How does rotation affect plant growth

They found that rotation rates of 2—4 rpm corresponded to a more effective randomization because it minimized the path length of statolith sedimentation. Figure 1 A standard two-dimensional clinostat used to grow Medicago seedlings green arrows at 1 rpm. Blue arrow indicated the direction of rotation. Scale bar , In addition to their use as a means of minimizing the unidirectional effects of gravity, several studies have incorporated modified versions of the clinostat that expose the axial organ to a fractional g treatment either through a programmed rotation pattern or through the incorporation of a centrifuge as the innermost rotating axis of the clinostat.

These instruments have been key in estimating the threshold acceleration necessary to activate gravity perception and growth responses Shen-Miller et al. Despite their usefulness for temporarily changing the unidirectional force of gravity, there is also evidence that such treatments introduce their own sets of stimuli that may compete with or confound interpretation of those pathways of most interest to the user Hasenstein and van Loon, Centrifugal forces resulting from rotation about one or two axes varies as a function of the position of the organ under study along the radius and speed of rotation, and the organ will experience variable g levels across its axis.

Because the force of gravity itself is never altered, the bending due to differential growth will cause the position of the organ with respect to the radius of rotation to change over the course of an experiment.

For example, the growth of an axial organ over the course of a long-term experiment will result in the organ experiencing a change in acceleration if the growth direction is away from the center of the axis of rotation.

This factor is one source of complexity when interpreting the results of clinostat experiments, as indicated by the observation that plants respond differently when rotated around one axis versus the other John and Hasenstein, ; Hasenstein and van Loon, Long-term experiments on clinostats are particularly challenging because as the organs increase in mass, the changing weight distribution will cause bending stresses and other non-random mechanical stimulation that will vary as a function of the specific load-bearing structure of each organ.

Thus, growing plants on a rotating clinostat can result in mechanical stress van Loon, ; Manzano et al. The use of clinostats to study developmental effects of gravity are also limited because of their inability to control for constantly changing loads and rotational forces, thus restricting their usefulness with plants mainly to studies of directional growth responses.

Thus, many factors, such as weighting distribution and rotation velocity, need to be considered when designing clinostats for life science experiment Brown et al. Despite these disadvantages, the simplicity and availability of clinostats are the main reasons that these devices are the most common approach to attempt to simulate altered gravity conditions.

Although the artifacts associated with clinostats require caution of the assessment of gravitational effects, they can provide valuable comparisons with space experiments and have been widely used by many researchers e. In liquids, sedimentation and a relatively slow rotation result in potentially significant artifacts including spirally movements from centrifugation, sedimentation, and a viscosity-dependent Coriolis force.

When the speed of rotation is increased as in a fast-rotating clinostat, sedimentation of a particle will be less than the movement of the liquid, thereby resulting in a reduced radius that finally produces a smaller diameter than the size of the particle or a cell.

Thus, in the conditions as found in the fast-rotating clinostat, the rotation stabilizes the fluid around the particle, which in turn eliminates the gravity effects for all practical purposes.

While the fast-rotating clinostat can provide conditions that mimic weightlessness very well, it is limited to small organisms such as unicells or bacteria but, generally, not applicable for plant studies Cogoli, The bottom dwell time determines the effective residual acceleration. Because additional acceleration or deceleration needs to be minimized, the movement requires precise algorithms and motor control. The advantage of such designs is that fractional g -levels can be established.

Nonetheless, this principle also depends on rotation and therefore suffers from the same shortcomings as standard clinostats Hasenstein and van Loon, Figure 2 Motion profile of a an object experiencing fractional gravity as a result of non-uniform rotation.

The trace of a point rotating around an axis shows an extended resting position during phase T 1. The ratio between the dwell time in the bottom position T 1 and complete rotation T 0 corresponds to the fractional gravity experienced by plants, provided that the dwell time does not exceed the gravity perception time.

The limited ability to average gravity effects by horizontal rotation led to the evolution of RPMs in order not to generate constant accelerations in any particular direction Kraft et al.

The idea is to provide a more complex motion patterns than constant rotation around one or two axes such that no directional preference remains. Ideally rotation should occur around all three spatial axes x , y , and z , i. This arrangement is sufficient to position any object on the experimental platform in any desirable direction i. Thus, seedlings that develop on an RPM appear to grow randomly as achieved in spaceflight Figure 4.

Figure 3 Three random positioning machines RPMs each with two independently driven perpendicular frames. The discrete rotation axes allow the implementation of slip rings to provide power and exchange data with the experiment that can be mounted onto the inner frame.

The diameter of the disk asterisk on the full-sized RPM is 40 cm and provides the generation of partial gravity. Figure 4 Arabidopsis seedlings grown in spaceflight hardware for 3.

Arrowheads indicate the hypocotyl apex. A Seedlings that germinated and developed on the random positioning machine RPM are disoriented. B Ground controls GR are oriented to the gravity vector which is toward the bottom of the photograph. Scale bar , 6 mm. Figure is from Kraft et al. Randomness is achieved when the rotational angle differs between the two axes and changes over time.

While these systems provide the best gravity compensation, they do so despite apparently exceeding the maximum permissible angular acceleration approx. While this puzzling observation deserves future studies, it also highlights some postulated gravisensing mechanisms, namely that the movement of the suspected gravity sensors starch-filled amyloplasts; Kiss, is sensitive to mechanostimulation, and thus describes dynamic gravisensing Hasenstein, Nevertheless, in gravity-perceiving root columella cells, the position of amyloplasts was similar in weightlessness in spaceflight and on the RPM, but was significantly different between spaceflight and two-axial clinostats Figure 5.

For in vitro systems like Arabidopsis cell cultures, it is important to realize that in fluid-filled experimental containers, there is also a fluid shear applied to the cells Leguy et al. This issue can be mitigated by increasing the cell substrate viscosity Kamal et al. These cells are involved in gravity perception Kiss, Thus, the RPM can be a useful proxy for weightlessness for certain biological parameters, as shown in studies with plant cells, Drosophila , and mammalian cell cultures Kraft et al.

This scenario is true especially when the changes in direction are faster than the response time of the object e. In contrast to the various clinostats that attempt to randomize the effect of gravity by changing the direction of its vector, magnetic forces counteract the gravity force by a magnetic force that results from a magnetic gradient and the diamagnetic susceptibility of the object which together generate a force that can be equal to gravity Geim et al.

Interestingly, based on the orientation of the magnetic core, this gradient exists in two opposing directions such that in a vertically oriented magnetic field the top gradient balances gravity effects on biological, i. The opposite pole of the magnetic gradient also generates a 1- g force equivalent and therefore provides a 2- g equivalent 1 g attributed to the magnetic gradient in addition to the original gravity.

While the effect of magnetic gradients and diamagnetic properties of the levitated object e. Additional research is needed to determine which systems best mimic reduced gravity conditions, especially for plants that occupy a large volume and are therefore affected by any gradient of rotational, inertial, or magnetic conditions.

Despite the above-mentioned complications, the ability to produce partial or even excess gravity forces makes magnetic gradients an attractive alternative to clinostat-based research. As indicated earlier, the precise and narrow space that corresponds to the desired level makes studies on whole plants problematic because the compensation point averages all forces acting on the levitated object by susceptibility, density, and distance.

Thus, the most valuable aspect of high-gradient magnetic fields is the ability to precisely move levitate cellular organelles, such as statoliths in roots Kuznetsov and Hasenstein, , hypocotyls Kuznetsov and Hasenstein, , rhizoids Kuznetsov and Hasenstein, , and seedlings Hasenstein and Kuznetsov, In addition, magnetic levitation has been shown to be a useful ground-based proxy for microgravity in a number of other systems including osteoblast cells Hammer et al.

Although it sounds somewhat counterintuitive, we can also explore the effects of microgravity by the application of centrifuges. This reduced gravity paradigm RGP is based on the premise that adaptations seen going from a hypergravity level to a lower gravity level are similar to changes seen going from 1 g to microgravity van Loon, Using such a paradigm, we are not focusing on the absolute acceleration values but rather on the responses generated due to the change between the two accelerations levels.

The premise of such an experiment is that the plant sample has to be adapted and stable to a higher gravity level such as 2 g. Then, as the g -level is lowered to 1 g , the plant will respond to this reduced gravity level. It is hypothesized that the processes in such adaptations are of the same type as one would see going from 1 g into free fall, although the magnitude might be different.

Thus, this reduced gravity paradigm is best used for stable and steady systems at a certain higher g level combined with measuring a relatively fast responding phenomenon when reducing the acceleration load.

Numerous studies on plant growth and development have been performed in space Wolverton and Kiss, ; Vandenbrink and Kiss, In contrast, we know little about plant physiology in reduced gravity environments, which are less than the normal 1 g that characterizes Earth-based studies. Reduced gravity can also be termed partial- g or fractional- g.

The exploration of the Moon and Mars will be important in the future and will rely upon optimized plant cultivation because plants will be essential for life support systems Kiss, Therefore, it is important to develop new knowledge about the biology of plants at the lunar and Martian g -levels, 0.

Studies on plants in partial gravity environments also can provide new information on basic biological questions such as what is the threshold of gravisensing in plants e. To establish partial gravity on-board sounding rockets or orbiting laboratories, a centrifuge is needed to produce the desired accelerations.

Centrifuges can be used to generate any acceleration from near zero to 1 g. Especially 1- g experiments are valuable as in-flight controls, which provide context for the analyses of spaceflight experiments Vandenbrink and Kiss, Fortunately, there are several facilities on the International Space Station ISS that are equipped with centrifuges, and the ISS can be used to study partial gravity effects on plant development. A series of experiments have recently been performed on the ISS with Arabidopsis thaliana and have focused on 1 the interaction between gravitropism and phototropism in microgravity and fractional gravity Kiss et al.

These experiments utilized the European Modular Cultivation System EMCS which had onboard centrifuges allowing for gravitational ranges from microgravity to small fractions of a g up to 1 g Kiss et al. In experiments in which directional light and fractional gravity were applied simultaneously, Kiss and colleagues reported strong positive phototropism in response to unilateral red light in the stem-like hypocotyls and roots of plants grown in microgravity Millar et al. In time course studies, shoots had positive phototropism in response to red light in microgravity and at 0.

Left, initial position. The sample stage SS is rotated about a horizontal axis in a conventional clinostat. Right, after 3-D rotation with two motors M placed at right angles. On the other hand, plant morphogenesis is greatly modified on a clinostat. Since the studies of Sachs and Pfeffer more than a century ago, spontaneous morphogenesis, termed automorphogenesis, has been observed in various plant materials on the clinostat [ 1 , 3 , 6 ].

Spontaneous curvature occurs in growing regions in the direction explained by the presence of dorsiventrality having distinct back and front structures [ 3 , 5 , 7 ]. The angle of automorphic curvature increases with organ growth in the early growth phase on the clinostat, but seedlings lose dorsiventrality and show a radial form after approximately a week of growth.

Automorphic curvature differs in various parameters from tropistic curvatures [ 1 ]. For instance, the rate of automorphic curvature is only one-tenth of that of gravitropic curvature. Plant roots on the 3-D clinostat first grow along the direction of the tips of the root primordia and then in random directions [ 3 ].

This transition of growth pattern occurs in the early growth phase in some species, such as maize and pea, but in the late growth phase in other species, such as rice and garden cress. Clinorotated roots also show automorphic curvature. The automorphic curvature of roots occurs in random directions, without any dorsiventrality [ 8 ]. Thus, the automorphogenesis of roots on the 3-D Clinostat also consists of altered growth direction and spontaneous curvature.

The mechanisms by which automorphic curvature is induced on the 3-D clinostat have been studied with maize and rice seedlings. The rate of plant cell growth is most directly regulated by the osmotic potential of the cell sap and the mechanical extensibility of the cell wall.

Previous studies have shown the direct cause of differential growth in clinorotated coleoptiles and roots to be the difference in cell wall extensibility between the convex and concave sides, not the difference in cellular osmotic potential [ 1 , 7 ].

A number of differences in the metabolic turnover of the cell wall constituents have been detected between the two sides [ 1 , 7 ]. Changes in microtubule orientation may also be involved in the automorphic curvature of shoot organs. The cortical microtubules of epidermal cells on the convex side are oriented more transversely than those on the concave side in clinorotated rice coleoptiles [ 9 ].

These changes preceded the automorphic curvature when rice seedlings were transferred from a stationary orientation to the 3-D clinostat [ 1 ]. Clear structural anisotropy exists between the back and front sides of young shoot organs [ 1 , 7 ]. For instance, the back, convex side of a rice coleoptile consists of small, extensible cells, as compared with its front, concave side.

In the presence of a gravity vector, this difference in growth capacity is likely diminished, and the shoot organs are forced to grow upward evenly along the vector; in the absence of this vector, the organs show automorphic curvature following their inherent anisotropy Figure 2. The involvement of reduced polar auxin transport in automorphogenesis has been demonstrated in pea seedlings grown on a clinostat [ 10 ]. The mechanism inducing automorphic curvature.

Under microgravity conditions, the inherent structural and physiological anisotropies of plant organs induce spontaneous automorphic curvature, whereas in the presence of gravity, the organs are forced to grow along the vector of gravity, leading to gravitropic curvature.

Growth of plant seedlings is not significantly influenced by clinostat rotation, as mentioned above, and the magnitude of gravitational acceleration must be changed to examine the effects of gravity on growth processes. Water immersion has been used to simulate microgravity in the development of space science and technologies.

Submergence is recognized as a kind of microgravity, because the apparent strength of gravity, as measured by the weight of materials, is reduced due to buoyancy, even if gaseous and other factors are also different and no changes in cellular events occur underwater [ 1 , 11 ].

Land plants are generally unable to survive underwater. However, aquatic or semi-aquatic plants such as rice can grow for long periods under conditions of submergence.

Rice coleoptiles elongate rapidly and are growing slender when growing underwater Figure 3 , as submergence induces the stimulation of elongation and suppression of lateral expansion [ 1 , 12 ].

Because the modification of growth anisotropy is suppressed only partially by air bubbling, the microgravity effect due to buoyancy, in addition to gaseous factors, may be involved.

Similar changes have been observed in deepwater rice and Regnillidium grown underwater [ 1 , 12 ]. Rice seedlings grown in air with only roots immersed in water or underwater. Submergence induces the stimulation of elongation and the suppression of lateral expansion in rice coleoptiles. The cell wall extensibility of rice coleoptiles grown under submergence is higher than that of coleoptiles grown in air [ 1 , 12 ]. Submergence also causes diverse changes in the levels and metabolism of the cell wall constituents of rice coleoptiles.

These changes in cell wall properties may be responsible for the modifications of growth and morphogenesis observed in plant seedlings under submergence. Another practical method to modify the magnitude of gravitational acceleration is centrifugal hypergravity. Waldron and Brett first analyzed the effects of hypergravity on the growth and cell wall compositions of pea seedlings [ 13 ]. The responses of various other plant materials to centrifugal acceleration have since been analyzed, and hypergravity has been shown to decrease the longitudinal growth rate of various shoot organs and roots [ 1 , 13 , 14 ].

Hypergravity not only suppresses elongation growth but promotes the lateral expansion of organs [ 15 , 16 ]. Plant organs are highly resistant to gravitational acceleration, and growth parameters vary in proportion to the logarithm of the magnitude of gravitational acceleration up to g Figure 4. Linear dose-response relationships to the logarithm of a dose are common among biotic responses in which proteinaceous signal receptors, such as photoreceptors and hormone receptors, are involved.

When plant seedlings grown under hypergravity conditions at g for several hours are transferred to 1 g conditions, the growth rates recover fully within a couple of hours, indicating that the effects of gravitational acceleration on growth properties are prompt and reversible [ 1 ]. Dose-response relationship between the magnitude of gravity and growth parameters. The magnitude of gravity is plotted on a logarithm basis. Regarding the cell wall properties that regulate the growth rate, hypergravity has been shown to decrease the cell wall extensibility [ 1 , 17 ].

Hypergravity also induces the accumulation of cell wall constituents, polymerization of certain matrix polysaccharides due to suppression of breakdown, stimulation of cross-link formation, and increase in cell wall pH, depending on the magnitude of the gravitational acceleration; all of these factors decrease the cell wall extensibility.

Thus, the modifications of the growth and cell wall properties of plant seedlings caused by hypergravity are opposite to those caused by submergence. Cortical microtubules are likely involved in the modification of growth anisotropy by hypergravity through the regulation of the orientation of cellulose microfibrils. In the growing regions of stem organs at 1 g , cells with transverse cortical microtubules are predominant. As gravitational acceleration increases, the percentage of cells with transverse microtubules decreases, whereas that of cells with longitudinal microtubules increases [ 15 ].

On the other hand, the stem organs of tubulin mutants are shorter and thicker than those of the wild type and display helical growth at 1 g. The degree of this twisting phenotype is high under hypergravity conditions [ 20 ]. Thus, hypergravity intensifies the morphological defects of certain mutants related to gravity responses. The ground-based experiments used to clarify the effects of gravity on plant growth and morphogenesis all possess specific fundamental weaknesses, as mentioned above.

Thus, studies must be conducted under true microgravity conditions in space. Previous opportunities for space experiments have been limited, but the situation was greatly improved by the initiation of scientific operations on the International Space Station in From the results of experiments using water submergence and centrifugal hypergravity, the elongation of seedlings was hypothesized to be stimulated under true microgravity conditions in space.

Researchers observed a 33 percent increase in soil carbon by increasing rotational diversity. An indication of soil organic matter, the carbon content of soil is a major factor in its overall health and improves the physical properties of soil. Researchers also found that as crop diversity increased, so did total nitrogen concentrations, a sign of soil fertility.

A byproduct of increased pressure on soils due to agricultural intensification is a negative impact on microbial diversity and function. This is a problem worldwide and can lessen soil's ability to perform important ecosystem functions. Results may include threats to long-term food security, increases in greenhouse gas emission, and a reduction in water quality.

Even increasing rotation by one or two crops, especially if cover crops are used, will improve soil physical, chemical, and biological processes that help regulate yields and environmental quality," Grandy said.

The research findings are presented in the journal Ecology Letters in the article "Crop rotational diversity enhances belowground communities and functions in an agroecosystem. Generally, crops may be characterized as having low, medium, or high nutrient demands based on their nutrient uptake efficiency table 3.

Different varieties within any crop may be more or less efficient at taking up nutrients. Those crops with a high nutrient demand predominately N require higher levels of those nutrients to be present in the soil solution.

Green manures and soil fertility amendments have the most benefit when they target the crops with high nutrient demands. On inherently fertile soils, crops with low nutrient requirements often achieve good yields from residual soil fertility alone.

Crop rooting depth can have important implications for nutrient availability as well as soil physical characteristics. Crop rotations that integrate deep-rooting crops with less nutrient-efficient crops can help cycle nutrients in the soil profile.

The deep-rooted crops listed in table 3. Deep-rooted crops also create channels into the soil that later can improve water infiltration. Although most of the listed crops are typical of grain rotations, the data are also relevant to vegetable producers, since grain and forage crops are integrated into vegetable rotations as cover crops. Note: Ten tons of manure were applied every other year. The cumulative balances are based on the difference between the export and the input of nutrients.

Not all of the nutrient inputs are available in the first year. Soil tests may suggest the need for additional inputs of particular nutrients. In some cases, soils are naturally low in nutrients; in other cases, export of nutrients in crops has led to soil depletion. Organic soil amendments such as composts, trace element mixes, plant and animal meals, and rock powders can be used to meet some of these needs.

Many organic soil amendments become available only slowly; in some cases application to the previous cover crop improves availability to the cash crop. Since some of these amendments can be expensive, they should be applied strategically within a rotation. Prior to the application of any of these materials, adjust soil pH to the desired range for the majority of crops within the rotation generally 6. High or low pH will reduce the availability of phosphorus and many micronutrients.

Most composts contain relatively stable forms of organic matter and low levels of readily available nutrients. Some types, such as poultry compost, may contain high levels of nutrients compared to other organic fertility amendments, but not compared to commercial fertilizers.

Good composts applied at specific points in a rotation can improve soil fertility in the long term by enhancing soil structure and tilth, improving soil water movement, and providing a slow-release fertility source.

Usually, meeting the complete nitrogen needs of a crop by using only compost is difficult without also adding excessive phosphorus. These can provide low levels of nitrogen, calcium, magnesium, boron, zinc, and iron. Foliar fertilizing must be managed carefully, since effectiveness depends on uptake of the micronutrients through the plant cuticle. Depending on application rates, environmental conditions, and plant maturity, foliar feeding can sometimes result in burning of leaves.

Rock powders ground limestone, gypsum, granite dust, rock phosphate and trace element mixes slowly release nutrients to plants. The more finely ground the powder, the sooner the minerals will be available to the crop due to a greater surface area of the powder available for microbial digestion and physical weathering.

Like composts, rock powders cannot be used to provide immediate crop needs. They should be used as long-term sources of crop nutrients.