Darkness mediates different senescence-related responses depending on the targeting of dark treatment (whole plants or individual leaves) and on the organs that perceive the signal (leaves or cotyledons). As no data are available on the potential role of darkness to promote senescence when applied to individual cotyledons, we have investigated how darkness affects the progression of senescence in either a single or both individually darkened cotyledons of young 10-day-old Cucurbita pepo (zucchini) seedlings. Strong acceleration of senescence was observed when both cotyledons were darkened as judged by the damage in their anatomical structure, deterioration of chloroplast ultrastructure in parallel with decreased photosynthetic rate and photochemical quantum efficiency of PSII. In addition, the endogenous levels of cytokinins (CKs) and IAA were strongly reduced. In a single individually darkened cotyledon, the structure and function of the photosynthetic apparatus as well as the contents of endogenous CKs and IAA were much less affected by darkness, thus suggesting inhibitory effect of the illuminated cotyledon on the senescence of the darkened one. Apparently, the effect of darkness to accelerate/delay senescence in a single darkened cotyledon depends on the light status of the other cotyledon from the pair. The close positive correlation between CK content and the activity of CK oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) suggested that CKX was essentially involved in the mechanisms of downregulation of endogenous CK levels. Our results indicated that CKX-regulated CK signaling could be a possible regulatory mechanism controlling senescence in individually darkened cotyledons.

The effects of Cd, Pb, and Zn uptake on plant morphology, photosynthetic parameters, antioxidant potential, and essential oil yield and quality in Ocimum basilicum L. and Origanum vulgare L. plants were evaluated. The plants were grown as a pot experiment in soil heavily polluted with Cd and Pb and on unpolluted soil. Both plants accumulated Cd, mainly in the roots, while Pb occurred in the oregano shoots only. The leaf blade thickness of both plants increased when grown in polluted soil. Basil plants responded to the action of heavy metals with increases in gas exchange , stomatal conductivity, and transpiration, but water-use efficiency declined. Gas exchange and transpiration were reduced in treated oregano plants, but stomatal conductivity and water-use efficiency increased significantly. In basil, the increased levels of low molecular weight antioxidants such as phenols and flavonoids were observed, while in oregano, ascorbate, glutathione, and phenols were enhanced. Enzymatic antioxidant defense was observed in both plants when grown in contaminated soil, mainly with glutathione peroxidase, quaiacol peroxidase, glutathione S-transferase, and glutathione reductase. Accumulated levels of Cd, Pb, and Zn in plant organs resulted in a reduction in essential oil yield in basil only.

Manganese (Mn) is an essential micronutrient in all organisms, but may become toxic when present in excess. Four maize (Zea mays L.) varieties, Kneja 605, Kneja 434, Kneja 509 and Kneja 537, were studied with respect to their responses to excess Mn in hydroponic solution. In the varieties Kneja 605, Kneja 509 and Kneja 537, increasing Mn concentrations in the nutrient solution negatively affected biomass accumulation, photosynthetic rate, transpiration, stomatal conductance and chlorophyll content. In addition, these varieties showed increased electrolyte leakage and lipid peroxidation (malondialdehyde [MDA] content). Increased Mn leaf concentrations, higher contents of chlorophyll a and chlorophyll b, higher photosynthetic rate and transpiration, lower concentrations of MDA and insignificant changes in the electrolyte leakage in the leaves were found in var. Kneja 434 compared with the other maize varieties studied. This variety appeared to possess a stronger ability to cope with Mn phytotoxicity, suggesting high potential for Mn detoxification and var. Kneja 434 could be a good candidate for improving maize productivity on acid soils under non-tropical conditions.

Natural senescence of Cucurbita pepo (zucchini) cotyledons was accompanied by a gradual degradation of reserve proteins (globulins) and an intensive decrease in the content of both large subunit (LSU) and small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). The net photosynthetic rate, the primary photochemical activity of PSII, estimated by the variable fluorescence (F v )/maximal fluorescence (F m ) ratio (F v /F m ) and the actual quantum yield of PSII electron transport in the light-adapted state (UPSII) also progressively decreased during natural senescence. In contrast, the fraction of the absorbed light energy, which is not used for photochemistry (LNU) increased with progression of senescence. The decline in the photosynthetic rate started earlier in ontogenesis compared with the down-regulation of the functional activity of PSII, thus suggesting the existence of protective mechanisms which maintain higher efficiency of the photochemical electron transport reactions of photosynthesis compared with the dark reactions of the Calvin cycle during earlier stages of natural senescence. Decapitation of the epicotyl above the senescing cotyledons resulted in full recovery of the polypeptide profile in the rejuvenated cotyledons. In addition, the photosynthetic rate increased reaching values that exceeded those measured in juvenile cotyledons. The photochemical efficiency of PSII also gradually recovered, although it did not reach the maximum values measured in the presenescent cotyledons. KeywordsCotyledons of Cucurbita pepo L. (zucchini) Á Natural senescence Á Photosynthesis Á Polypeptide pattern Á Rejuvenation Abbreviations PSIIPhotosystem II UPSIIThe actual quantum yield of PSII electron transport in the light-adapted state LSU

The effects of cadmium (Cd) and/or paraquat (PQ) toxicity on photosynthesis in maize leaves were examined by measurement of gas exchange and chlorophyll content in hydroponically cultured plants. It was found that growth rate was distinctly influenced only by 100 µM Cd treatment. Chlorophyll a and chlorophyll b decreased along with the increase of Cd concentration, while PQ spraying, alone and combined with Cd, increased chlorophyll a content on the third and seventh experimental days. Generally, carotenoid content increased in response to Cd and PQ and reached the highest levels at 100 µM Cd. Rate of photosynthesis in maize decreased after Cd treatment. CO 2 assimilation was approximately 60% reduced at 50 µM Cd and 70% reduced in the presence of 100 µM Cd. PQ toxicity was partly overcome after the third day of exposure. Transpiration and stomatal conductance in maize leaves decreased on the third day along with Cd concentration and PQ spraying, except for the 25-µM Cdtreated plants. On the tenth day, the 25-µM Cd-treated plants and those from PQ-treated variants showed an increase of transpiration and stomatal conductance. Maize exhibited an ability to accumulate Cd in high quantities, especially in the roots-over 4,500 mg Cd/kg dry weight.