Anatomical alterations of Phaseolus vulgaris L. mature leaves irradiated with X‐rays

Abstract

The cultivation of higher plants in Space involves not only the development of new agro‐technologies for the design of ecologically closed Space greenhouses, but also understanding of the effects of Space factors on biological systems. Among Space factors, ionising radiation is one of the main constraints to the growth of organisms. In this paper, we analyse the effect of low‐LET radiation on leaf histology and cytology in Phaseolus vulgaris L. plants subjected to increasing doses of X‐rays (0.3, 10, 50, 100 Gy). Leaves irradiated at tissue maturity were compared with not‐irradiated controls. Semi‐thin sections of leaves were analysed through light and epi‐fluorescence microscopy. Digital image analysis was applied to quantify anatomical parameters, with a specific focus on the occurrence of signs of structural damage as well as alterations at subcellular level, such as the accumulation of phenolic compounds and chloroplast size. Results showed that even at high levels of radiation, general anatomical structure was not severely perturbed. Slight changes in mesophyll density and cell enlargement were detected at the highest level of radiation. However, at 100 Gy, higher levels of phenolic compounds accumulated along chloroplast membranes: this accompanied an increase in number of chloroplasts. The reduced content of chlorophylls at high levels of radiation was associated with reduced size of the chloroplasts. All data are discussed in terms of the possible role of cellular modifications in the maintenance of high radioresistance and photosynthetic efficiency.

Leaf Anatomy and Photochemical Behaviour ofSolanum lycopersicumL. Plants from Seeds Irradiated with Low-LET Ionising Radiation

“…Altered mesophyll, characterized by partly collapsed cells with irregular cell shape, was found only sporadically after the exposure of seeds to 50 and 100 Gy in the Microtom cultivar. Such modifications were less marked than those found in leaves of bean where the target organs for irradiation were the leaves themselves [30]; leaves directly exposed to X-rays in bean showed the formation of more loose parenchyma with larger cells, likely due to a radiation-induced damage to cell walls that reduced mechanical constraints to cell enlargement [30, 52]. Further investigations on cell wall composition would be useful to elucidate whether a possible alteration of cell walls was responsible for cell enlargement in the spongy parenchyma and epidermis of leaves from seeds of “Microtom” irradiated with the highest dose of X-rays.…”

“…Some plants possess natural screens against radiation when their tissues contain phenolic compounds with antioxidant functions and can counteract photoinhibitory processes [29, 57]. An increase of phenolic compounds linked to the chloroplast membranes has been recently found in Phaseolus vulgaris irradiated leaves exposed to high levels of X-rays [28, 30]. Changes in phenolic content after the exposure to Space can be attributed either to direct effects on the synthesis or mobilization of phenolic compounds or to indirect effects due to changes in ultrastructural organization, such as the increase in the number of chloroplasts per cell [30, 58].…”

“…An increase of phenolic compounds linked to the chloroplast membranes has been recently found in Phaseolus vulgaris irradiated leaves exposed to high levels of X-rays [28, 30]. Changes in phenolic content after the exposure to Space can be attributed either to direct effects on the synthesis or mobilization of phenolic compounds or to indirect effects due to changes in ultrastructural organization, such as the increase in the number of chloroplasts per cell [30, 58]. The lowering of phenolic content at high levels of X-rays in “Microtom” leaves is not in agreement with general trends, indicating enhanced phenolics production in plants grown in Space [58] or in various plant tissues directly irradiated with gamma rays [59].…”

“…A set of three Petri dishes with 15 seeds each was used for each irradiation treatment and for a nonirradiated control. Doses up to 20 Gy were chosen in order to build a reference-response range to explore plant sensitivity, as generally set in experiments to evaluate the effect of radiation on biological systems [13, 14, 28, 30]. The highest doses (50 Gy and 100 Gy) are generally applied since they can be considered as positive controls, as there is more likely a plant reaction at these doses.…”

“…Photoprotection mechanisms can rely on physiological adjustments [14, 28] and/or on the intensification of structural and biochemical barriers such as trichomes and phenolic compounds acting as natural screens against radiation [29]. Indeed, the increased content of phenolic compounds linked to chloroplast membranes has been shown in bean leaves irradiated with high doses of X-rays [30]. …”