Comparative Studies on Effects of Simulated Microgravity on Growth and Photosynthetic Parameters in Rice and Mungbean

Comparative Studies on Effects of Simulated Microgravity on Growth and Photosynthetic Parameters in Rice and Mungbean

Abstract

Microgravity or low gravity environment of space can be simulated on Earth by using clinorotation with appropriate speed of rotation. Previous reports show that microgravity environment (real or simulated) affects growth, chlorophyll content and photosynthetic performance in different plants. However, most of the reports showed variation in results. Earlier studies carried out on simulated microgravity using clinorotation in our laboratory showed increase in growth, chlorophyll content and various yield attributes in case of laboratory and field grown rice. Subsequent studies were carried out on mungbean seedlings. In the present work, comparative aspects of these studies are reported. Results of the present study showed increase in growth, chlorophyll content and chlorophyll fluorescence parameters in simulated microgravity samples as compared to control samples in rice and mungbean seedlings. Interestingly, percentage of enhancement in growth, chlorophyll content and chlorophyll fluorescence parameters of simulated microgravity samples is significantly higher in rice (monocotyledonous) seedlings as compared to mungbean (dicotyledonous).

Introduction

On Earth, all living being experiences gravitational acceleration of nine point eight meters per second squared, termed as one Earth's gravity or simply one G. If this value is reduced to ten to the negative three to ten to the negative six times one G then it is called as microgravity or low gravity or simply micro G. To achieve the condition of microgravity one has to either travel into deep space (real microgravity) or it can be simulated on Earth by using different platforms such as parabolic flights, sounding rockets, drop towers or more commonly used instruments by plant biologists, clinostat (simulated microgravity or S M G). Recent studies revealed that gravity has had a profound impact on the form, structure and function of plants. The instrument called clinostat developed by Von Sachs has been widely used to simulate microgravity condition on ground. Previous reports show that microgravity (real or simulated) affects growth, chlorophyll content and photosynthetic performance in different plants. However, variation in results was obtained in studies carried out by using clinostat.

For instance, increase in shoot length in rice seeds (Sasanishiki) under clinorotation was also observed by Takakura et al. The enhancement in growth as measured by root, shoot length and weight are consistent with similar observations on soybean, azuki bean as well as pea and maize using three-D clinostat. Jagtap et al. showed similar increase in root shoot length and weight as well as chlorophyll content in rice seedlings when rotated at two revolutions per minute in one-D clinostat. The photosynthesis process might also affected as observed in four to ten days old garden cress seedlings when exposed to three-D clinostat rotated at lower revolutions per minute. The proton permeability in pea chloroplasts grown under slow clinorotation (two revolutions per minute) was increased. The rate of carbon dioxide assimilation was found to be greater in Pisum sativum grown on a horizontal axis clinostat as compared to control.

Though the enhancement in growth, chlorophyll content and photosynthetic performance was observed in different plants under microgravity, some of the previous reports showed no change or decrease in plant growth and development, chlorophyll content and photosynthetic performance in space and clinorotation.

Thus, these results show variation and do not show consistency. The discrepancies in the results may be due to the difference in methodologies, different environmental conditions and use of different species. These inconsistent results could also be due to the duration of exposure to clinorotation. Besides these factors, it would also be interesting to study whether clinorotation has similar effects on monocotyledonous and dicotyledonous plants. Therefore, in the present work, the attempt has been made to study and compare the seed germination, growth and especially the chlorophyll a fluorescence (O-J-I-P) transient and other important photosynthetic parameters in rice and mungbean seedlings grown under slow rotating (two revolutions per minute) clinostat under similar environmental conditions. The response against simulated microgravity was compared in two plants viz., monocotyledonous rice and dicotyledonous mungbean.