Responses of chlorophyll b-less chlorina 3613 mutant of barley to a prolonged decrease in illuminance: 2. Dynamics of carotenoids in leaf chloroplasts

Barley (Hordeum vulgare L.) mutant chlorina 3613 is notable for a lack of chlorophyll b (Chl b), low content of chlorophyll a (Chl a) and carotenoids in the chloroplasts, as well as reduction in the majority of components of LHCI and LHCII. Incompletely developed photosynthetic machinery of chlorina 3613 results in suppressed growth, lower biomass, and the declined rate of photosynthesis (as compared with the wild-type cv. Donaria). The lack of Chl b and greater part of peripheral antenna suggests that this mutant will have difficulties during acclimation to long-term shading because the light-harvesting role of Chl b-containing antenna becomes more important under the shortage of light. Earlier, our experiments with the mature chlorina 3613 plants shaded for one week at PAR photon flux density of 60 and 40% of that in full sunlight showed a stimulating effect of shading on growth, biomass accumulation, and Chl a synthesis in chlorina 3613 when biosynthesis of Chl b did not occur [1]. In this work, we investigated in more detail the changes in the content of carotenoids in chlorina 3613. We found that in Donaria at both investigated levels of illumination (60 and 40% of full sunlight) and in chlorina 3613 at 60% illumination, moderate reversible changes typical of shade-enduring plants occur in the content of carotenoids. In chlorina 3613 at 40% illumination, the content of β-carotene increased considerably (by 3 times) with simultaneous accumulation of Chl a. When full illumination was restored, the content of β-carotene decreased and remained on the level, which exceeded its initial content in the plants without shading by 38%; this level, was maintained by the end of vegetation. The changes in the contents of β-carotene and Chl a in chlorina 3613 were not accompanied by any accumulation of xanthophylls or changes in the relative content of active violaxanthin. The obtained results suggest that a long-term shading of the leaves of mature chlorina 3613 plants induced the formation of certain components of photosynthetic apparatus: reactive centers and core parts of photosystems' antennae as well as proteins CP26 and CP29 and in this way contributed to partial restoration of photosynthetic activity and production process in the mutant lacking Chl b.