Vegetative Growth and Phenology of Hop Cultivars in Successive Growing Seasons with Supplemental Artificial Lighting in a Subtropical Climate

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

-1. The Introduction should state more explicitly that this is not a comparison between supplemented and non-supplemented photoperiods.
1) The claim that supplemental lighting can overcome subtropical photoperiod limitations is plausible, but in this manuscript, it is not experimentally tested against an unsupplemented treatment.
2)The objective should be narrowed to 'characterizing cultivar growth and phenology under a supplemental-lighting management system' rather than implying that the effect of supplemental lighting itself was evaluated.
3) If the central limitation is short photoperiod, why was no non-supplemented control included to quantify the actual contribution of the 17-h photoperiod treatment?

- 2. The lighting description is not sufficient to establish uniform treatment exposure. 
1) The lamps were installed every 10 m along the row, with a reported photon flux of 25 μmol m^-2 s^-1, but the manuscript does not report spatial uniformity, measurement height, nighttime light intensity at canopy positions, or possible differences among cultivars / blocks in actual light exposure.

-3. The calculation and interpretation of hop growth rate require reconsideration. 
1) The manuscript defines HGR as Ht / GD, where Ht is height at a given time, and GD is accumulated degree days. This is closer to cumulative height per cumulative thermal time than to an actual growth rate. 
2) A growth rate should generally be calculated as an increment, such as Δheight / ΔGDD, or derived from the fitted growth curve.
3) The base temperature of 0 °C for degree-day accumulation should be justified. This assumption may strongly affect GDD values, HGR estimates, and stage classification. If this value was adopted from previous hop studies, the authors should explain whether it is appropriate for subtropical production and for the cultivars evaluated here.
4)The experiment used a randomized block design with 15 cultivars, 4 replications, and 3 plants per plot, but the analysis section only states that fresh mass and lateral shoots were analyzed by ANOVA and grouped by Scott-Knott. 
5)It is not clear whether plants or plot means were used, whether blocks were included, and how repeated measurements across successive seasons on the same planting were handled.

- 4. Was actual nighttime irradiance measured at multiple positions within each block and cultivar row?

- 5. Were the same plants followed across seasons, and if so, how was repeated seasonal dependence handled statistically?

-6. The Results contain a large amount of cultivar-level information, but the interpretation of cultivar performance is complicated by incomplete cycles. 
1) Many cultivars did not complete their phenological cycle in one or more seasons, yet the analysis of model parameters and earliness is performed only for cultivars that completed the cycle. 
2)This creates a potential selection bias: cultivars that failed to complete the cycle are excluded from the fitted comparisons, even though failure to complete the cycle is itself one of the most important adaptation outcomes.

- 7. The abbreviation 'NT' is potentially misleading. 
1) In several cases, cultivars were not simply 'not tested'; rather, they did not complete the phenological cycle. This should be clearly distinguished.
2) The classification of cultivars as early, mid-season, or late should include the category of 'incomplete cycle' as a biologically meaningful outcome.
3) The authors should avoid presenting earliness only among successful cultivars, because this may overstate adaptability.

- 8. The statement that all evaluated cultivars reached a standard maximum height also needs caution. 
1) This statement applies only to cultivars with fitted models in a given season, not to all 15 cultivars across all seasons. 
2) Several cultivars did not complete the cycle in specific seasons.

- 9. The fresh mass interpretation is also problematic. 
1) The manuscript defines plant fresh mass as including stems, lateral shoots, leaves, and cones, and then treats it partly as a proxy for yield. 
2) However, actual cone yield was not separately reported, and cone quality was not measured. 
3)The statement that early cultivars likely had higher cone yield because plant fresh mass correlates with cone production is inferential, not directly demonstrated by the data.

- 10. Seasonal effects are confounded with plant age and establishment status: the summer season was the first crop after planting, while the winter and fall seasons were the second and third cycles, when rhizome reserves may have increased. 
1)The authors themselves acknowledge this possibility, but the Abstract still states that air temperature was the primary modulating factor.
2) Temperature may be an important factor, but it is not experimentally separated from plant age, reserve accumulation, pruning season, rainfall, and solar radiation. The phrase 'air temperature was the primary modulating factor' should be softened unless a formal analysis demonstrates this relationship.

- 11. The discussion of Sorachi Ace should also be more cautious. 
1)The cultivar completed all three cycles and showed relatively stable growth parameters, which is important. 
2) However, 'superior performance' should not be based only on growth stability, especially because lateral shoot number and fresh mass varied among seasons, and cone yield / quality were not reported separately.

- 12. The conclusion that hop development in successive seasons is viable under subtropical conditions with supplemental lighting is promising, but it should be substantially qualified. 
1) Only Sorachi Ace, Triple Pearl, and Zeus completed the phenological cycle in all three evaluated seasons, while many cultivars failed to complete at least one cycle. 
2) The manuscript also notes that these results were obtained during the first productive years of the planting, which limits generalization to mature hop yards. The conclusion should not imply broad viability for all cultivars.
3)The recommendation should be cultivar-specific and season-specific.
4) Since cone yield and quality are not reported separately, the study should not imply production success beyond vegetative and phenological feasibility.
5)The conclusion should emphasize that further studies with mature plants, unsupplemented controls, cone yield, cone quality, and multi-year validation are needed.

Thank you.

Author Response

Dear Reviewer 1,
We sincerely thank you for the careful evaluation of our manuscript and for the valuable comments and suggestions provided. Your insightful observations greatly contributed to improving the quality and clarity of the manuscript. We have carefully considered all comments and revised the manuscript accordingly. All modifications have been incorporated throughout the text, and the changes are highlighted and addressed point-by-point in the response provided below.
The authors.
________________________________
Comments and Suggestions for Authors
Comment 1: The Introduction should state more explicitly that this is not a comparison between supplemented and non-supplemented photoperiods.
1) The claim that supplemental lighting can overcome subtropical photoperiod limitations is plausible, but in this manuscript, it is not experimentally tested against an unsupplemented treatment.
2)The objective should be narrowed to 'characterizing cultivar growth and phenology under a supplemental-lighting management system' rather than implying that the effect of supplemental lighting itself was evaluated.
3) If the central limitation is short photoperiod, why was no non-supplemented control included to quantify the actual contribution of the 17-h photoperiod treatment?
Response 1: Thank you for pointing that out. We agree with that comment. 1) In fact, the objective of the work was not to test the efficiency of supplemental lighting itself (which would require a no-light control), but rather to characterize the behavior of different cultivars within a system that adopts this technology as a management premise in the subtropical region. 2) To avoid any ambiguity, the Introduction and Objective have been reformulated (Page 2, Lines 50-51; 56-60; 69-71; 75-77), making it clear that supplemental lighting is a fixed condition of the evaluated cultivation system, and the focus of the study is restricted to the characterization of the growth and phenology of the cultivars under this management. 3) Supplementation was not evaluated in this study due to space limitations in the experimental area, which prevented the housing of all the analyzed cultivars under no-light conditions.
Comment 2. The lighting description is not sufficient to establish uniform treatment exposure. 
1) The lamps were installed every 10 m along the row, with a reported photon flux of 25 μmol m-2 s-1, but the manuscript does not report spatial uniformity, measurement height, nighttime light intensity at canopy positions, or possible differences among cultivars / blocks in actual light exposure..
Response 2: Thank you for pointing that out. We agree with the comment. Photosynthetically Active Radiation (PAR) measurements were possible last year after the implementation of an automatic weather station equipped with specific sensors. The monitoring is recent and does not cover the entire study period. However, complementary analyses were performed with the LI-180 spectrometer, which allows for the characterization of the spectral distribution of radiation emitted by the LED lamps used in the experiment. This assessment was added as supplementary material (page 3, lines 100-104).
Comment 3. The calculation and interpretation of hop growth rate require reconsideration. 
1) The manuscript defines HGR as Ht / GD, where Ht is height at a given time, and GD is accumulated degree days. This is closer to cumulative height per cumulative thermal time than to an actual growth rate. 
2) A growth rate should generally be calculated as an increment, such as Δheight / ΔGDD, or derived from the fitted growth curve.
3) The base temperature of 0 °C for degree-day accumulation should be justified. This assumption may strongly affect GDD values, HGR estimates, and stage classification. If this value was adopted from previous hop studies, the authors should explain whether it is appropriate for subtropical production and for the cultivars evaluated here.
4)The experiment used a randomized block design with 15 cultivars, 4 replications, and 3 plants per plot, but the analysis section only states that fresh mass and lateral shoots were analyzed by ANOVA and grouped by Scott-Knott. 
5)It is not clear whether plants or plot means were used, whether blocks were included, and how repeated measurements across successive seasons on the same planting were handled.
Response 3: Thank you for pointing this out. We agree with the comment. 1 and 2) We acknowledge that there was an error in the mathematical description of the HGR calculation in the original manuscript. The procedure has been revised in the text to correctly express that the growth rate was calculated as an increment every 7 days (Δ height / ΔGDD) (page 4, lines 155-159). 3) The adoption of Tbase = 0 was based on previous studies on hop cultivation conducted under similar subtropical conditions; this justification has been added to the manuscript (Page X, Line Y). 4 and 5) For the variables of plant fresh mass and number of lateral branches, data from all individual plants in the experiment (3 plants per plot) were collected and subjected to analysis of variance (ANOVA), fully considering the randomized block design with the cultivar and block factors included in the statistical model, followed by Scott-Knott mean grouping. For the variables of plant height and vertical growth rate (HGR), which were evaluated over time and subjected to non-linear regression analyses, all plants in the experiment were also measured; then, the average per cultivar was calculated on each specific evaluation date to proceed with the adjustment of the regression models. Regarding the treatment of successive crops, the analyses were performed separately for each season (summer, winter, and autumn), allowing for the evaluation of the behavior and adaptation of the cultivars in the face of the seasonal climatic variations of each growing period. The text of the Statistical Analysis section in the manuscript has been reformulated (Page 5, Lines 182-189).
Comment 4. Was actual nighttime irradiance measured at multiple positions within each block and cultivar row?
Response 4: Thank you for pointing that out. The distribution of photosynthetically active radiation (PAR) emitted by the LEDs was characterized using the LI-180 spectrometer (LI-COR Biosciences, Lincoln, Nebraska, USA). Measurements were taken at ground level in a representative sample area consisting of six rows. To avoid the shading effect of the canopy, the assessments were carried out before plant sprouting in the 2024-2025 autumn crop (page 3, lines 100-104).
Comment 5. Were the same plants followed across seasons, and if so, how was repeated seasonal dependence handled statistically?
Response 5: Thank you for pointing that out. We clarify that the same plants and plots were, in fact, monitored over time. However, the statistical treatment adopted circumvented the classic seasonal and temporal dependence in two main ways. Regarding the level of data grouping, regression analyses for vegetative growth were not applied to the individual measurements of each plant, but rather using the average of the plots and blocks for each cultivar on each evaluation date. This procedure reduces the individual variation of each experimental unit over time. Furthermore, the seasons were not included as a fixed factor in a factorial arrangement due to a biological and adaptive limitation of the plants. There was a marked variation in cultivar development between harvests, such that few cultivars managed to complete the phenological cycle in all three seasons evaluated. Imposing a factorial model would cause a massive loss of data due to imbalance and would drastically reduce the number of genotypes with results to be presented.
Comment 6. The Results contain a large amount of cultivar-level information, but the interpretation of cultivar performance is complicated by incomplete cycles. 
1) Many cultivars did not complete their phenological cycle in one or more seasons, yet the analysis of model parameters and earliness is performed only for cultivars that completed the cycle. 
2)This creates a potential selection bias: cultivars that failed to complete the cycle are excluded from the fitted comparisons, even though failure to complete the cycle is itself one of the most important adaptation outcomes.
Response 6: Thank you for pointing that out. We agree with the comment. To correct this approach, we have reformulated the Results and Discussion section to integrate the performance of cultivars with incomplete cycles (page 5, lines 192-204).
Comment 7. The abbreviation 'NT' is potentially misleading. 
1) In several cases, cultivars were not simply 'not tested'; rather, they did not complete the phenological cycle. This should be clearly distinguished.
2) The classification of cultivars as early, mid-season, or late should include the category of 'incomplete cycle' as a biologically meaningful outcome.
3) The authors should avoid presenting earliness only among successful cultivars, because this may overstate adaptability.
Response 7: Thank you for pointing this out. The abbreviation ‘NT’ has been replaced by ‘IC’ Incomplete Cycle in all tables (page 6, lines 224-226; page 10, lines 352-354; page 12, lines 413-418; page 14, lines 449-452). An explanatory note listing the incomplete cycle cultivars in each crop season has been added to the captions of the corresponding figures (page 7, lines 250-253; page 11, lines 372-375; page 16, lines 519-522).
Comment 8. The statement that all evaluated cultivars reached a standard maximum height also needs caution. 
1) This statement applies only to cultivars with fitted models in a given season, not to all 15 cultivars across all seasons. 
2) Several cultivars did not complete the cycle in specific seasons.
Response 8: Thank you for pointing this out. We agree with the comment. To correct this inaccuracy, we have reviewed and refined the wording of the corresponding paragraphs in the Results and Discussion section (Page 8, Lines 264-267; 271-274).
Comment 9. The fresh mass interpretation is also problematic. 
1) The manuscript defines plant fresh mass as including stems, lateral shoots, leaves, and cones, and then treats it partly as a proxy for yield. 
2) However, actual cone yield was not separately reported, and cone quality was not measured. 
3)The statement that early cultivars likely had higher cone yield because plant fresh mass correlates with cone production is inferential, not directly demonstrated by the data.
Response 9: Thank you for pointing that out. We agree with the comment. To avoid indirect inferences about productivity, we have reformulated the corresponding passage in the Results and Discussion section (Page 14, Lines 455-456; Page 14-15, Lines 467-470; Page 15, Lines 472-477).
Comment 10. Seasonal effects are confounded with plant age and establishment status: the summer season was the first crop after planting, while the winter and fall seasons were the second and third cycles, when rhizome reserves may have increased. 
1)The authors themselves acknowledge this possibility, but the Abstract still states that air temperature was the primary modulating factor.
2) Temperature may be an important factor, but it is not experimentally separated from plant age, reserve accumulation, pruning season, rainfall, and solar radiation. The phrase 'air temperature was the primary modulating factor' should be softened unless a formal analysis demonstrates this relationship.
Response 10: Thank you for pointing this out. We acknowledge that, since this is a field experiment conducted in successive cycles with a perennial species, the effects of climatic variables (such as temperature) occur concomitantly and are intertwined with plant aging, pruning times, and the progressive accumulation of carbon reserves in the rhizome over time. To address your recommendation, we have reformulated the corresponding sentence in the Abstract (Page 1, Lines 25-27) and in the Conclusion (Pages 18, Lines 597-600), softening the statement and contextualizing the interaction between climate and crop development.
Comment 11. The discussion of Sorachi Ace should also be more cautious. 
1)The cultivar completed all three cycles and showed relatively stable growth parameters, which is important. 
2) However, 'superior performance' should not be based only on growth stability, especially because lateral shoot number and fresh mass varied among seasons, and cone yield / quality were not reported separately.
Response 11: Thank you for pointing this out. To correct the interpretation with due caution, we have reformulated the final part of the paragraph in the Results and Discussion section (Page 9, Lines 332-336) and in the Conclusion (Page 18, Lines 609-613).
Comment 12. The conclusion that hop development in successive seasons is viable under subtropical conditions with supplemental lighting is promising, but it should be substantially qualified. 
1) Only Sorachi Ace, Triple Pearl, and Zeus completed the phenological cycle in all three evaluated seasons, while many cultivars failed to complete at least one cycle. 
2) The manuscript also notes that these results were obtained during the first productive years of the planting, which limits generalization to mature hop yards. The conclusion should not imply broad viability for all cultivars.
3)The recommendation should be cultivar-specific and season-specific.
4) Since cone yield and quality are not reported separately, the study should not imply production success beyond vegetative and phenological feasibility.
5)The conclusion should emphasize that further studies with mature plants, unsupplemented controls, cone yield, cone quality, and multi-year validation are needed.
Response 12: Thank you for pointing this out. We agree with the comment. In response to your comments, the Conclusions section has been substantially restructured to reflect the actual limitations of the study, eliminating generalizations (Page 18, Lines 593-617).

Reviewer 2 Report

Comments and Suggestions for Authors

 

I find the article interesting because it studies and analyzes several hop varieties and the possibility of obtaining successive harvests in emerging hop-growing areas. To achieve this, they used supplemental artificial light to complement the daily photoperiod and extend the plants' growth phase.

The objective is well-defined and aims to characterize the vegetative growth and phenology of hop cultivars grown in successive harvests under supplemental (artificial) lighting in a subtropical climate region.

In Figure 1, the rainfall data is not clearly represented.

I would like to know how the varieties were selected. Also, if any of the 15 varieties tested are grown in the area as a single crop. Looking at the bibliographic references, it appears the authors have conducted previous work in this area. 

The results show that some varieties did not complete the cycle, leading me to believe that these varieties did not adapt to the subtropical climate of the area and did not respond to supplemental artificial light. However, were there any climatic factors that could have also affected them? Which factor do you think was most limiting in the development of successive harvests: temperature, light, or crop management?

The conclusions are consistent with the stated objectives and conclude with the recommendation that the Sorachi Ace cultivar showed promise for the successive cropping system, exhibiting a uniform and stable growth pattern that remained constant and independent of seasonality and air temperature variations observed in each growing season.

Author Response

Dear Reviewer 2,
We sincerely thank you for the careful evaluation of our manuscript and for the valuable comments and suggestions provided. Your insightful observations greatly contributed to improving the quality and clarity of the manuscript. We have carefully considered all comments and revised the manuscript accordingly. All modifications have been incorporated throughout the text, and the changes are highlighted and addressed point-by-point in the response provided below.
The authors.
________________________________

Comments and Suggestions for Authors:
Comment 1: I find the article interesting because it studies and analyzes several hop varieties and the possibility of obtaining successive harvests in emerging hop-growing areas. To achieve this, they used supplemental artificial light to complement the daily photoperiod and extend the plants' growth phase.
Response 1: Thank you for pointing that out. We are very pleased to know that the manuscript was considered interesting and relevant to emerging areas of hop cultivation.
Comment 2: The objective is well-defined and aims to characterize the vegetative growth and phenology of hop cultivars grown in successive harvests under supplemental (artificial) lighting in a subtropical climate region.
Response 2: Thank you for pointing that out. We are pleased that the scope of the work has been clearly understood.
Comment 3: In Figure 1, the rainfall data is not clearly represented.
Response 3: Thank you for pointing that out. We clarify that Figure 1 was constructed using a dual ‘y’ axis layout to combine the different meteorological variables in the same graph. Precipitation data are represented by the vertical bars, and their corresponding values should be read on the right vertical axis. On the other hand, the left vertical axis indicates the temperature values, represented by the lines. To avoid any ambiguity or difficulty in reading, we have revised the legend for Figure 1 (Page 3, Lines 119-120).
Comment 4: I would like to know how the varieties were selected. Also, if any of the 15 varieties tested are grown in the area as a single crop. Looking at the bibliographic references, it appears the authors have conducted previous work in this area. 
Response 4: We thank the reviewer for their interest in the history of the introduction of plant materials. We clarify that the choice of the 15 cultivars did not follow a prior selection criterion based on agronomic performance, but rather an exploratory screening process. Initially, we tested only 5 cultivars in our area, which generated the results published previously. With the receipt of seedlings of new cultivars through donations for research purposes, the experimental area was completely restructured into a new statistical layout to accommodate all 15 genotypes simultaneously. The central objective is to test the largest possible number of genetic materials to identify biotic and abiotic limitations, providing unprecedented answers about the phenotypic plasticity and adaptation of hops in the region.
Comment 5: The results show that some varieties did not complete the cycle, leading me to believe that these varieties did not adapt to the subtropical climate of the area and did not respond to supplemental artificial light. However, were there any climatic factors that could have also affected them? Which factor do you think was most limiting in the development of successive harvests: temperature, light, or crop management?
Response 5: Thank you for pointing that out. We agree with the premise that incomplete cycle cultivars demonstrated less phenotypic plasticity under the evaluated conditions. When considering the most limiting factor for the success of successive harvests, we believe that the interaction between light (artificial photoperiod) and crop management constitutes the main bottleneck, overshadowing temperature alone. As hops are essentially photoperiodic plants, artificial light management becomes the most challenging factor in the subtropical region. Since light supplementation was identical for all cultivars, variables such as seasonal temperature fluctuations, added to the age and establishment of plants throughout the cycles, acted as important modulators of vegetative growth and phenology in this study.
Comment 6: The conclusions are consistent with the stated objectives and conclude with the recommendation that the Sorachi Ace cultivar showed promise for the successive cropping system, exhibiting a uniform and stable growth pattern that remained constant and independent of seasonality and air temperature variations observed in each growing season.
Response 6: Thank you for pointing that out. The Conclusions section has been substantially restructured to reflect the actual limitations of the study (Page 18, Lines 593-617).

 

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The overinterpretation claimed in the original manuscript has been significantly mitigated.

There are no further comments.

Thank you.