Restorative processes in perennial plants growing in the

It is difficult to assess the impact of the radiation on perennial plants in the exclusion zone, as they were subjected to a period of intense exposure in the spring of 1986, followed

by less intense chronic irradiation, and also because their radiosensitivity changed during their annual development cycle. Interpreting the results is also complicated by the fact that during irradiation, restorative processes take place along with development of the radiation effects.

After the period of severe irradiation, restorative processes in perennial tree and bush species were observed for 1–1.5 years. The intensity of such processes as the formation of roots, shoots and young growth, as well as physiological reactions of a compensatory nature, were used as criteria of the restoration process.

Perennial pine trees do not tend to form young growth. A pine crown has powerful restorative potential of a vegetative nature, having a large number of dormant buds which are protected all the year round by coniferous branches. Observations during the post-accident period 1986–1992 showed that, if a pine tree had received a dose greater than 14 Gy (a sublethal dose for a pine tree during the intense spring irradiation period), restoration could only occur in the form of the additional formation of shoots from previously dormant buds.

This process began in 1987, a year after acute irradiation started, and was manifest by the appearance in the highest part of the crown of individual shoots in bunches on a very long branch situated on a short, thick growth centre. The role of these shoots is thought to be the provision of the minimum level of photoassimilation required to support the vital functions of the tree's aggregating organs and tissue (the roots, wood of the trunk and branches). This is confirmed by the excessively high intensity of photosynthesis in such coniferous branch shoots, three to four times higher than normal, suggesting the compensatory nature of this process. With each successive year, the number of such shoots in the crown of the tree has increased, indicating the possibility of the gradual restoration of the initial functions of a perennial plant. Starting in the 1988 growing season, in the zone of severe and moderate damage to the forest, intensive restoration occurred in older trees, although the previous year's growth of shoots was higher in the upper whorls than in the lower ones. The survival rate of severely contaminated trees depends on the previous year's growth of shoots, and if the growth is about 20% of the normal assimilation at the observed rates of growth, two to three years are required for the restoration of such trees. Survival is therefore possible for individual trees which currently have 5–10% of the usual number of shoots on green branches.

Changes in tree height are a post-radiation reaction to the death of the axially topmost shoot; its role is taken over by one of the side shoots in the whorl, usually the strongest one.

This effect was noted in approximately 30% of pine trees (Pinus excelsa) which had received doses of 11–15 Gy. Data on the dynamics of compensatory and reproductive generation at different levels of radiation doses also suggest the possibility of the restoration of perennial plants in the exclusion zone which have exhibited new growth. Such indices as the healing of damage to branches and of saw cuts on branches of a certain diameter, and the amount of secondary growth on perennial plants after removing the topmost buds, were used as criteria of compensatory regeneration. The intensity of rhizogenesis in black currant (Ribes nigrum) and holly ivy (Salix acutifolia) cuttings was used as a criterion of the occurrence of reproductive regeneration in plants sprouting in soils with different levels of radioactive contamination. The results obtained show, at the dose levels observed in the exclusion zone, a real increase in the intensity of all types of regenerative processes in perennial plants.

The short lived beta emitters were the main contributors to the absorbed dose, and the major radiobiological effects appeared in contaminated plantations of ordinary pines and fir trees. However, in the first year, the maximum absorbed doses also had an effect on deciduous birch, alder and aspen, with their subsequent full recovery.

The number of buds, the growth of phytomass of the crown and the state of development of seeds can be used as indicators for evaluating damage to trees. Minor damage was found in terms of these indicators when the cumulative dose in the period of severe irradiation was up to 0.5 Gy. No visible signs of damage to the trees were observed. In some cases, the effects of the radiostimulation of growth appeared, but these were not of a clearly defined character. The situation returned to normal within the first year, and the state of the plantations could not be distinguished from that of the tree stock which had not been irradiated (Table XXVI).

In the 'zone of minor damage to the forest', a slowing down in the growth of shoots and branches and morphological changes in the vegetative organs were noted during the first year after the accident. The effect of the radiation was observed for two years but in the following year, the growth of the trees returned to normal. In the moderately contaminated tree stocks, the cumulative absorbed dose amounted to 5–10 Gy in the needles and 0.5–5 Gy in the apical meristem. A significant depression in the growth of shoots and branches was noted, and also a decrease in root size, stunting of the crown, multiple morphological changes and the death of parts of trees, predominantly those parts with low growth rates (Table XXVII).

TABLE XXVI. THE REACTION OF 30-YEAR-OLD PINE TREES TO DIFFERENT LEVELS OF RADIATION

Radiation levels Number of buds on average tree

Growth of phytomass of crown of single tree (%)

1986 1994 1986 1987 1994

Severe <5 55–75 <5 10–25 40–60 Moderate 5–25 >200 5–30 25–50 100 Minor 25–75 >200 30–70 50–95 100 No signs of

radiation damage >75 >200 >70 > 95 100

TABLE XXVII. THE RELATIVE SIZES OF BRANCHES IN TREE STOCKS AT DIFFERENT LEVELS OF RADIATION

Radiation levels Sizes of axial shoots, as % of size before irradiation of branches

1986 1987 1994

Severe 66 182 115

Moderate 65 158 100

Minor 91 142 99

No signs of radiation

damage 95 97 99

At the moment, no morphological signs of the contamination of the stocks of pine trees are to be seen over the entire zone. Different signs of damage to the growth of severely contaminated tree stocks have been caused by forestry management, rather than by the effects of radiation.

Damage to the generative organs of pines was noted. In the first year, damage in the structure of meristematic tissue was observed. The repair processes in these plantations took up to three years. The irradiation accelerated the processes of differentiation of trees in the tree stock.

Ionizing radiation has had a much greater effect on reproductive capability. The evaluation of the reproductive capability of trees subjected to different levels of radiation has shown that, when there is ¹³⁷Cs contamination of land of 185–3700 kBq.m^-2 (5–100 Ci.km^-2), no significant drop in fertility is observed. In tree stocks where contamination exceeds this level, a sharp decrease in the production of live seeds occurs, and when ¹³⁷Cs exceeds 18 500 kBq.m^-2 (500 Ci.km^-2), ordinary pine trees lose their ability to reproduce for two to three years. Five years after the accident, in the trees in plantations with a moderate cumulative dose (6–10 Gy), fertility had reached a normal level (Table XXVIII).

TABLE XXVIII. THE DYNAMICS OF LIVE SEEDS IN PLANTATIONS WITH A MODERATE ACCUMULATED DOSE (6–10 Gy.)*

Years 1987 1988 1989 1990 1991 1994

Average number of

seeds in a cone 2.1 4.9 14.2 17.1 16.3 15.2

* The standard average number of live seeds in a cone is 17.2.

In plantations where the absorbed dose exceeded 20 Gy, the death of trees occurred in a very short period. This led to a sudden invasion of vermin, which later spread to adjoining areas. These tree stocks have now ceased to exist. In their place, grassland has been formed, and deciduous species have begun self-seeding, so a new tree stock is being formed. To summarize, four main zones of damage to the coniferous forests of pine and fir trees can be differentiated (Table XXIX).

Short term changes in growth were also observed in a ‘zone of perceptible change’

(doses to the apical meristem of 0.1–0.5 Gy).

Currently, processes of renewal of natural biocenoses (secondary succession) are ongoing. In cases where the self-sowing of agricultural cultures appeared in the exclusion zone within three years, sufficiently stable associations of Arthropoda have formed.

Favourable conditions for the development and reproduction of useful entomological fauna, such as the presence of nectar-yielding plants and the absence of the use of toxic chemicals, has led to an increase in their regulatory role in natural and agricultural cenoses in the zone.

(a) Zone of complete forest death, with absorbed doses of 80–100 Gy in the needles and exceeding 20 Gy in the apical meristem.

(b) Zone of severe damage to the forest, with death of parts of trees, dessication of the needles, loss of buds, with typical absorbed doses of 30–80 Gy in the needles and 10–20 Gy in the apical meristem.

(c) Zone of moderate damage to the forest, with partial dessication of needles, decrease in growth, and morphological changes, with absorbed doses of around 20 Gy in the needles and 5–10 Gy in the apical meristem.

(d) Zone of minor damage to the forest with damage to reproductive organs of the trees, and morphological changes, with absorbed doses around 5–10 Gy in the needles and 0.5–5 Gy in the apical meristem.

TABLE XXIX. THE DYNAMICS OF THE CONDITIONS OF THE PINE FORESTS WITHIN THE EXCLUSION ZONE OF THE CHERNOBYL NPP IN 1986–1994

Level of impact,

An extensive chain of succession processes constantly takes place in the vegetable world. Anthropogenic factors such as radioactive pollution can intensify these processes. One element which plays an important part in these territories is the interruption of land cultivation. Species of flora which disappear are replaced by species better adapted to the new ecological conditions. For example, dicotyledonous plants are replaced by cereals. Generally speaking, every ecological niche is filled and the ecosystem acquires a new level of equilibrium.

The main diseases in cereals growing wild in the exclusion zone are helminthosporiosis (striated and reticular leaf spot), black spot and stem (linear) rust. In conditions where widespread rehabilitation is under way in the exclusion zone, any alternariosis which has appeared on wild Cruciferae can cause a significant drop in the harvesting of seeds from cultivated Cruciferae, especially cabbage.

Monitoring of the different vegetable populations shows that numerous plants have in many ways appeared resistant to the effects of radiation. Most representatives of the vegetable world located in the exclusion zone were able to survive, due to the activation of regenerating processes.