Photosynthetic adaptations in grasses to abnormal levels of soil cations

Abstract

This research investigated adaptation of photosynthesis manifest in populations of grasses adapted to growth on abnormal
levels of cations. The main investigative plants belong to the Agrostis genus. Plants were sampled from habitats with soi.ls
which provided a range of abnormal levels of cations; mine spoil sites with heavy metal contaminations, calcareous and acidic sites with contrasting levels of cations particularly calcium and serpentine sites with low calcium and high magnesium and nickel levels were sampled.
Barley and lettuce were used as comparative material as they are more routinely used in photosynthesis investigations. A
commercially available seed stock of a heavy metal tolerant cultivar of Festuca rubra was compared with a non—tolerant
cultivar.
The comparative responses of plants to metals were examined using a range of monitors of photosynthetic reactions including fluorescence, oxygen evolution and light—induced electron transport.
Techniques involving the measurement of steady state emission fluorescence spectra of excised leaves were standardised and applied. The soil conditiens were shovn to have a marked effect on the flucrescence spectra of leaves of barley plants grown in a rccge of soil typcs.
The assay of light—induced oxygen evolution from excised leaf segments of the Merlin cultivar of Festuca clearly showed that increased resistance to metal inhibition is also present in the leaves as well as in the roots. The increased tolerance could be due to modifications of the cell wall or cytoplasm as has been extensively studied in the root. However a greater relative metal tolerance was also demonstrated in this cultivar at the level of the chioroplast membranes.
Isolated, broken (type D) chloroplasts from ten Agrostis ecotypes showed a variation in their sensitivities to metals in the light—induced photosynthetic electron transport through photosystem 2. The chloroplasts from the mine spoil Agrostis races were more resistant to inhibition by the heavy metals (Pb, Zn, Cu, Ni, Cd) in general, although the degree of tolerance did
not always relate directly to the root toiernr.ce indices or exchangeable soil metal content.
A co—sensitivity between calcium and cadmium was revealed in the roots and in the chloroplasts which is probably related to the similarity in the two cations ionic radii. Plants from high calcium soils showed an increased tolerance to cadmium.
The results recorded from work on chlorcplasts isolated from lettuce, barley and Agrostis ecotypes showed that Ca" can have fundamental effects on photosynthesis, both stimulatory and inhibitory. Ca 2+ was shown to stimulate the light—induced photosynthetic electron transport rates through PS2 and to protect isolated chloroplasts from loss of acti'Qity with ageing. Hcwever the degree and nature of the Ca2+ effect was dependent on plant species, ecotype, growing conditions and experimental protocol of CaZ+ addition. 'A marked inhibition with incubation in Ca' 4 after IL extraction was measured in four Agrostis races which originated from soils with low exchangeable calcium levels. However not all low soil_CaZ+ Azrostis ecot ypes were inhibited and this was interpreted to indicate a division between distributional calciftges and physiological calciophcbes.. SE! PAGE profiles of chloroplasts from within Agrostis and Festuca were compared for differences in the band regions reported in the literature to be associated with metal tolerance and Ca2+ binding in other species. Variations in the levels of proteins in the 30 and 36 kDa ranges have been obsened in the Pb— and Znresistant Festuca. In the Agrostis, Ca 2 /Cd'' sensitivity in three races is co—incidental with reduction in a band La the region of 30-33 kDa. A loss or gain of a specific band could not be associated with heavy metal tolerance in the Agrostis chloroplasts.
It is therefore clear from this work that photosynthetic activity of isolated chloroplasts depends on (a) the heavy metalard Ca2+ content of the soils at the collection site (b) the Ca2+ level of the growth medium (c) the level of Ca 2 in the extraction
buffer. The variable ebloroplast activity is probably a reflection of genetic adaptations of the plants to variable growth
conditions. This is reflected in variations in proteins in the chloroplasts.