Dried seeds of plants carried by recoverable satellites to near-earth orbit are subjected to the effects of manifold space mutagens, such as microgravity, cosmic radiation, high vacuum, etc., and variations emerge among the progenies of plants grown from the seeds. So far, the experiments in this field in China and abroad mainly concentrated on morphological effects, physiological and biochemical mechanisms of induced variations, and selection of mutants, while reports about changes of plant chloroplast ultrastructure and photosynthetic pigments were very few, and no information about maize (Zea mays L.) was found. The objective of this experiment was to study changes of chloroplast ultrastructure and photosynthetic pigment contents of maize leaves after space flight, and try to elucidate the cytological and cytogenetical bases of the changes.
Plant materials and satellite carrying treatments. Dried seeds of two maize inbred lines, YiZi24 and Yi141, were divided into two parts, one of which was used as ground control, the other was carried by the Recoverable Scientific Exploration and Technological Experimentation Satellite of China, launched on October 20, 1996 and returned to earth after 15 days. The satellite conditions were: flight altitude, Perigee 175±5 km, Apogee 235±5 km; period around the earth, 90 min, total time of space flight, 360 h; temperature in the biocabin, 10~20 C; vacuum degree, 10-3~10-6 Pa; gravity level, 10-3~10-5 G; total dose of radiation (15 d), 1.92 mGy.
Effects of space flight on photosynthetic pigment contents in maize leaves. Determination of absorption spectra of acetone extracts of maize leaves revealed that the absorption curves of space-flight treated were quite similar to those of their corresponding ground controls at the same time of sampling, only that the absorbance values of the former were slightly lower near 663 nm and 645 nm (maximum absorption peaks of chlorophyll a and b, respectively). This showed that their light-absorption characteristics didnít change after space flight.
After space flight, the contents of chlorophyll a and chlorophyll b were significantly reduced, and the relative reduction range of chloroplayll b far exceeded that of chlorophyll a. The result suggested that the former was more sensitive to space mutagens. As shown in Table 1 (A, B), there was a substantial decrease of chlorophyll (a+b) contents after space flight that lowered the total amount of their photosynthetic pigments (Ca+b+k), except for that of YiZi24 60 days after sowing for its carotenoid contents were far higher than those of ground control.
Effects of space flight on chloroplast ultrastructure of maize leaves Electron micrographs of chloroplasts from ground controls showed the shape of a typical convex lens composed of alternately arranged grana and stroma lamellae. The double outer envelopes and intima systems were observed clearly. The thylakoid membranes in grana and stromata were rich in number and arranged in an orderly fashion. Each granum disc was stacked with several to more than twenty thylakoids which made the section of granum discs approximating a rectangular shape. There were plenty of ribosomes and some osmiophilic globules in the stromata. The fact that the chloroplasts had integrated structure, and rich and clear photosynthetic membrane systems, indicate that they possess a powerful photosynthetic capacity.
After space flight, a series of chloroplast configuration changes, such as strip shape, circular shape, and irregular amoeba shape, etc, were often observed. The main changes in the internal structure of chloroplasts were incompletely developed systems of grana and stroma lamellae, a marked decrease in the number of grana per chloroplast and thylakoids per granum, a notch or notches on the envelope membrane, and blurring of granum lamellae. Twisted and disorderly arranged stroma lamellae were often arranged in a radiating fashion with centers of obscure granum discs , or arranged in a parallel manner without stacking into granum discs. Swollen thylakoids were seen, resulting in an increase of intrathylakoid space (see Fig. 1, Fig. 2).
The most obvious difference between bundle sheath chloroplasts of space-flown plants and ground control was that the former possessed a far greater number of starch grains. However, further studies are needed to make sure whether or not it was caused by microgravity as some authors postulated.
Figure 1. Chloroplasts of maize mesophyll cells from ground control, showing more granum discs and thylakoid membranes arranged in orderly fashion. (1) X 30000 X 0.4; (2) X 49500 X 0.4; (3) ~ (6) Chloroplasts of maize mesophyll cells after space flight. (3) Showing changes of chloroplast contours X 7500 X 0.4; (4) Showing grana lamellae decreased in number, X 9900 X 0.4; (5) Showing changes of chloroplast contours, X 5000 X 0.4; (6) Showing notch of chloroplast envelope.
2. Chloroplasts of maize mesophyll cells after space flight. (7), (8)
Showing thylakoids arranged in a radiating fashion, with centers of obscure
grana discs, (7) X 24000 X 0.4, (8) X 49500 X 0.4; (9) Showing thylakoids
arranged parallelly, no grana discs, X 49500 X 0.4; (10) Showing swelling
of thylakoids and notch of envelope membrane (arrow), X 49500 X 0.4; (11)
of corn bundle sheath cells from ground control, X 13000 X 0.4; (12) cells
of corn bundle sheath cells after space flight, showing more starch.
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