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Chlorophyll fluorescence parameters of Quercus pubescens Willd. as response to heat shock (HS) by immersing leaves for 5 and 15 min in water of temperatures between 38 and 59 o C were examined. Fluorescence was measured after different periods of recovery (15, 30, 90, 210, and 1 440 min at 24/26 ºC night/day temperature and 100 % humidity). The effective quantum yield of photosystem 2 (Y) in control and HS-treated leaves was always measured after previous 15 min irradiation. Under a 5 min HS, Y did not change after using temperatures below 44 o C, was rapidly restored after HS of moderate temperatures (44-48 o C), and progressively decreased and recovered eventually to the initial value after HS of high temperatures (48-52 o C). Y did not recover after HS with temperatures higher than 52 o C. Increase in the duration of HS from 5 to 15 min lead to change of the initial Y at each HS temperature, but the recovery processes were similar to those characteristic after 5 min incubation. The processes of recovery may depend mainly on the specificity of injuries caused by different heat shock temperatures. Thus Q. pubescens is able to preserve and recover the functional potential of its photosynthetic apparatus in response to HS up to 52 o C.Additional key words: photosystem 2; temperature. --The growth and survival of plants can be determined by the thermotolerance capabilities of photosynthesis (Sharkey 2000). The composition of thylakoid membranes of chloroplasts, which contain the light-absorbing system, electron transport chain, and ATP synthase, is important in determining a plant's ability for growth and photosynthesis at high temperatures (Murakami et al. 2000). Heating followed by cooling causes deleterious changes in many physiological processes and disrupts the intricate organization of the thylakoid membrane, which has a high content of unsaturated fatty acids (Berry and Björkman 1980). These membranes must be sufficiently fluid to allow the "spinning" of ATP synthase, yet solid enough to produce the proton-motive force needed to propel this spinning (Gounaris et al. 1984). The thylakoid membrane is heterogeneous, and the lipids must be kept properly dispersed to prevent them from concentrating and forming non-bilayer structures.We examined the influence of the temperature of HS and the period of recovery after shock on the fluorescence of Quercus pubescens leaves. The experiment was set up in a completely randomized block design with 6 replicates. Leaves were collected in August 2005 from the white oak trees that grow in forests of the Leova area of Moldova. As humidity strongly influences the sensitivity of plants to the growing temperature, we incubated the isolated leaves in glass crystallizers on wet filter paper, at a relative humidity of 100 %. The crystallizers were exposed to 60 PAR from luminescent lamps with 16 h daylength, and 26/24 o C day/night temperature for the duration of the experiments. In order to assess the effect of temperature on the photosynthetic machinery, leaves were pre-treated by...

The preparations obtained by extraction of active substances from different organisms, when processing plants, are influencing their growth and response to abiotic and biotic stress factors. Their action is different from that of nutrients and synthetic preparations for plants protection. In the scientific literature, to distinguish them from other preparations, the term biostimulant was proposed. In this article we are included some examples of the effects of the biostimulant Reglalg, extracted from the biomass of algae Spirogyra sp., on the productivity and resistance of wheat plants to frost. Plants respond to the action of abiotic and biotic stress with sophisticated innate reactions induced by the signals being modified plant-derived or invading organism derived molecules [4].

The changes in the seeds germination capacity of the hexaploid wheat variety Odesskaya 267 after different periods of immersion in water at a temperature of 4 C were studied. The seeds immersion in water for up to 104 hours was accompanied by rapid absorption of water in the first 32 hours (phase I), followed by a slow increase in seed moisture over the next 72 hours (phase II). The seeds sensitivity to the application of heat shock (4852 C) increased with the immersion time, which, together with the improvement of their germination parameters during phase I and the first part of phase II, suggests the seeds coming out of dormancy and initiating processes of germination. The sensitivity to heat shock of the seminal roots initials, especially of the radicle, gradually increased with the imbibition time expansion. Their meristems initials cells have progressed more in exit from dormancy and germination processes intensification during this period. The elaborated method of assessing the seed resistance to heat shock is prospective to compare the primary resistance (without the involvement of adaptation processes) of different wheat genotypes to high temperatures.

În condiţiile actuale de încălzire a climei rolul cercetărilor care se referă la aprecierea acţiunii factorilor de stres termic asupra posibilelor schimbării în răspândirea speciilor de stejar va spori în viitor. Răspunsul plantelor la acţiunea şocului termic include evitarea factorilor de stres şi mecanismele fiziologice şi biochimice de recuperare a leziunilor pricinuite ţesuturilor frunzelor. Recuperarea stării funcționale a fotosistemului II în frunzele speciilor de stejar expuse șocului termic s-a manifestat la nivel maxim atunci când probele au fost incubate în condiții optime de păstrare. Speciile de stejar au elaborat posibilităţi diferite de adaptare la acţiunea temperaturilor ridicate. Dintre cvercinee, stejarul pufos este cea mai termotolerantă specie spontană răspândită în Republica Moldova. După excluderea influenței mecanismelor care determină evitarea / diminuarea acţiunii temperaturilor ridicate, frunzele stejarului pufos au arătat o sensibilitate mai sporită la șocul termic decât cele ale stejarului pedunculat și gorunului. În funcție de aclimarea frunzelor la temperaturi ridicate, speciile de stejar au arătat o adaptare avansată, după cum urmează: stejarul pedunculat în zona de nord, gorunul – în zona de centru și stejarul pufos – în condițiile ecologice ale zonei de sud a Republicii Moldova.