Comparison of photosynthesis recovery dynamics in floating leaves of Trapa natans after inhibition by manganese or molybdenum: effects on Photosystem II.

The aquatic plant Trapa natans L. is highly resistant to Mn and moderately resistant to Mo, mainly thanks to its ability to sequestrate the metals by chelation in the vacuole. Excess of Mn and Mo causes somewhat aspecific toxicity symptoms in plants, but the main target of their toxicity seems to be the photosynthetic process. In this work, we aimed at understanding how the effect on photosynthesis caused by Mn (130 μM, full recovery) or Mo (50 μM, partial recovery) in T. natans is linked to changes occurring in the photosynthetic apparatus, with emphasis on Photosystem II (PSII), during a 10 day treatment with these metals. The time-course of net photosynthesis, photosynthetic pigment content, amount of PSII and its peripheral antenna LHCII, and room-temperature fluorescence emission ratios F694/F680 and F700/(F685 + F695) showed that the early inhibiting effect of Mo and Mn (one day exposure) was essentially non-specific with respect to the metal, though more marked in Mo- than in Mn-treated plants. During the subsequent recovery phase, Mo still impaired PSII assembly and, consequently, photosynthesis could not reach the control values. Conversely, in Mn-treated plants the amount of PSII was fully re-established, as was photosynthesis, but the metal induced the accumulation of LHCII. The extent of inhibition and the effectiveness of photosynthesis recovery are proposed to reflect the different ability of T. natans to sequestrate safely excess Mn or Mo in vacuoles.