It seems evident that a gale ripping through a wheat field is bad news for a harvest, but what about gentler breezes? Rebecca Hindhaug and colleagues at Aberystwyth University investigated mechanical stimulation in wheat to find out. They focussed on three questions. First, does plant age affect the response to mechanical stimulation? Second, is there a minimum threshold for the perception of mechanical stimuli? Third, is the effect of manual brushing different to natural wind stimulation? They found that even slight stimulation had an impact, with just one daily brushstroke being noticeable. They also found that age did indeed matter.
Hindhaug and colleagues found that there had been studies of mechanical stimulation on wheat, but this had focussed on lodging. Lodging is when the crops’ stems are permanently moved from a vertical position usually due to buckling of the stem, or else displacement of the roots. However, what happens with transient shifts due to wind or animals brushing past the stems?
The team examined how the plants reacted to wind and brushing by taking two-week-old plants and randomly assigning them to one of three groups. One group were exposed to a domestic fan running at a wind speed of 3.5 m/s. The scientists rotated this group to make sure all the plants got exposed to the breeze. For the second group, they were brushed on a special rig with 20 brushstrokes each morning. The rig had a height-adjustable bar to make sure the brushing was always at half the canopy height. The third group was the control, with no treatment to see if either wind or brushing was having an effect.
“The number of tillers, leaves, and biomass measures showed a significant increase for both treatments while the length of the main tiller top leaf was significantly reduced. These findings showed that mechanical stimulation of 2-week-old wheat seedlings by brushing and wind similarly affected the phenotypic parameters evaluated,” write Hindhaug and colleagues.
“Since brushing utilizing the rig allowed for a more controllable, even and reproducible treatment for larger numbers of wheat plants, brushing was used in subsequent mechanical stimulation experiments.”
As far as age-response went, the younger the plant, the more significant the effect, write the authors. “The largest height reduction (41%) was observed for the 2-week-old age-group, followed by the 4-week and 6-week age-groups (height reductions of 16% and 5%, respectively). The length of the main tiller, measured at the end of flowering, was also significantly reduced across the age groups, although with no clear age- response effect (reductions of 11%, 16% and 6%, respectively for the 2-, 4- and 6-week age- groups).”
With the two-week-old plants having the greatest response, the team set out to see what the dose response was. How many strokes did they need to give a plant to find an effect? They took plants from two weeks and brushed them daily for the next month. Some plants had just one stroke, but others had more, with one group receiving up to twenty strokes a day.
“The application of just one brushstroke per day for four weeks reduced the plant height by 146 mm (24%) compared with untreated plants at the end of the treatment. Increasing the number of daily brushstrokes reduced plant height further (Fig. 3A) with the mean plant height after 20 strokes again significantly lower (by 28%) compared with just one daily brushstroke and 46% lower compared to untreated controls,” write Hindhaug and colleagues.
The team also found that brushing could either increase or decrease spike count for the wheat plants. Total spike weight was reduced with brushing but with no clear dose response. In contrast, brushing increased the number of tillers.
Having gained these results in controlled conditions, Hindhaug and colleagues took their work outside to see if it held up in the real world. This time the team protected the control sample with baffles while the experiments were exposed to the wind. They found, as expected, that wind exposure reduced tiller height, with brushing causing a further loss on height, but both treatments also gained tillers. While the control plants had an average of 5.8 tillers, they had 8 tillers if exposed to the wind and 9.2 tillers if also brushed in the wind.
“This study revealed a remarkable age- and dose-response of wheat to mechanical stimulation. Besides affecting plant phenological traits, the treatment of 2-week-old wheat plants significantly affected spike and grain development. Together with the results from outdoor experiments, the outcomes highlight the complexity of the response to mechanical stimulation,” write Hindhaug and colleagues.
“While excess mechano-stimulation can clearly be harmful, traditional Japanese farmers have applied mechanical stimulation for centuries to wheat seedlings by trampling on them, a process called “mugifumi”, in order to make them more resilient and improve yields. Although there is little knowledge on the dose response nor the molecular mechanisms underlying mechano-sensing and mechano- transduction in cereals, our results showing that mechanical stimulation can increase aboveground biomass, increase tiller numbers, and affect grain development, corroborated by the tradition of “mugifumi”, highlight the importance of mechanical stimulation for wheat productivity.”