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

Figure 4 illustrates the length of fertile internodes 20–40 within the various treatments of: FC, FN, F45, T45, N45, and B90. The FC, FN, and F45 treatments grew lengthier internodes from node 20–40 than the T45, N45, and B45 treatments (Fig. 4). Internode width, however, was greater in T45, N45, and B90 as compared to the undisturbed FC, FN, and F45 treatments (Table 1). The T45 and N45 treatments had a 27.9% and 26.6% reduction in internode elongation compared to the FC, FN, and F45 treatments. Of the treatments, B90 had the shortest internodes and widest internode thickness between node 20–40 (Tables 1, 2). Due to the shorter internode lengths in the mechanically affected treatments, the density of nodes per unit area was ~ 25% greater in T45 and N45 from node 20–40 and an additional 28% shorter in B90. In other words, B90 internodes were ~ 54% shorter between nodes 20–40 as compared to the untouched treatments and had the densest node concentration (cf Fig. 4 and Table 2). Both touched and bent bines were significantly reduced in elongation (Table 1; P < 0.01 and P < 0.01 respectively). We note that although interspecific thigmomorphogenic variation has been shown in rainforest tree species¹⁷, the intraspecific internode variation within this study was not significant (Table 2). Regarding internode width, only cultivar ‘Centennial’ was significantly different from ‘Cascade’ and ‘Cashmere’ (Table 1). This may partly be due to the shared genetics of the cultivars in this study where ‘Cascade’ and Centennial’ share ‘Fuggle’ parentage and ‘Cascade’ is a parent of ‘Cashmere’.

One of the main criteria for controlled environment production in a confined space is compact plant growth. As compared to ornamental crop production where the focus is on visual quality, efficient production of edible flower crops focuses on flower quality and yield per given cultivated area. For hop production specifically, the primary goal of mechanostimulation is to control internode length under confined greenhouse space conditions without causing a reduction in the node development rate, bine yield, or flower quality¹⁸. Chemical growth regulators are one means to control plant compactness, but their toxicity to human health could potentially be carried into the hop strobili acids and oils, a primary beer ingredient e.g.¹⁹. Being that hop flowers are consumed in beer as a flavor and bitterness ingredient, we deployed mechanical perturbation as an alternative to potentially toxic synthetic growth substances. Although internode length decreased with touch, the combination of touching and bending approximately doubled the decrease in internode length in all three cultivars studied (Table 2). One could argue that the response is attributable to an “observer” effect²⁰. However, treatments T45 and N45 allowed us to tease out the potential for an “observer” effect from B90 by maintaining an identical touch stimuli among T45, N45, and B90. Furthermore, to minimize variation in the pressure applied via touch, the stimuli was consistently implemented by the same observer²¹. Thus, variation in internode lengths and widths within a cultivar and treatment was minimal; possibly a result of a consistent observer, alternatively the response we observed in hops was in the form of a gradual morphogenetic response resulting in a decrease in internode length and increase in radial expansion. This type of gradual response is the most common thigmomorphogenic response observed in plants⁴. Hence, although there is a strong correlation between the degree of longitudinal strain experienced and the extent of the thigmomorphogenic response²², reduced internode elongation and increased plant compactness are known primary thigmomorphogenic plant responses among many species²³.

No significant differences were observed among internode lengths among the FC, FN, and F45 treatments (Table 2). This is one indication that the 45° slope in the untouched F45 treatment did not impart a gravitropic effect on internode length and node development. Secondly, when a plant stem is artificially bent, the plant’s orientation to the gravity field is changed as opposed to when a plant is tilted and bends under its own weight²⁴. The resulting change in the gravity field normally makes it hard to disentangle thigmomorphogenesis from gravitropism. Hops, however, lack a self-supporting stem and thus rely on an independent physical support structure for vertical ascent. The independent support structure/trellis prevents the bine from experiencing gravity when tilted or bent at an angle because the structure counteracts the effects of gravity (e.g. F45 and B90). Thus, this study attempted to remove the gravity component as a potential culprit of decreased internode length or increased width, leaving only thigmomorphogenisis as the primary stimuli. In so doing, it was quantitatively apparent that the yields did not suffer with the introduction of a 45° bine slope or multiple mechanically induced internode bends (Fig. 5). To illustrate, Fig. 3 shows a subsection of cultivar ‘Centennial’ canopy with bent internodes supported by netting at a mean slope of 45°, the quantity of cones produced was copious (Fig. 3). Figure 4 illustrates the non-bent versus bent node elongation zone, showing a shorter internode that results in a higher density of cones along the length of the bine.

There was not a significant difference between treatments T45 and N45 (two tailed t-test; P > 0.14) where on average the intraspecific internode difference between T45 and N45 was 1.2%, 2.3%, and 5.6% for ‘Cashmere’, ‘Cascade’ and ‘Centennial’ respectively. Bending a stem introduces a local strain^14,22,25,26. It is the local strain, not force to which plants respond, signaling a modification to the active elongation zone (local perception)¹⁴. Although this would indicate that the touching and bending treatments solely have a local influence on hop node elongation, more research is needed to substantiate that assertion.

In nature, wind exerts force and a resulting strain on plants during shoot elongation²⁷. This study minimized the added effects of widespread wind strain in a controlled environment in an attempt to isolate long distance strain from that perceived at an internode. Although the elongation zone of a hop bine autonomously coils in search of vertical support, human touch and applied strain within the youngest main bine apical tissue (the youngest ~ 0.5 m of apical bine) resulted in local internode biomass reallocation from length to width. Therefore, new growth of hop bines appears to respond to both components of mechanical perturbation (touch and strain). This finding illustrates that the new growth at the apical end of the hop bine does not desensitize from repeated touching and bending, at least not when the stimulus occurs once per 24 h. Moreover, it is likely that this zone of growth would be the most affected by the stimuli e.g.^1,28.

With respect to cone yield, the FT, FN, F45, T45, N45, and B90 treatments for cultivars ‘Cascade’, ‘Centennial’, and ‘Cashmere’ were not different (Fig. 5; two-way ANOVA; P > 0.32). Moreover, the flower yield of treatments T45, N45, and B90 was not significantly different than FC, FN, and F45 (Fig. 5) nor was the mean flower dry weight affected by touch or bending treatments. Regardless of similar yields among treatments, the benchmark for comparison of hops grown under any type of controlled-environment conditions are the α and β brewing characteristics as compared to field-grown hops. The analysis of α and β acids present in hop flowers describes hop cone chemical quality for brewers. The acid concentrations determine market value²⁹. The percentages of α and β acids in the cone are used in the brewing process to estimate the bitterness profile in beer. Comparing measured values in this study with field grown literature values showed that α and β acids were not adversely impacted by controlled-environment conditions nor were they impacted by the thigmomorphogenic stimuli (Table 3). Furthermore, the concentrations of α and β acids grown under controlled-environment conditions with mechanical perturbation were within the typical cultivars’ field-grown α and β-acid range (Table 3)^{30,31,32,33,34,35}.

Future experiments should look at the combination of mechanical perturbation with an artificially extended vegetative cycle because hop bine node quantity is directly related to total bine yield. Our controlled environment space constraints did not permit variation in the quantity of nodes developed among treatments prior to photoperiod induced flower initiation. However, hop bines grew to 22.5 m when artificially extending the vegetative phase via photoperiod extension³⁶. Given that hop bines in the vegetative cycle under controlled environment conditions produce a new visible node approximately every 1–2 d⁻¹ (depending on variety), it could be economically valuable to extend the vegetative portion of the crop cycle in order to increase overall crop yield. The reasons are hop production cycles necessitate a protracted juvenile phase during which they are incapable of flowering⁷. This stipulation not only results in bines that are of significant length prior to reaching maturity, it reduces their productivity to only the adult phase nodes that are present in a confined space i.e. nodes > 12–25. Thus, amassing many fertile nodes per vertical distance within a high sidewall greenhouse (e.g. ≥ 6 m) would be one viable means to increase the yield potential of hop in controlled environment production as long as plant resources did not become limiting. What’s more the 15.25 cm rise over run staircase created by the B90 internode bending treatment would allow for approximately double the bine length from the container to the top of a high sidewall greenhouse as compared to a vertically trellised bine (an additional direct step toward increasing node quantity per unit vertical production area). Secondly, the time and resource investment in overcoming the hop cultivar specific 11–24 infertile juvenile phase adds approximately three weeks to a single hop crop cycle e.g.^11,36. Thus, it would be more time and space efficient to grow fewer crop cycles per annum that contain larger amounts of fertile nodes within a cycle as compared to additional cycles that contain the unfertile juvenile phase.

In conclusion, repeated touch and/or bine bending within the active elongation zone of hop bines resulted in shortened internode length with higher cone production per given area. Mechanical stimuli did not reduce cone yield or flower quality. The results demonstrate that successive local internode strain can aid the control of internode elongation. Moreover, the study provides evidence that thigmomorphogenic cues can be used as a management tool to increase bine compactness and increase node density per unit area. This finding is especially important for growth control when production space is limiting and/or of high-value (e.g. greenhouse production)¹. Hence, mechanical perturbation was an effective non-chemical means to control hop internode length. Nonetheless, models aimed at predicting internode length of hop bines in response to strain should still take into account a cultivar parameter. The results are practical on a commercial scale because the methods of touch and bending used in this study are easy to apply with minimal investment in labor, have a short time interval of application (approximately 5–10 s⁻¹ per bine per 24 h), and the application duration is relatively short ~ 30 days out of the 90–120 day crop cycle, making this a practical endeavor when one considers that high value vine crops are already repeatedly handled by humans throughout their production cycle (e.g. viticulture grape and controlled environment cucumber production).

Source: Ecology - nature.com