Short-Term Hypergravity-Induced Changes in Growth, Photo synthetic Parameters, and Assessment of Threshold Values in Wheat (Triticum aestivum L.)

Plants can respond to many abiotic stress conditions, such as temperature, drought, salinity, heavy metals, and so on in several ways. It has been hypothesized that, for almost every biological system there exists a threshold for physiological response and behavior, above or below which the responses are significantly affected. Identifying potential threshold values in plants is important as it indicates their sustainability in extreme abiotic environments, and significant changes in plant growth and development may occur around this value. For instance, the water potential threshold that started to negatively affect the rice growth was observed to be between −0.046 and −0.056 MPa (Santos et. al., 2018). Plants subjected to mild heat stress (1°C to 4°C above optimal growth temperature) had reduced yield (Timlin et al., 2006; Tesfaendrias et al., 2010); however, more intense heat stress (generally greater than 4°C above optimum) resulted in severe yield loss extending to complete crop failure (Ghosh et al., 2000; Tesfaendriasetal., 2010). The optimum temperature range in citrus (Citrus sinensis L. Osbeck) is 22°C–27°C, and temperatures greater than 30°C increased significant fruit drop (Cole and McCloud, 1985).

Gravity, a constant abiotic physical factor on Earth, plays a very important role in functioning of many biological systems such as bacteria, plants, animal, etc. Recent advances in space and gravitational biology have shown that an alteration in the gravity value above or below Earth's gravity (1 g) produces measurable changes in biological systems. Indeed, it is well documented that microgravity has positive effects while hypergravity has negative effects on plant growth and development (Waldron and Brett, 1990; Hoson et al., 1996; Jagtap et al., 2011; Vidyasagar et al., 2014). Although several studies have been conducted on ground-based low gravity (simulated microgravity), the use of plant systems for studies under a higher range of hypergravity conditions is limited, which may be due to the negative impact of high g forces on plants. Efforts have been made to calculate the threshold of gravi response for unicellular organisms such flagellates, ciliates, etc. (Häder et al., 2005), but the threshold values of hypergravity effects for multicellular organisms such as plants were not reported clearly. The understanding and assessment of the threshold value of hypergravity in plants is important, as it would provide the lower and upper limit of the g-value where the change in plant growth and development are more prominent.

Higher-range g forces retarded growth and development in plants as seen from earlier reports, for example, pea seedlings when centrifuged at 140×g, 370×g, and 1,050×g for 5 days (Waldron and Brett, 1990), cress hypocotyls at 35×g, and 135×g for 12 hours (Hoson et al. 1996), azuki bean epicotyls at 30×g and 300×g for 10 hours (Soga et al., 1999), Arabidopsis thaliana at 300×g for 24 hours (Nakabayashi et al., 2006). An increase in the content of cell wall polysaccharides due to hypergravity exposure was observed in cress hypocotyls (Hoson et al., 1996) and in azuki bean epicotyls (Soga et al., 1999a). The elongation growth and lignin deposition in secondary cell walls of the Arabidopsis thaliana (L.) grown for three days after exposure to hypergravity of 300 g for 24 hours suggest the role of mechanoreceptors in hyper gravity-induced lignin deposition in secondary cell walls (Tamaoki et al., 2006). The loss of starch and soluble carbohydrates during seed development was found to be accelerated due to hyper gravity (Musgrave et al., 2009). Thus, different plants were exposed to different hypergravity levels (selection of g-value unclear) and effects were studied. Despite adequate information available onthe effects of hyper gravity on plant growth and development, much less is known about photosynthetic parameters such as the rate of photosynthesis, stomatal conductance, the rate of transpiration, and intracellular CO₂ concentration that plays an important role in driving photosynthesis. In addition, the information on hypergravity-induced changes in chlorophyll fluorescence parameters characterized by the Kautsky fluorescence induction curve (photochemical activity of PS II) is limited. Only few reports have documented the effects of hypergravity on photosynthetic and chlorophyll fluorescence parameters; however, these studies were not aimed at threshold values (Vidyasagar et al., 2014; Takemura et al., 2017). This could be due to the negative impact of hypergravity on plant growth and development, which might have discouraged researchers from extending hypergravity studies for understanding photosynthesis.

Therefore, in addition to growth, the photosynthetic and chlorophyll fluorescence parameters were also considered to assess the threshold values of hypergravity in wheat (Triticum aestivumL) in this study. The study of the threshold value of high g on growth and photosynthetic parameters will not only give us an idea about the response of these parameters under high g but also will indicate the optimum range of high g for future studies.

The effects of short-term hypergravity exposure on growth, photosynthetic and chlorophyll fluorescence parameters in wheat seedlings raised from hypergravity-exposed seeds were investigated to understand the threshold response of the effects of hypergravity. The significant reduction in growth begins at 400 g and declined to its maximum at 1,000 g. The rate of reduction was found to be 10% more pronounced in seedling length as compared with its weight. The reduction in growth is consistent with the previous studies on various vascular plants such as peas, azuki beans, cress, etc., due to long-term moderate exposure to hypergravity (Hoson et al., 1996; Waldron and Brett, 1990; Soga et al., 1999; Takemura et al., 2017). The decrease in cell wall extensibility (Hoson et al. 1996) or lower enzyme activity (Vidyasagar et al., 2014) could be the possible reasons for the observed decrease in growth as reported earlier.

Photosynthesis is one of the major physiological processes in plants and is highly sensitive to changes in environmental conditions. Intracellular CO₂ is one of the important factors that determine plants ability to perform photosynthetic assimilation. In the present study, it was found that intracellular CO₂ concentration (C_int) gradually decreased from 400 g to 1,000 g. The significant decrease in rate of photosynthesis (P_N) was also observed from 400 gupto 1,000 g. The observed lower rate of photosynthesis in hypergravity samples compared with control could be due to the relatively reduced availability of C_int (Tominaga et al., 2018). The reduced availability of CO₂ could be attributed to decrease in stomatal conductance (Gs), i.e., the ability of the stomatal pore to release water vapor from the leaf to the atmosphere and allow CO₂ to enter from the atmosphere. The decrease in stomatal conductance may also cause a decline in the transpiration rate (Farquhar and Sharkey, 1982; Jeanguenin et al., 2017). In addition to the above argument, contributions from other factors such as reductions in chlorophyll content (Vidyasagar et al., 2014) and changes in chloroplast size (Takemura et al., 2017) might also be responsible for the reduced rate of photosynthesis in the present study.

Chlorophyll fluorescence gives the information about change in the efficiency of photochemistry of photosystem II (Maxwell and Johnson, 2000). The fluorescence curve starts from minimal fluorescence F_o (O) to maximal fluorescence F_m (P) via two intermediate steps J and I with respect to time. This entire curve divides into three phases: O-J, J-I and I-P (Figure 4). As seen from the results, the initial minimal level of fluorescence F_o (fluorescence origin) was found to be same for the controls as for all the hypergravity samples(Figure 3). This indicates that all the PSII reaction centers were in open state and the first stable electron acceptor of PSII called Q_A oxidized equally in control as well as in all hypergravity samples. However, the maximal fluorescence (F_m) value was found to be reduced in hypergravity samples (Figure 3). Maximal fluorescence is observed when all the reaction centers are closed and Q_A is fully reduced. This, in turn, is governed by the relative rates of PSII photochemistry and oxidation of plastoquinol by electron transfer to photosystem I (PSI). The area above the chlorophyll fluorescence curve between F_o and F_m is proportional to the pool size of the electron acceptors Q_A on the reducing side of PSII. It can beseen from Figure 3 that the average area above the Kautsky curve gradually decreased for the hypergravity samples ranging from 400 g to 1,000 g. The decrease in the F_m value in the hypergravity samples (Figure 3), as Q_A was not fully reduced, possibly indicates an imbalance between the rate of PSII photochemistry and plastoquinol oxidation with an increase in the g-value from 400 g to 1,000 g. The important parameter of chlorophyll fluorescence, the performance index (PI), indicating plant vitality under stress decreased from 400 g to 1,000 g. The decrease in PI could be a consequence of reduced reaction center density or declined trapping efficiency and electron transfer efficiency (Tripathy et al., 1996; Stribet and Govindjee, 2011).

Figure 4

The present study has revealed that when seeds were exposed to a short duration of 10 minutes, the reduction of growth, photosynthetic and chlorophyll fluorescence parameters started at 400 g and was at the maximum at 1,000 g. This indicates that 400 g may be considered as a lower threshold and 1,000 g a higher threshold of hypergravity for growth, as well as photosynthetic and chlorophyll fluorescence parameters in wheat (Figure 5). The value of hypergravity at which these effects have just started can be termed the lower threshold g (TH_L-g) while at which these effects are at the maximum can be termed the higher threshold g (TH_H-g).

Figure 5

In our previous study, the reduction in growth parameters due to short-term hyper gravity were seen at 500 g and the maximum at 2,000 g in wheat (Vidyasagar et al., 2014). The difference in threshold g values between the present and the previous studies could be due to the difference in the environmental conditions, especially temperature (25°C and 30°C) and humidity (60% and 40%). Several alterations are possible in growth and developmental response under hypergravity due to the difference in the physiological state of the plants and environmental conditions during and/or after exposure as reported earlier (Tamaoki et al., 2006; Kozeko and Kordyum, 2009; Takemura et al., 2017), which may lead to a different threshold response. Further, it can be assumed that the protective layer (coating) of seeds might have opposed hypergravity below 400 g but failed to resist it at and above 400 g. It would be interesting to explore the threshold values of the effects of hyper gravity for different cereals display in g variation in the density of the seed coat. In addition, to find the exact threshold value of high g, we recommend that the acceleration steps in g-value should be as small as possible. Keeping this in mind, we hypothesize that, plants possess the threshold value of hyper gravity effects, which may vary depending upon the time of exposure, the sensitivity of seeds or seedlings to a given g-value, and the environmental conditions under which they are exposed to hypergravity and grown. We encourage future studies to investigate underlying the physiological mechanisms responsible for observed effects at threshold g-values.

The present study aimed to analyze the effects of short-term hyper gravity exposure ranging from 200 g to 1,000 g on plant growth and photosynthetic parameters and to assess threshold g-values at which these effects were observed. No significant change was seen at 200 g in terms of growth and photosynthetic parameters. However, a significant reduction in these parameters was observed from 400 g onward and found to be at the maximum at 1,000 g. This implies that plants do not perceive short-term exposure of 200 g as a stress and respond normally, while from 400 g onward, plants experience a stressful hyper gravity environment leading to the beginning of a reduction in growth and photosynthetic parameters. We therefore propose that short-term (10 minute) hypergravity at 400 g can be treated as a lower threshold and 1,000 g as higher threshold for wheat seeds when they are exposed and grown under the environmental conditions described above. The present study could help researchers to choose proper g-values to prevent excessive exposure of plants to hypergravity. According to the available literature, this is the first study to report the threshold values of short-term hypergravity for growth and photosynthetic parameters.