Photoinhibition, carotenoid composition and the co-regulation of photochemical and non-photochemical quenching in neotropical savanna trees.

Plants in the neotropical savannas of central Brazil are exposed to high irradiances, high air temperatures and low relative humidities. These conditions impose a selection pressure on plants for strong stomatal regulation of transpiration to maintain water balance. Diurnal adjustments of non-photochemical energy dissipation in photosystem II (PSII) provide a dynamic mechanism to reduce the risk of photoinhibitory damage during the middle of the day when irradiances and leaf temperatures are high and partial closure of the stomata results in considerable reductions in internal CO(2) concentration. At the end of the dry season, we measured diurnal changes in gas exchange, chlorophyll fluorescence parameters and carotenoid composition in two savanna tree species differing in photosynthetic capacity and in the duration and extent of the midday depression of photosynthesis. Non-photochemical quenching and its quantum yield were tightly correlated with zeaxanthin concentrations on a total chlorophyll basis, indicating that the reversible de-epoxidation of violaxanthin to antheraxanthin and zeaxanthin within the xanthophyll cycle plays a key role in the regulation of thermal energy dissipation. In both cases, a single linear relationship fitted both species. Although efficient regulation of photochemical and non-photochemical quenching and adjustments in the partitioning of electron flow between assimilative and non-assimilative processes were operating, these trees could not fully cope with the rapid increase in irradiance after sunrise, suggesting high vulnerability to photoinhibitory damage in the morning. However, both species were able to recover quickly. The effects of photoinhibitory quenching were largely reversed by midday, and zeaxanthin rapidly converted back to violaxanthin as irradiance decreased in late afternoon, resulting in the maximal quantum yield of PSII of around 0.8 just before sunrise.

Constraints to seedling success of savanna and forest trees across the savanna-forest boundary.

Tropical savannas and closed forests are characterized by distinct tree communities, with most species occurring almost exclusively in only one of the two environments. The ecology of these two groups of species will largely determine the structure and dynamics of the savanna-forest boundary, but little is known about the ecological and physiological differences that might control their distributions. We performed field and nursery experiments to compare seedling establishment success, predawn leaf water potential, biomass allocation, and root carbohydrate concentration of congeneric species, each composed of one savanna species and one forest species. Seedling establishment of savanna and forest species responded differently to vegetation cover, with forest species having lowest establishment success in the open savanna and savanna species having lowest success in forest. Subsequent survival followed similar patterns, resulting in even greater differences in cumulative success. The low survival of forest species in the savanna appears related to drought stress, as seedlings of forest species had lower predawn leaf water potential than savanna species. Seedlings of savanna species had greater root: shoot ratios and root total nonstructural carbohydrate (TNC) concentration, particularly among evergreen genera. Among evergreen genera, root TNC per shoot mass, which may largely determine resprout capacity, was seven times higher in savanna species than forest species. Although water availability and microclimate may reduce the success of forest species, these factors appear unable to completely exclude forest seedling establishment in savanna. Fire, on the other hand, appears to be a much more absolute constraint to success of forest species in savanna.

Seasonal dynamics of non-structural carbohydrates in bulbs and shoots of the geophyte Galanthus nivalis.

Seasonal dynamics of non-structural carbohydrates were studied in Galanthus nivalis L. over a 2-year period. The plants were collected in the field and separated into above- and below-ground biomass. The polysaccharide fraction of the bulbs consisted of fructans and starch. Seasonal variations suggest that the polysaccharides were utilized for carbon and energy supply for re-growth and flower development. With the re-sprouting of the bulbs in autumn the fructans within the bulbs were depolymerized and an increase of low degree of polymerization fructans as well as sucrose was observable. Within shoots the major polysaccharides were fructans, the starch content was much lower. Gas liquid chromatography and high-performance, anion-exchange chromatographyanalysis of the fructan fraction revealed that the fructans within the shoots were predominantly those with a low degree of polymerization. In addition to the two polysaccharides the other dominant sugar in shoots was sucrose. During the period of slow re-growth and flowering, fructan and starch pools were depleted to different degrees. Calculation of the difference between the carbohydrate content at the start of visible growth and at the time of lowest content revealed that the starch pool showed a higher depletion than the fructan pool. During the re-growth periods in 1996/97 and 1997/98 fructans were catabolized by 39 and 32% only, whereas the starch pool was depleted by 92% (1996/97) and 79% (1997/98), respectively. During rapid shoot growth and fruiting, the bulbs and above-ground organs appeared to be competing sinks for the photosynthetically fixed carbon. Refilling of the bulbs carbohydrate reserve started in February/March In shoots, the period of refilling the bulbs was characterized by a low content of oligosaccarides and a high content of hexoses.

Occurrence of the lutein-epoxide cycle in mistletoes of the Loranthaceae and Viscaceae.

The lutein-epoxide cycle (Lx cycle) is an auxiliary xanthophyll cycle known to operate only in some higher-plant species. It occurs in parallel with the common violaxanthin cycle (V cycle) and involves the same epoxidation and de-epoxidation reactions as in the V cycle. In this study, the occurrence of the Lx cycle was investigated in the two major families of mistletoe, the Loranthaceae and the Viscaceae. In an attempt to find the limiting factor(s) for the occurrence of the Lx cycle, pigment profiles of mistletoes with and without the Lx cycle were compared. The availability of lutein as a substrate for the zeaxanthin epoxidase appeared not to be critical. This was supported by the absence of the Lx cycle in the transgenic Arabidopsis plant lutOE, in which synthesis of lutein was increased at the expense of V by overexpression of epsilon-cyclase, a key enzyme for lutein synthesis. Furthermore, analysis of pigment distribution within the mistletoe thylakoids excluded the possibility of different localizations for the Lx- and V-cycle pigments. From these findings, together with previous reports on the substrate specificity of the two enzymes in the V cycle, we propose that mutation to zeaxanthin epoxidase could have resulted in altered regulation and/or substrate specificity of the enzyme that gave rise to the parallel operation of two xanthophyll cycles in some plants. The distribution pattern of Lx in the mistletoe phylogeny inferred from 18S rRNA gene sequences also suggested that the occurrence of the Lx cycle is determined genetically. Possible molecular evolutionary processes that may have led to the operation of the Lx cycle in some mistletoes are discussed.

Sprouting of the fructan- and starch-storing geophyte Lachenalia minima: Effects on carbohydrate and water content within the bulbs.

The carbohydrate pool within the bulbs of Lachenalia minima W.F. Barker (Hyacinthaceae) consists of similar amounts of fructans and starch. This study was conducted to examine the changes within the pool of non-structural carbohydrates that occur during sprouting under field conditions. The bulbs were watered over a period of 23 days to simulate the onset of the rainy season. Even though there was no significant change of the total fructan content, the distribution and the composition of the fructan fraction within the different leaf scales of the bulbs altered during sprouting. The major changes occurred in the innermost scales, the total fructan content increased from 300 (day 0) to 607 (day 23) g kg-1 dry mass and high-performance anion-exchange chromatography analysis revealed a significant increase of fructans with low degree of polymerization (DP). With respect to starch, the most pronounced difference accompanying the transition to growth was also in the innermost scales. In contrast to fructans, starch content decreased from 241 (day 0) to 60 (day 14) g kg-1 dry mass. These results demonstrate that starch, and not fructan, is used as the carbon and energy source for sprouting. The water content data suggest the involvement of fructans in water relations. The preferential accumulation of low DP fructans and sucrose within the innermost scales directs the water flow to where it is most needed for growth. Similar changes were obtained for bulbs in the dry soil, but transformation rates were much slower and occurred to a lesser extent, indicating that these reactions were not triggered but were accelerated by water.