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),
Therefore, in addition to growth, the photosynthetic and chlorophyll fluorescence parameters were also considered to assess the threshold values of hypergravity in wheat (
Wheat seeds (
Growth parameters such as shoot length, root length, fresh shoot weight, and fresh root weight of five-day-old seedlings raised from controls and hypergravity-exposed seeds were measured.
On the fifth day, seedlings were taken out of the 0.8% (w/v) agar gel, and shoots were removed for measurement of the photosynthesis parameters. The shoots were immediately placedin the leaf cuvette (PLC4-B) for the measurement of photosynthesis parameters, i.e., the rate of photosynthesis (
Chlorophyll fluorescence has been routinely used for many years to monitor the photosystem II (PSII) behavior of plants noninvasively (Strasser et al., 1995; Baker and Rosenqvist, 2004). For the recording of chlorophyll fluorescence, a portable instrument Handy PEA (Plant Efficiency Analyser, Hansatech Instruments, Kings Lynn, UK) was used. Shoots from five–day-old seedlings raised from control and hypergravity-treated seeds were isolated and immediately enclosed in leaf clips for 10 minutes of adaptation to darkness. After 10 minutes, a single strong 1 second light pulse (3,500 μmolm−2s−1) was applied to the shoots with the help of three light-emitting diodes (650 nm). The fast fluorescence kinetics (
The data presented in this research paper gives the mean values of three sets of experiments with
The images of five-day-old shoots raised from controls and higher g-treated seeds are shown in Figure 1. No statistically significant difference was observed between controls and 200 g. The significant decrease in growth parameters such as shoot length (SL), root length (RL), fresh shoot weight (FSW), and fresh root weight (FRW), was observed in hypergravity-treated seeds from 400 g to 1,000 g. The decrease in shoot length and root length was 30% at 400 g and 90% at 1,000 g. The decrease in FSW and FRW was 19% at 400 g and 80 % at 1,000 g(Table 1).
The photosynthesis parameters, i.e., the rate of photosynthesis (
The Kautsky fluorescence induction curves for five days old seedlings raised from control and hypergravity exposed seeds are shown in Figure 3. The fluorescence intensity of Kautsky induction curves decreased with increase in value of hypergravity from 400 g to 1,000 g compared with controls.
The area above the fluorescence OJIP curve between
The performance index (PI) showed a significant fall in five-day-old seedlings ranging from 400 g to 1,000 g as compared with controls (Table 1) except at 200 g. The percentage fall in PI was 9% at 400 g and 33% at 1,000 g. No change in the ratio
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 (Cint) gradually decreased from 400 g to 1,000 g. The significant decrease in rate of photosynthesis (
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
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
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.
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