The influence of excess irradiance on resistance of wheat (Triticum aestivum L.) photosynthetic apparatus to heating in darkness and in the light was investigated and compared with changes in leaf cell ultra-structure and composition of cell lipids and fatty acids. The leaves of 14- to 16-day-old plants grown at low irradiance (about 20 W/m2) were exposed for 1 h to irradiance of 370 or 600 W/m2 PAR. Using infrared gas analysis, we found that the preexposure of leaves to excess irradiation elevated resistance of apparent photosynthesis to 10-min heat treatment at 40–45°C. The rate of Hill reaction (reduction of 2,6-dichlorophenolindophenol by isolated chloroplasts) was higher for leaves heated at high irradiance than for leaves heated in darkness. During illumination of leaves with strong light, mesophyll cells became more abundant in mitochondria and peroxysomes, as well as in cisternae of endoplasmic reticulum and Golgi complex. The chloroplast thylakoids and grana became more extensive and numerous. At the same time, the leaf content of main classes of membrane glycerolipids increased in parallel with the increase in the phospholipid/glycolipid and lipid/chlorophyll ratios. The unsaturation index of fatty acids of membrane lipids increased because of the elevated content of linolenic acid. Thus, excessive light (not fully utilized in photosynthesis) induced in wheat leaves a series of nonspecific adaptive changes that were similar to those occurring under the action of other environmental factors, such as heat shock, cooling, salinity, and osmotic stresses.

We conducted a detail study of the photosynthetic apparatus in assimilating organs of three introduced evergreen conifer species: Taxus cuspidate S. et Z. ex E. (Far-Eastern yew), Thuja occidentalis L. (arbovitae “green”), and Th. occidentalis f. “Reingold” (arbovitae “yellow”) at various times in their life cycle. We studied the potential photosynthesis rate; composition and ratios of pigments, including primary carotenoids; the violaxanthin cycle (VC) activity, the synthesis of a secondary carotenoid, rhodoxanthin; and chloroplast ultrastructure. In winter and spring, β-carotene and lutein (primary carotenoids) contents were relatively constant in yew and arbovitae “yellow”. In December, the VC in yew was balanced and in arbovitae “yellow” unbalanced. In arbovitae “yellow”, the zeaxanthin pool was heterogeneous, and only part of it took part in the VC. It can be assumed that the other part of the pool can be oxidized to form a secondary carotenoid, rhodoxanthin. This secondary carotenoid was also accumulated in arbovitae “green”; its synthesis took place during the season, when the photosynthesis rate of plants was the lowest, and a significant chloroplast reorganization occurred (the number of thylakoids in grana decreased and plastoglobules appeared). We suppose that rhodoxanthin forms a filter for the light under the conditions of high insolation in winter. Thus, the evergreen conifer plants studied, which are adapted to growing at high latitudes where temperature is low and insolation is high in winter and spring, have a system for protecting the photosynthetic apparatus against photodestruction. In the basis of this system, the primary and secondary carotenoids lie, whose content changes during the year.

Illumination of wheat ( Triticum aestivum L.) leaves during heat treatment produced either additional injury or protection of photosynthetic apparatus depending on irradiance and the heating dose. Furthermore, illumination of leaves during hyperthermia exerted differential impacts on thermal tolerances of photosynthesis and photosystem II-driven electron transport assessed from the reduction of 2,6-dichlorophenolindophenol (DCPIP). Measurements with infrared gas analyzer showed that mild heating of leaves in darkness (10 min at 38–40°C) had stronger inhibitory effect on CO 2 uptake than heating of leaves exposed to low and moderate complex irradiances (3–30 klx), as well as excessive irradiance (75–100 klx). When the leaves were heated at higher temperatures (42–44°C), the low and moderate irradiances had a protective action, while high-intensity light aggravated the inhibition of photosynthesis. Illumination of leaves with weak light during heat treatment mitigated the impairment of chloroplast ultrastructure, whereas irradiation with high-intensity light (100 klx) destroyed the sensitive population of chloroplasts. The heat-stimulated photoinhibition was stronger for leaf photosynthesis than for DCPIP reduction in chloroplasts isolated from heat-treated leaves. No correlation was observed between the extent of violaxanthin deepoxidation, zeaxanthin accumulation, and the protective effect of light on photosynthetic apparatus during heat treatments.

Preliminary heating of 15-16-day-old wheat (Triticum aestivum L.) plants for 3 h at 37–38°C (heat shock, HS) increased the tolerance of photosynthetic electron transport (determined as the reduction of 2,6-dichlorophenol indophenol by isolated chloroplasts) toward heating of leaves at 42–48°C in high light (100 klx). At the same time, HS did not affect the activity of the xanthophyll cycle reactions in the 30–48°C temperature range. HS exposure induced an increase in the thylakoid length, the number of grana, and the average number of thylakoids per granum. The volume of the thylakoid system increased 1.4-fold. Such indices as the total content of chlorophylls (a + b), the chlorophyll a/b ratio, as well as the contents of individual carotenoids, chloroplast membrane proteins, and the soluble leaf proteins remained unchanged. The de novo photosynthetic membrane formation was accompanied by the 1.5-fold increase in major chloroplast lipids. It was concluded that, in mature wheat chloroplasts, HS induced the formation of thylakoids characterized by a changed molecular structure and by increased lipid/protein and lipid/chlorophyll ratios.

Heating of the leaves of 15-day-old wheat (Triticum aestivum L.) plants at 42°C in the light (370 W/m2 PAR) suppressed their ability to fix CO2 twice stronger than heating in darkness. Heat hardening (3 h at 38–39°C) improved the tolerance of photosynthesis to combined action of high light and temperature but did not affect the tolerance to photoinhibition at 30°C. Hardening did not induce changes in the levels of photosynthetic pigments and their ratios. De-epoxidation of violaxanthin turned out to be more tolerant to photoinhibition at 42°C than CO2 fixation. Protective effect of hardening was not related to the accumulation of zeaxanthin and activation of the xanthophyll cycle. Hardening protected the most sensitive population of chloroplasts against heat-induced photodamage and simultaneously increased the number and length of thylakoids. An increase in the volume of the thylakoid system was also induced by heating at 42°C and exposure to high light at 30°C. The formation of additional thylakoids and grana of shade type was not associated with improved tolerance of photosynthesis to heat and light stresses.

A 3 hours heating at 39 degrees C of 14-day old wheat plants increases the termotolerance of photosynthesis, and also the length and number of thylakoids in chloroplast in mature leaves. The acquired termotolerance disappears within 10 days. Simultaneously the intensity of photosynthesis and the length of thylakoids decrease. Reduction of photosynthesis ability and of thylakoid membranes occurs in the first leaves of non-hardened plants during 14-29 days after sowing. The intensity of photosynthesis in plants of both variants positively correlates with the length of grana membranes and with the total length of membranes of all thylakoids. Besides, a positive correlation was detected between the intensity of photosynthesis and the share of small (2-7 thylakoids) grana and the length of their membranes in non-hardened plants. The level of thermotolerance of photosynthesis in leaves in heat hardened plants correlates positively with the length of grana membranes and with the total length of all thylakoid membranes and the share of small grana.

The plants of winter wheat (Triticum aestivum L.) grown to the three leaf stage at 18/14 degrees C (day/night) were exposed to a temperature of 38 degrees C for 3 h. Such a treatment elevated the thermotolerance of potential photosynthesis measured at a saturating CO2 concentration and saturating light intensity, but had no effect on the rate of photosynthesis under optimal temperatures. Morphometric analysis of the median cross-sections of chloroplasts showed an increase in the total length of thylakoids and in the number and size of grana in mature leaves during heating. The number and length of granal thylakoids in heat-treated leaves increased by a factor of almost 1.5, whereas the length of intergranal thylakoids increased twofold; thus, the ratio of the length of appressed to nonappressed membranes decreased. The chlorophyll (a + b) content, calculated on the basis of both leaf area and dry weight, decreased 15% after heating, but the chlorophyll alb ratio remained unchanged. These data indicate that heat shock provokes the active formation of photosynthetic membranes in mature chloroplasts and that the composition of newly formed membranes is modified.

The recovery of photosynthesis and chloroplast ultrastructure after heating the shoots of intact spring wheat (Triticum aestivum L.) plants for 10 min at 40 and 42.5 degrees C was studied. Heating resulted in substantial depression of the rates of potential photosynthesis and the Hill reaction acid induced some changes in the arrangement of the thylakoid system concerning the orientation and shape of grana, the ratio of small to large grana, and the value of the stacking index. This index was estimated as the ratio of the total lengths of appressed to nonappressed (end granal and stromal) thylakoid membranes. The total length of photosynthetic membranes on chloroplast sections was not changed after plant heating. All characteristics were monitored over seven days after returning plants to he original growth conditions, After heating at 40 degrees C. photosynthetic activity and some indices of chloroplast ultrastructure completely restored, but the average length of thylakoid membranes was reduced, and the stacking index did not return to its initial value. The recovery of photosynthetic capacity and the strucutre of the thylakoid system in the chloroplasts was less evident after heating at 42.5 degrees C. The chlorophyll content in the leaves just after heating and during the subsequent days was similar to that in untreated plants.

The chloroplast structure in mesophyll cells of ii plant species inhabiting both arctic and boreal zones was compared. The chloroplasts in arctic plants are characterized by (I)a lower thylakoid number in the grana; (2) a lower ratio of the length of appressed thylakoid membranes in the grana to the length of nonappressed ones; and (3) a lower ratio of the volume fraction of the thylakoid system to that of the stroma. The chloroplasts of northern plants have a structure of the "light" type. The "light" chloroplast structure of arctic plants is suggested to be a result of the adaptation of the photosynthetic apparatus to low temperatures.

The thermotolerance of potential photosynthetic activity (measured by (CO2)-C-14 incorporation), cytoplasmic streaming, and leaf cell ultrastructure was monitored following the heating of intact wheat (Triticum aestivum L.) shoots at 40 or 42.5 degrees C for 10 min. Heating at 40 degrees C inhibited photosynthetic activity by 71%, but enhanced the thermotolerance of photosynthesis and cytoplasmic streaming, as was later revealed by the additional heating of excised leaves. Visible damage to cell ultrastructure was not observed after 40 degrees C heating, although sparse heat-shock granules appeared in 2% of the chloroplasts. After one day, these granules disappeared. After two days, the thermotolerance of photosynthesis declined, and its activity was restored to the original level. Heating at 42.5 degrees C resulted in the prevention of cytoplasmic streaming, almost complete inhibition of photosynthetic activity, and prominent destructive changes in fine subcellular structure. Abundant heat-shock granules were produced in the cytoplasm, chloroplasts, and mitochondria of all leaf tissues. Within two days after the heat shock, the thermotolerance of photosynthesis and cytoplasmic streaming was restored to that in untreated leaves. The time required for granule disappearance depended on their location inside the cell. Heat-shock granule accumulation in wheat leaves was correlated with some disturbances appearing in the cell ultrastructural organization. It is suggested that heat-shock granule accumulation was not directly related to the acquisition of thermotolerance by cells and the recovery of heat-induced disturbances in physiological processes and fine subcellular structure.

Biologia plantarum, an international journal for experimental botany founded in 1959 by Professor Bohumil Němec. Covers all branches of experimental botany ranging from molecular biology and biotechnology to whole-plant and stand functioning.

Heating strawberry plants of the Alexandria cultivar at 42-degrees-C for a period of 3 h leads to increase of the photosynthetic rate, raises the primary thermostability of photosynthesis, and stimulates the ability to repair heat injury caused by heating at 49-degrees for 30 min. Following return of the plants to normal temperature conditions (18/14-degrees, day/night), the primary thermostability of photosynthesis and photosynthetic rate declined to the starting level in approximately 12 days. However, elevated reparability was preserved throughout this period. After 3-h heating at 42-degrees, heat shock granules (HSG) were detected in the cytoplasm, chloroplasts, and mitochondria of mesophyll cells, and perichromatin granules were detected in the nucleoplasm. Heat shock granules were lacking in the cells at 24 h after hardening heating, when thermostability was highest. Stronger beating (49-degrees for a period of 30 min) caused appearance of HSG only in solitary cells.

The numbers of chloroplasts and mitochondria in the mesophyll cells of the plants studied were shown to increase on moving northward. The same pattern was observed for alpine plants with increasing altitude. An increased number of organelles in the mesophyll cells is considered to be one of the most important adaptive traits that allows cold-climate plants to maintain quite high photosynthesis and respiration rates at low temperatures.