Internode elongation and strobili production of Humulus lupulus cultivars in response to local strain sensing

Abstract

Three different cultivars of Humulus lupulus L. were subjected to a regime of internode touch and bending under greenhouse conditions. Experiments were performed to assess intraspecific variability in plant mechanosensing, flower quality, and yield to quantify the thigmomorphogenic impact on plant compactness and flowering performance. Touching and/or touching plus bending the plant shoot internodes located in the apical meristem zone decreased internode elongation and increased width. The growth responses were due partly to touching and/or touching plus bending perturbation, 25.6% and 28% respectively. Growth of new tissue within the local apical portion of the bine continued to remain mechanosensitive. The number of nodes and female flowers produced was unaffected by either type of mechanical stress. The study provides evidence that thigmomorphogenic cues can be used as a hop crop management tool to increase bine compactness and increase node density per unit area. The findings have broad implications for hop production; production can more readily take place in a confined greenhouse space with the aid of mechanical stimulation to control plant growth without sacrificing yield or flower quality.

Figure 1

67 days into the crop cycle a representative example of cultivar ‘Centennial’ hop cone production on a node where the internode was not previously bent (blue arrow and left side of photo) versus a subsequent node that was bent at 90° (red arrow and right side of photo). Moreover, the red arrow indicates a shorter internode that results in a higher density of cones along the length of the bine (90° bending), whereas to the left of the blue arrow indicates a sparser cone density sans bending.

Figure 2

The 24 h sequence of forming a 90° bend in the bine between the second and third visible node from the apical meristem (a) first the apical meristem is bent at approximately 90°, (b) secondly it is undisturbed for 24 h of growth, and (c) thirdly the second and third visible node from the apical meristem is artificially bent at 90° and repositioned clockwise around the netting so that it can proceed to grow up the 15.25 cm rise then run of netting (e.g. a 15.25 cm × 15.25 cm rise and run is sequentially bent into the shape of a bine staircase at an average slope of 45°). Note, images are black and white for color consistency to not distract the reader by the color variations brought about by the red and blue LED illumination under daytime versus nighttime conditions.

Figure 3

Representative example of cultivar ‘Centennial’ hop cone production within a subsection of crown guided through netting (67 days into the crop cycle). The red arrow indicates the local internode strain location at the junction of the netting 15.25 cm vertical rise transition to the 15.25 cm run and the blue arrow indicates the subsequent 15.25 cm horizontal run.

Figure 4

The kinetics of bine elongation from node 20–40 + /- standard deviation for (a), cultivar ‘Cascade’, (b), cultivar ‘Centennial’ and (c), cultivar ‘Cashmere’ control free to climb at 90° (FC; ●), free to climb a strand of 90° netting (FN; ○), free to climb twine at 45° (F45; ■), touch on twine at 45° (T45; □), touch on a strand of trellis netting at 45° slope (N45; ◆), and 90° internode bending on netting at a 45° mean slope (B90; ◇). Vertical bars represent standard deviations of six replicates (n = 6). Internode lengths among treatments are statistically different from each other (P < 0.01).

Figure 5

The hop mean dry cone yield (node 20–40; kg) across treatments for (a), cultivar ‘Cascade’. (b), cultivar ‘Cashmere’, and (c), cultivar ‘Centennial’. Control free to climb at 90° (FC), free to climb a strand of 90° netting (FN), free to climb twine at 45° (F45), touch on twine at 45° (T45), touch on a single strand of trellis netting at 45° slope (N45), and 90° internode bending on netting at a 45° mean slope (B90). Means and standard deviation of six replicates (n = 6). Yield means were not statistically different from each other (P > 0.32).