The role of GAs in promoting seed germination is well known and experiments with seeds from different species have suggested the requirement of de novo synthesis of GAs upon imbibition for germination. There are also strong indications that the enhancement of GA synthesis is part of the mechanism through which environmental signals (i.e. light) induce germination. Since along the GA biosynthetic pathway, oxidation at C-20 carried out by GA 20-oxidases is thought to be a site of regulation, a cDNA clone encoding a GA 20-oxidase was isolated from embryos of sorghum (SbGA 20ox). Expression analysis of this gene in embryos within imbibed caryopses with low dormancy showed detectable amounts of the specific mRNA early upon incubation, increasing thereafter. In contrast, it remained barely detectable in embryos from dormant caryopses. Changes in endogenous GA4 levels were in agreement with those of SbGA 20ox mRNA, suggesting that GA production might be regulated differentially at the level of transcription of this gene. The expression of SbGA 20ox was enhanced in incubated embryos isolated from either type of caryopses, illustrating a physiological control exerted by the surrounding seed tissues on gene expression. The results also show that ABA leads to a suppression of transcription of this gene.

The effect was studied of sunlight and far‐red (FR) light during seed development, on seed quality and germination of Sicyos deppei G. Don. Seeds exposed to FR during development were lighter in colour and their weight, size and water content were significantly lower. Less than 10% of non‐scarified freshly harvested seeds germinated. Scarified, freshly harvested seeds developed under sunlight had a partially negative photoblastic response; both red (R) and FR light inhibited germination. The highest and fastest germination occurred in darkness, probably due to the effect of the high photon flux densities on the phytochrome during seed development. Scarified seeds ripened under FR light, germinated well in FR light and in darkness, but R light inhibited germination. After 6 months of storage, the permeability of S. deppei seeds increased, the partially negative photoblastic response was lost and germination of scarified seeds increased. Specifically, in seeds developed under FR, germination in darkness was faster than for the other light treatments, but slow in darkness for seeds developed under sunlight. The physiological and morphological heteroblastic responses in S. deppei probably extend its seed germination and seedling recruitment periods.

Resumen. Se determinaron las características y calidad de las semi llas de Ch. pentadactylon, de dos localidades en Guerrero y Oaxaca, su germinación bajo los factores de escarificación, temperatura y luz y la viabilidad d e semillas de Gu errero almacenadas 6 meses a 5º C. Las semillas resultaron ortodoxas y con latencia por cubie rta sem in al impermeable. Las semillas de Guerrero presentaron mayor peso, longitud y contenido de humedad respecto a las de Oaxaca , pero las semi llas escarificadas de Oaxaca germinaron con mayor porcen taje. La germinación de sem illas escarificadas de ambos siti os fue mejor en temperatura flu ctuante y se inhibió bajo rojo lej ano . No hu bo diferencias en la capacidad germinativa bajo obscuridad, luz roja y b lanca. Una fracción d e la población de semi llas podría permanecer latente en bajas relaciones de rojo:rojo lej a no transmitidas por la hojarasca. Las diferencias entre poblaciones pudieron deberse a un efecto materno inducido por temperatura y la humedad de sus localidad es. Palabras clave: germinación, almacenamiento, fotoblastismo, semi llas ortodoxas, temperatura de germ in ación. Abstract. Seed quality and seed germination with scarificatio n and n on scarification and variab le temperature and light were determined in Ch. pentadactylon seeds form two pladces of Guerrero and Oaxaca. Also seed viability was determined in seeds from Guerrero stored 6 months at 5º C. Seeds showed to be orthodox a nd dorman t dueto an impervious seed coat. The seeds from Guerrero had higher weight, length and water content tha n those from Oaxaca, however the scarified seeds from Oaxaca reached a higher germination percentages. Germination of scarified seeds of the two popu lations was enhanced by alternated temperatures and in hibite d by far red light. Germ in ation under darkness, red light and wh ite light did not differ significan tly. In soil sorne seeds could be dormant under a low ratio red:far red light transmitted by litter. The differences between the two populations might be due to a maternal effect induced by weather conditions in their geographic localiti es.

The plant vp1 gene encodes a transcription factor originally identified in maize that participates in the control of the transition from embryogenesis to seed germination. Different lines of evidence suggest that vp1 participates in preharvest sprouting (PHS) resistance in cereals, but the genetic relationship is not yet established. Thus, the aim of this study was to investigate if there is a connection between vp1 and the formerly documented quantitative trait loci for PHS in Sorghum bicolor. This gene was mapped in sorghum using two well-studied inbred lines displaying contrasting PHS phenotypes. Linkage analysis revealed that the S. bicolor vp1 (Sbvp1) locus is linked to markers located on chromosomes 3 and 8 in maize. Analysis indicated that this gene is not correlated with PHS at a statistically significant level. Structural analysis of the Sbvp1 gene from both inbred lines showed a high degree of overall conservation (low polymorphism), even in the promoter region. These findings, taken together with the previously observed correlation between the pattern of expression of this gene and PHS performance, suggest that Sbvp1 might act downstream in the signalling pathway that leads to seed germination, or even be differentially regulated as a consequence rather than as a cause of the phenotypic behaviour.