data, such as in cassava germplasm banks.