Seasonal variability of maximum quantum yield of PSII photochemistry (Fv/Fm) was studied in needles of Taxus baccata seedlings acclimated to full light (HL, 100% solar irradiance), medium light (ML, 18% irradiance) or low light (LL, 5% irradiance). In HL plants, Fv/Fm was below 0.8 (i.e. state of photoinhibition) throughout the whole experimental period from November to May, with the greatest decline in January and February (when Fv/Fm value reached 0.37). In ML seedlings, significant declines of Fv/Fm occurred in January (with the lowest level at 0.666), whereas the decline in LL seedlings (down to 0.750) was not significant. Full recovery of Fv/Fm in HL seedlings was delayed until the end of May, in contrast to ML and LL seedlings. Fv/Fm was significantly correlated with daily mean (T mean), maximal (T max) and minimal (T min) temperature and T min was consistently the best predictor of Fv/Fm in HL and ML needles. Temperature averages obtained over 3 or 5 days prior to measurement were better predictors of Fv/Fm than 1- or 30-day averages. Thus our results indicate a strong light-dependent seasonal photoinhibition in needles of T. baccata as well as suggest a coupling of Fv/Fm to cumulative temperature from several preceding days. The dependence of sustained winter photoinhibition on light level to which the plants are acclimated was further demonstrated when plants from the three light environments were exposed to full daylight over single days in December, February and April and Fv/Fm was followed throughout the day to determine residual sensitivity of electron transport to ambient irradiance. In February, the treatment revealed a considerable midday increase in photoinhibition in ML plants, much less in HL (already downregulated) and none in LL plants. This suggested a greater capacity for photosynthetic utilization of electrons in LL plants and a readiness for rapid induction of photoinhibition in ML plants. Further differences between plants acclimated to contrasting light regimes were revealed during springtime de-acclimation, when short term regeneration dynamics of Fv/Fm and the relaxation of nonphotochemical quenching (NPQ) indicated a stronger persistent thermal mechanism for energy dissipation in HL plants. The ability of Taxus baccata to sustain winter photoinhibition from autumn until late spring can be beneficial for protection against an excessive light occurring together with frosts but may also restrict photosynthetic carbon gain by this shade-tolerant species when growing in well illuminated sites. and P. Robakowski, T. Wyka.
Zinc (Zn) plays an important role in the physiological processes in plants and may mitigate trace element stress. The aim was to evaluate the morphophysiological responses of Alternanthera tenella plants exposed to cadmium (Cd) and Zn under in vitro conditions. Segments of A. tenella were transferred to flasks containing medium supplemented with different combinations of Cd (0, 75, or 150 μM) and Zn (0, 750, or 1,500 μM) concentrations, totalizing nine treatments. We assessed the growth traits, anatomy, chlorophyll a fluorescence by OJIPs, and tolerance index (TI). With exposure only to Cd, the plants showed physiological disorders. Zn supplementation in the medium had a positive effect on the physiological performance of plants. At concentrations ≤ 750 μM, it can partially mitigate the deleterious effects of Cd. Plants grown with Cd and Zn showed intermediate TI. The results proved the potential of Zn as a mitigator of Cd-induced stress in plants.