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Senescence is an important developmental process in the life span of a plant. It is affected by several endogenous and exogenous factors such as hormones, darkness, and drought. Hormones are major endogenous factors that affect senescence intensively. In this study, our aim was to investigate the effects of different plant growth regulators; benzyl adenin (BA), spermine (Spm) and spermidine (Spd) on both natural and dark-induced senescence and on γ-aminobutyric acid (GABA) content in wheat leaf segments. Following the incubation, fresh and dry weight, chlorophyll and carotenoid contents; peroxidase activity, soluble protein, GABA and malondialdehyde (MDA) contents and cell death amount were determined. Our results demonstrate that Spm and Spd accelerated senescence under light and delayed it under dark conditions. BA has delayed leaf senescence under both conditions. Cell death and MDA content showed a correlation with the senescence process. GABA content was enhanced considerably by BA, Spm and Spd applications under light conditions. According to our results we can conclude that Spm and Spd could promote senescence on wheat segments under light conditions, whereas inhibit senescence under dark conditions. In addition, BA could retard the deleterious effects on senescence under both light and dark conditions.   ********* In press - Online First. Article has been peer reviewed, accepted for publication and published online without pagination. It will receive pagination when the issue will be ready for publishing as a complete number (Volume 47, Issue 4, 2019). The article is searchable and citable by Digital Object Identifier (DOI). DOI link will become active after the article will be included in the complete issue. *********

Abiotic stresses (e.g., heavy metals, drought, cold, or combinations) induce oxidative stress with overproduction of reactive oxygen species (ROS). We have investigated the antioxidative responses of Albizia julibrissin Durazz. (silk tree, Fabaceae). Four-month-old plants grown in sand cultures were subjected to various single or sequential treatments involving exposure to cadmium (50-250 lmol L-1 Cd), lead (1000-5000 lmol L-1 Pb), chilling at 4°C (CH), or drought (DR), for a period of 7-45 days. Leaf extracts were assayed for glutathione peroxidase (GPX), glutathione-disulfide reductase (GR), catalase (CAT), soluble proline (Pro), ascorbate peroxidase (APX), and guaiacol peroxidase (GUAPX). Cd and Pb accumulation in the leaves was also measured. CAT activity decreased strongly with increasing Pb exposure and after CH. It was also found to be reduced after Cd and DR treatments. GR activity increased highly in nearly all treatments, most strongly at high Cd or Pb, after DR ? CH, and after CH followed by Cd. GUAPX and GPX showed similar trends of increase. APX activity dropped after CH, but increased after low Cd treatment and in CH ? DR sequential stresses. Massive accumulation of soluble Pro occurred after 14-21 days in highly Cd-or Pbstressed plants. CH or DR acclimation led to some alterations of antioxidative responses, particularly for CAT, GR, and APX. Our data indicate that GSH, GSHlinked redox systems, peroxidases, and Pro are possibly the more important antioxidants under severe stress.

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