No abstract

Polyethylene glycol (PEG 6000)-induced water deficit causes physiological as well as biochemical changes in plants. The present study reports on the results of such changes in hydroponically grown tomato plants (Lycopersicon esculentum Mill. cv. Nikita). Plants were subjected to moderate and severe levels of water stress (i.e. water potentials in the nutrient solution of -0.51 and -1.22 MPa, respectively). Water stress markedly affected the parameters of gas exchange. Net photosynthetic rate (Pn) decreased with the induction of water stress. Accordingly, a decrease in the transpiration rate (E) was observed. The ratio of both (Pn/E) resulted in a decrease in water use efficiency. One of the possible reasons for the reduction in Pn is structural damage to the thylakoids, which affects the photosynthetic transport of electrons. This was indicated by an increase in non-photochemical quenching and a reduction in the quantum yield of photosystem II. Furthermore, a decrease in both leaf water potential and leaf osmotic potential was observed, which resulted in a significant osmotic adjustment during stress conditions. Analysis of the physiological responses was complemented with a study on changes in proline content. In stressed plants, a 10-fold increase in proline content was detected compared with control plants. It is clear that water stress tolerance is the result of a cumulative action of various physiological and biochemical processes, all of which were affected by PEG 6000-induced water stress.

A greenhouse experiment was performed in order to investigate the effects of different levels of water stress on leaf water potential (Ψ Ψ Ψ Ψ Ψ w ), stomatal resistance (r s ), protein content and chlorophyll (Chl) content of tomato plants (Lycopersicon esculentum Mill. cv. Nikita). Water stress was induced by adding polyethylene glycol (PEG 6 000) to the nutrient solution to reduce the osmotic potential (Ψ Ψ Ψ Ψ Ψ s ). We investigated the behavior of anti-oxidant enzymes, such as catalase (CAT) and superoxide dismutase (SOD), during the development of water stress. Moderate and severe water stress (i.e. Ψ Ψ Ψ Ψ Ψ s = -0.51 and -1.22 MPa, respectively) caused a decrease in Ψ Ψ Ψ Ψ Ψ w for all treated (water-stressed) plants compared with control plants, with the reduction being more pronounced for severely stressed plants. In addition, r s was significantly affected by the induced water stress and a decrease in leaf soluble proteins and Chl content was observed. Whereas CAT activity remained constant, SOD activity was increased in water-stressed plants compared with unstressed plants. These results indicate the possible role of SOD as an anti-oxidant protector system for plants under water stress conditions. Moreover, it suggests the possibility of using this enzyme as an additional screening criterion for detecting water stress in plants.

For Tunisian olive tree orchards, nitrogen deficiency is an important nutritional problem, in addition to the availability of water. Establishment of relationships between nutrients such as nitrogen and ecophysiological parameters is a promising method to manage fertilisation at orchard level. Therefore, a nitrogen stress experiment with one-year-old olive trees (Olea europaea L. 'Koroneiki' and 'Meski') was conducted with trees respectively subjected to four nitrogen supply regimes (23.96 meq l(-1), 9.58 meq l(-1), 4.79 meq l(-1) and 0 meq l(-1) NO(3)(-)). The current paper focuses on the use of the SPAD-502 portable chlorophyll (Chl) meter, a nondestructive method for fertilisation management under nitrogen stress conditions of olive trees. Maximum net photosynthetic assimilation rates, chlorophyll fluorescence parameters and the SPAD Chl index were therefore measured simultaneously and the Chl and nitrogen content of the leaves were analysed. Significant correlations were established in the olive tree leaves between SPAD-502 readings on the one hand and Chl content, nitrogen content, photosynthetic assimilation rate, and Chl fluorescence parameters (Phi(PSII) and ETR) on the other hand