Effect of selenium and soil pH on cadmium phytoextraction by Urochloa decumbens grown in Oxisol.

It has been speculated that selenium (Se) supply can affect cadmium (Cd) 'availability' and increase the Cd tolerance of plants used for phytoextraction, in a pH-dependent process. Thus, we evaluated the interaction Cd-Se and the effects of soil pH in this interaction on plant availability of Cd and phytoextraction efficiency of Urochloa decumbens cv. Basilisk grown in Oxisol. Two soil concentrations of Cd (0.93 and 3.6 mg kg-1) and Se (<0.2 and 1 mg kg-1) and two soil pH (0.01 mol L-1 CaCl2) conditions (4.1 and 5.7) were considered. At both pH, Se supply increased the exchangeable fraction of Cd and decreased the residual Cd fraction. At pH 4.1, the growth of U. decumbens was impaired by Se addition, regardless of Cd exposure. The lower root growth and tillering of U. decumbens exposed to Cd disappeared at pH 5.7 due to uptake of low Se concentrations. Thus, the toxic or beneficial effects of Se on growth of U. decumbens used for Cd phytoextraction depend on the amount of Se assimilated. The Cd phytoextraction efficiency of U. decumbens was not improved by Se supply, regardless of soil pH. Therefore, we cannot recommend the application of Se to increase Cd phytoextraction by this grass.

Pollen morphology and viability of Tillandsia (Bromeliaceae) species by light microscopy and scanning electron microscopy.

Tillandsia is the bromeliad genus containing the largest number of species, with wide geographic dispersion and an important ecological role in the ecosystems. Investigations of pollen morphology are important to support taxonomic and conservation studies as well as to choose promising species for cross-pollination to obtain hybrids. The objective of this study was to evaluate the morphology, production, and viability of pollen grains of 24 Tillandsia species that naturally occur in the state of Bahia, Brazil. Pollen grains were acetolized and analyzed by scanning electron microscopy. The pollen were germinated in vitro in two culture media (BM and BKM) and collected at three floral development stages (pre-anthesis, anthesis and post-anthesis). Pollen viability also was analyzed by staining with Alexander's solution and acetocarmine in three floral development stages. Variations in the ornamentation and size of the pollen were observed among the species studied, with the majority having medium size, while T. polystachia and T. juncea had large grains and the subgenera Diaphoranthema and Phytarrhiza had small grains. The pollen of the majority of species had bilateral symmetry, with a single irregularly shaped colpus, semitectate exine and reticulated-heterobrochate surface. The pollen presented high germination percentage, tube length and viability according to the histochemistry, except for T. recurvata, T. usneoides and T. loliacea. The BKM medium and collection of pollen during anthesis produced the best results for the Tillandsia species. The results can support studies of the taxonomy and conservation of these species, which have great environmental importance and high value for ornamental purposes.

Selenium improves photosynthesis and induces ultrastructural changes but does not alleviate cadmium-stress damages in tomato plants.

The application of Se to plants growing under Cd contamination may become an alternative strategy to minimize Cd damage. However, there is no specific information available regarding whether Se can affect the anatomical structure and photosynthetic rates of plants under Cd stress. To address questions related to Se-protective responses under Cd stress, we evaluated the structural and ultrastructural aspects, photosynthetic rates and growth of tomato cv. Micro-Tom plants. Plants were exposed to 0.5 mM CdCl2 and further supplemented with 1.0 µM of selenite or selenate. The overall results revealed different trends according to the Se source and Cd application. Both Se sources improved growth, photosynthesis, leaf characteristics and middle lamella thickness between mesophyll cells. In contrast, Cd caused decreases in photosynthesis and growth and damage to the ultrastructure of the chloroplast. The number of mitochondria, peroxisomes, starch grains and plastogloboli and the disorganization of the thylakoids and the middle lamella in plants increased in the presence of Cd or Cd + Se. Se plays an important role in plant cultivation under normal conditions. This finding was corroborated by the identification of specific structural changes in Se-treated plants, which could benefit plant development. However, a reversal of Cd stress effects was not observed in the presence of Se.