What Research Actually Supports About Light, CO₂, Water, Tipburn and Leaf Quality
Bok choy, also called pak choi, is a fast-growing leafy vegetable belonging to Brassica rapa subsp. chinensis.
It often appears forgiving.
Large green leaves and thick petioles can develop quickly, which makes it tempting to manage bok choy with a simple recipe:
more light + more CO₂ + enough humidity = faster production.
Current research shows that the crop is more complicated than that.
There is no scientifically established stage-by-stage combination of:
PAR
CO₂
and:
VPD
that guarantees:
- maximum fresh yield
- firm petioles
- tender leaves
- no tipburn
- long shelf life
Instead, direct pak choi research shows important interactions among:
- Daily Light Integral
- PPFD
- photoperiod
- light spectrum
- CO₂
- temperature
- root-zone water
- cultivar
- calcium transport
- airflow around young leaves
The better greenhouse strategy is therefore to measure the actual crop environment and connect those measurements with the production trait that matters.
Quick Reference
| Variable | What It Tells You | What Pak Choi Research Supports |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the leaves now | Recent direct research used roughly 167–350 µmol/m²/s; higher PPFD was not always the most efficient way to increase biomass |
| DLI | Total photosynthetic light accumulated during the day | Direct 2025 research tested 10.8–18.9 mol/m²/day and found growth and several nutrients increased with DLI |
| Photoperiod | Hours of light per day | At the same DLI, longer photoperiod with lower PPFD produced more biomass than shorter photoperiod with higher PPFD |
| CO₂ | Carbon available for photosynthesis | Direct pak choi research at ~800 vs ~400 ppm increased growth and several leaf-quality traits |
| VPD | Atmospheric evaporative demand | Useful for water-demand diagnosis, but no validated stage-specific bok choy optimum is established |
| Root-zone water | Water available to support growth | Greenhouse research shows water-management strategy strongly affects yield, roots and water-use efficiency |
| Temperature | Influences growth, water demand and heat injury | Direct heat-stress research shows strong cultivar-dependent losses at high temperature |
| Airflow / calcium transport | Important for developing leaves | Tipburn risk rises under rapid high-DLI growth and should not be interpreted as a simple VPD problem |
These are research references rather than universal crop specifications.
1. Bok Choy and Pak Choi Refer to the Same Crop Group
“Bok choy” and “pak choi” are commonly used names for non-heading Chinese cabbage within:
Brassica rapa subsp. chinensis.
Cultivars differ substantially in:
- leaf color
- petiole color
- plant size
- heat tolerance
- growth rate
- response to light
That cultivar effect matters throughout this article.
A number that works well for one pak choi cultivar is not automatically optimal for every other cultivar.
2. PAR and DLI Answer Different Questions
PPFD measures photosynthetic photon flux at one moment.
It is expressed in:
µmol/m²/s
It answers:
How much photosynthetic light is reaching the bok choy canopy right now?
DLI measures the amount of PAR accumulated during an entire day.
It is expressed in:
mol/m²/day
It answers:
How much photosynthetic light did the crop receive today?
For constant artificial light:
DLI = PPFD × photoperiod × 0.0036
For example:
167 µmol/m²/s × 18 h
≈ 10.8 mol/m²/day
208 µmol/m²/s × 18 h
≈ 13.5 mol/m²/day
250 µmol/m²/s × 18 h
≈ 16.2 mol/m²/day
292 µmol/m²/s × 18 h
≈ 18.9 mol/m²/day
These values are particularly useful because they correspond directly to a modern pak choi experiment.
3. Pak Choi Now Has Excellent Direct DLI Research
A 2025 Wageningen University study examined three pak choi cultivars:
- ‘Hybrid Special’
- ‘Red Summer’
- ‘Shanghai Green’
under four DLIs:
10.8
13.5
16.2
and:
18.9 mol/m²/day.
The researchers created those DLIs in two different ways.
Method A
Keep PPFD around:
250 µmol/m²/s
and extend photoperiod from:
12 → 21 hours.
Method B
Keep photoperiod at:
18 hours
and increase PPFD from:
167 → 292 µmol/m²/s.
This experimental design tells us something much more useful than a normal DLI study.
4. The Same DLI Did Not Always Produce the Same Growth
At equal daily photon totals, plants generally produced more biomass when the light was distributed across a:
longer photoperiod at lower PPFD
rather than compressed into:
shorter periods at higher PPFD.
That means:
DLI alone does not describe the complete light environment.
How those photons are delivered also matters.
5. Longer Photoperiod Produced Much More Fresh-Weight Gain
Across the tested DLI range:
increasing DLI by extending photoperiod increased fresh weight by approximately:
67%.
Increasing DLI primarily by raising PPFD produced only about:
26%
fresh-weight gain.
This is one of the strongest recent bok-choy-specific lighting results.
It directly challenges the assumption:
If I want faster bok choy, I should simply increase PPFD.
6. The Highest PPFD Was Not the Best Treatment at the Same DLI
The study also included:
350 µmol/m²/s × 15 h
which produced approximately the same:
18.9 mol/m²/day
as other high-DLI treatments.
Yet the:
350 PPFD treatment produced the lowest fresh weight among treatments receiving the same 18.9 DLI.
This is extremely important.
It means:
more instantaneous light was less efficient than spreading the same photons across a longer day.
7. Why Can Lower PPFD Be More Efficient?
Photosynthesis does not remain perfectly linear as PPFD rises.
At relatively high light:
- photosynthetic efficiency per photon can decrease
- more photons can arrive after parts of the photosynthetic system are becoming saturated
- additional electrical input may produce smaller biomass gains
A longer photoperiod with lower intensity can sometimes allow the crop to use daily photons more efficiently.
The exact response remains cultivar-dependent.
8. Light-Use Efficiency Differed Between Cultivars
The 2025 study showed clear cultivar differences.
For example, ‘Hybrid Special’ became less light-efficient as PPFD increased from approximately:
167 → 292 µmol/m²/s.
‘Shanghai Green’ showed a more stable response.
‘Red Summer’ also behaved differently, potentially partly because its anthocyanin-rich leaves handle light differently.
This is why AquaHorti should avoid presenting one:
“Bok Choy PPFD = X”
as if the entire crop category behaves identically.
9. Increasing DLI Still Increased Growth Overall
The fact that higher PPFD was not always the most efficient strategy does not mean daily light is unimportant.
Across:
10.8 → 18.9 DLI
fresh and dry biomass generally increased.
So a useful broad conclusion is:
Bok choy responds positively to increasing DLI across approximately 11–19 mol/m²/day in direct controlled-environment research.
But:
how the DLI is delivered matters.
10. A Practical DLI Reference
Based on the 2025 direct experiment:
approximately 11–19 mol/m²/day
is a strong research-based comparison range for actively growing pak choi under controlled production.
This should not be described as:
the universal commercial optimum.
Cultivar, temperature, CO₂, production stage and lighting economics still matter.
11. Higher DLI Also Changed Leaf Structure
The study measured:
specific leaf area — SLA.
SLA decreased as DLI increased.
Lower SLA generally means:
more leaf dry matter per unit leaf area
and often a denser or thicker leaf structure.
This gives us direct evidence that the light environment can affect leaf anatomy.
But it does not justify saying:
“250 PPFD gives firm leaves and 450 PPFD makes them soft.”
The relationship is more complex.
12. Leaf Texture Is Not Controlled by PAR Alone
Commercial bok choy texture depends on:
- tissue water status
- leaf maturity
- petiole structure
- temperature
- cultivar
- mineral nutrition
- calcium movement
- harvest timing
- postharvest water loss
PPFD is only one part of that system.
Therefore:
PAR is not a direct crispness meter.
13. Higher DLI Also Increased Nutritional Compounds
The 2025 study found higher DLI increased several valuable components.
These included:
vitamin C
total soluble sugars
and important glucosinolates such as:
- glucobrassicanapin
- glucobrassicin
This creates another important production tradeoff.
Lighting influences:
yield
and:
nutritional chemistry.
The best strategy depends on which characteristic matters commercially.
14. Higher DLI Did Not Improve Shelf Life
This is particularly relevant because the old AquaHorti article directly linked a preharvest environmental range to better shelf life.
The 2025 study stored pak choi leaves at:
4°C
and evaluated visual quality for:
21 days.
Despite differences in production DLI:
DLI did not significantly affect shelf life or overall visual quality.
That directly argues against claiming:
a specific preharvest PAR or VPD automatically improves bok choy shelf life.
15. Shelf Life Is a Separate Production Problem
After harvest, bok choy quality is strongly influenced by:
- temperature
- moisture loss
- respiration
- packaging
- mechanical damage
- decay
- harvest maturity
UC Davis postharvest guidance describes good bok-choy quality as:
- glossy
- firm
- adequately hydrated
- free from yellowing and wilting
and recommends cold storage around:
2°C
while noting that lower temperatures near freezing carry freezing risk.
So:
preharvest environment matters, but postharvest handling remains critical.
16. High DLI Increased Tipburn Risk
There was one important negative response to high DLI in the 2025 experiment:
tipburn incidence increased as DLI increased.
All three cultivars showed greater tipburn tendency at high daily light.
The effect was especially noticeable when higher DLI was produced through:
longer photoperiod.
This demonstrates a classic high-productivity tradeoff.
17. Faster Growth Can Increase Tipburn Risk
Tipburn is generally associated with local calcium deficiency in rapidly expanding young leaves.
A plant can contain adequate calcium overall while the growing leaf margin receives insufficient calcium at the critical moment.
Rapid growth can therefore increase risk.
That means:
the lighting strategy producing the fastest biomass accumulation is not automatically the strategy producing the highest marketable percentage.
18. Tipburn Is Not Simply “High VPD”
The old type of crop guide can easily make this mistake.
Tipburn risk involves factors such as:
- rapid leaf expansion
- calcium transport
- transpiration distribution
- humidity
- airflow
- root-zone calcium availability
- temperature
- cultivar
A single VPD number cannot fully describe that system.
19. High Humidity Can Sometimes Help Tipburn — Which Shows Why the Story Is Complex
Research cited in the 2025 pak choi study notes that very high nighttime humidity can sometimes improve calcium transport to young leaf margins.
Other leafy-vegetable research shows that targeted airflow around the growing point can reduce tipburn under:
- high PPFD
- high temperature
- elevated CO₂
conditions.
This is important because it shows why the statement:
“higher VPD near harvest improves leaf quality”
is far too simplistic.
20. Airflow and VPD Are Not the Same Variable
Airflow influences:
- boundary-layer resistance
- local transpiration
- canopy temperature
- leaf drying
VPD describes:
the atmospheric vapor-pressure difference.
A grower can change airflow without changing room VPD very much.
Likewise, two greenhouses with the same VPD can have very different canopy airflow.
They should not be treated as interchangeable.
21. Blue Supplemental Light Has Direct Greenhouse Evidence
A 2018 greenhouse study tested supplemental blue light at:
0
50
100
and:
150 µmol/m²/s
during the final:
10 days before harvest.
The study included both:
- green-leaf
- red-leaf
pak choi.
The results again showed:
maximum intensity did not optimize every trait.
22. About 50 PPFD Supplemental Blue Favored Yield
In both cultivars:
50 µmol/m²/s supplemental blue
produced the highest yield in that experiment.
It also favored:
- chlorophyll
- sugars
Increasing blue intensity beyond that did not simply continue increasing yield.
23. Higher Blue Light Favored Some Quality Compounds
As supplemental blue intensity increased:
- vitamin C increased
- carotenoids increased
- nitrate decreased
Supplemental lighting also increased several:
- phenolics
- flavonoids
- anthocyanins
- glucosinolates
relative to no supplemental blue.
The authors therefore distinguished approximately:
50 PPFD blue for yield
from:
100 PPFD blue for quality-related objectives.
24. Again: Maximum Yield and Maximum Nutritional Quality Are Different Targets
This pattern now appears in multiple bok choy experiments.
One lighting treatment may favor:
fresh mass.
Another may favor:
vitamin C.
Another may reduce:
nitrate.
Another may improve:
glucosinolate concentration.
Therefore:
“best light” must always be tied to a specific production objective.
25. Spectrum Also Changes Pak Choi Sprout Chemistry
Pak choi sprout research under:
- white
- blue
- red
LEDs shows clear differences in carotenoid accumulation and metabolism.
Other research combining white and blue LEDs found lower fresh mass but higher dry-matter percentage compared with white light alone.
Again:
same crop + different spectrum = different morphology and chemistry.
PPFD alone cannot capture that difference.
26. CO₂ Has Strong Direct Pak-Choi-Specific Evidence
Unlike some microgreen crops, bok choy has very useful direct CO₂ research.
A 2021 greenhouse study grew:
‘Wuyueman’ pak choi
under approximately:
400 ppm CO₂
and:
800 ± 50 ppm CO₂.
CO₂ treatment began around the:
four-real-leaf stage
and continued for approximately:
40 days.
27. 800 ppm Increased Growth
Elevated CO₂ increased traits including:
- plant height
- leaf size
- leaf number
relative to ambient conditions.
It also increased:
net photosynthetic rate
and:
light saturation point.
At the same time:
- transpiration rate decreased
- light compensation point decreased
This is direct evidence that bok choy physiology responds strongly to elevated CO₂.
28. 800 ppm Also Changed Leaf Nutritional Quality
Compared with approximately 400 ppm, 800 ± 50 ppm increased:
vitamin C by about 7.7%
chlorophyll by about 33.5%
soluble protein by about 3.4%
soluble sugar by about 21.9%
and:
leaf water content by about 1.6%.
Organic acid content declined by approximately:
26%.
This is unusually detailed crop-specific CO₂ evidence.
29. Does This Mean Bok Choy Should Always Be Grown at 800 ppm?
No.
The study compared essentially:
ambient vs one elevated concentration.
It did not map a full commercial response curve across:
400 / 500 / 600 / 700 / 800 / 1000 / 1200 ppm.
Therefore:
800 ppm is a strong research reference.
It is not proof of:
a universal 800-ppm optimum.
30. Very High CO₂ Results in Related Brassica rapa Warn Against Simple Extrapolation
The same scientific literature also contains Brassica rapa experiments where very high CO₂ did not continue increasing shoot growth.
In some related cultivars, plants at approximately:
450 ppm
outperformed much higher:
900–1350 ppm
conditions.
These were not identical bok-choy systems.
But they reinforce the broader lesson:
CO₂ response is not guaranteed to increase linearly forever.
31. CO₂ Should Be Interpreted Together With Light
Additional CO₂ is most useful when the plant has sufficient:
- photons
- temperature conditions
- nutrients
- sink capacity
to use the additional carbon.
So a measurement such as:
800 ppm CO₂
does not tell you whether carbon is the current limiting factor.
That is why AquaHorti’s strongest measurement message is:
look at PAR and CO₂ together.
32. Monitor CO₂ Through the Day
A useful greenhouse question is:
What happens to crop-zone CO₂ as solar PAR rises?
Possible patterns include:
PAR rises → CO₂ falls
or:
PAR rises → CO₂ remains stable
depending on:
- ventilation
- enrichment
- crop density
- greenhouse leakage
Continuous measurements reveal much more than one snapshot.
33. Water Availability Has Direct Greenhouse Pak Choi Evidence
A two-season glass-greenhouse study in Beijing examined pak choi water management using:
- furrow irrigation
- micro-irrigation
- plastic mulch
- combinations of irrigation and mulch
with irrigation scheduling based on:
measured soil water content.
This is far more useful than assigning a universal VPD target.
34. Better Root-Zone Water Management Increased Yield
The micro-irrigation + plastic-mulch treatment produced approximately:
7.30 kg/m²
in spring
versus:
5.21 kg/m²
for conventional furrow irrigation.
In autumn:
6.86 kg/m²
versus:
4.21 kg/m².
The improved environment also increased:
- root development
- dry biomass
- plant water status
35. Water-Use Efficiency Improved Dramatically
Compared with micro-irrigation without mulch, treatments using plastic mulch reduced evaporation by approximately:
57–70%.
Irrigation water-use efficiency improved by:
84% in spring
and:
95% in autumn
for the micro-irrigation + mulch treatment.
This provides strong evidence that:
root-zone water management matters enormously.
36. This Does Not Give Us a Bok Choy VPD Optimum
The experiment measured and controlled:
soil water management.
It did not test:
0.6 vs 0.9 vs 1.2 vs 1.5 kPa VPD.
Therefore, it cannot be used to claim that one VPD produces:
- firmer petioles
- more tender leaves
- better shelf life
Those are separate questions.
37. Water Stress and Long Light Have Also Been Tested Together
A 2021 pak choi experiment compared:
- standard lighting
- long photoperiod of approximately 20 h/day
- water stress
- long photoperiod + water stress
over approximately:
six weeks.
Long light exposure increased:
- plant growth
- glucosinolate production
while drought reduced plant productivity.
38. Water Deficit Increased Glucoraphanin but Reduced Productivity
The drought treatment elevated:
glucoraphanin
under the study conditions.
But water deficit also severely reduced plant growth.
This is another classic crop-quality tradeoff.
Stress can increase a particular secondary metabolite while reducing:
commercial yield.
Therefore:
more stress does not automatically mean better bok choy.
39. VPD Cannot Be Used as a Glucosinolate Dial
A root-zone drought treatment is not equivalent to:
high atmospheric VPD.
Plant water status depends on:
- VPD
- substrate moisture
- roots
- irrigation
- nutrient EC
- plant size
- temperature
Therefore, the drought experiment does not justify:
“raise VPD to increase glucoraphanin.”
40. There Is No Validated Stage-Specific Bok Choy VPD Table
Current pak choi literature provides strong information about:
- DLI
- PPFD
- photoperiod
- CO₂
- water stress
- irrigation
- temperature
- tipburn
But there is not enough direct evidence to define one universal:
seedling VPD
vegetative VPD
and:
preharvest VPD
optimum.
The old 0.4–0.8 / 0.6–1.0 / 0.8–1.2 / 1.0–1.4 kPa stage table should therefore be removed.
41. What VPD Is Useful For
VPD describes:
atmospheric evaporative demand.
As:
- temperature increases
- relative humidity decreases
VPD usually rises.
This can increase:
- transpiration
- irrigation demand
- substrate drying
- canopy water loss
For bok choy, that is useful information.
But it should be treated as:
a diagnostic variable
rather than:
a direct leaf-quality specification.
42. What to Check When VPD Rises
If greenhouse VPD rises significantly, ask:
- Did air temperature increase?
- Did RH decrease?
- Did vents open?
- Is irrigation still adequate?
- Is the root zone drying faster?
- Is new growth developing tipburn?
- Has canopy airflow changed?
That gives VPD a practical role without inventing false precision.
43. High VPD Does Not Directly Mean Weak Texture
The old article claimed:
High PAR + high VPD → fast growth, weak texture.
Current evidence does not establish that causal formula.
Leaf and petiole quality depend on:
- plant water status
- dry-matter percentage
- cultivar
- maturity
- temperature
- nutrient availability
- calcium distribution
VPD is one component of the plant-water system.
It is not a texture meter.
44. Hollow Petioles Also Need a Separate Diagnosis
If bok choy develops hollow or abnormal petioles, possible causes can involve:
- rapid growth
- cultivar characteristics
- nutrient imbalance
- temperature
- physiological disorders
Current research does not justify using a high VPD reading as proof that VPD caused the hollow tissue.
If petiole quality is important, record it separately.
45. Temperature Is a Major Variable
Bok choy can grow quickly under moderate temperatures, but high heat creates real physiological stress.
A 2024 study screened:
26 bok choy cultivars
for heat tolerance.
Plants were compared under approximately:
25/18°C day/night
and:
35/25°C day/night
conditions, with additional severe heat exposure used in the heat-response evaluation.
46. High Temperature Reduced Growth — and Cultivars Responded Differently
For the heat-tolerant cultivar:
Jinmei
high-temperature treatment reduced approximately:
- dry shoot weight 24%
- fresh shoot weight 23%
- leaf area 12%
- plant height 13%
For the heat-sensitive:
Sanyueman
reductions were larger:
- dry shoot weight 37%
- fresh shoot weight 35%
- leaf area 28%
- plant height 24%
This is strong direct evidence that:
heat tolerance is cultivar-dependent.
47. Therefore “18–26°C Is Fine” Is Too Broad Without Context
The old page gave stage temperatures as if crop response were predictable from a simple band.
Research shows that:
- temperature matters
- genotype matters
- duration matters
- extreme events matter
A crop exposed to prolonged 35°C daytime conditions is not experiencing the same physiology as one growing around the low 20s.
48. Temperature Also Changes VPD
Even if RH remains unchanged:
warmer air increases vapor-pressure capacity.
So rising greenhouse temperature can increase VPD automatically.
This means a VPD change may simply be the consequence of a heat event.
Always interpret:
temperature + RH + VPD
together.
49. Temperature Can Also Change Light Interpretation
The same:
300 PPFD
does not create the same plant condition at:
20°C
and:
35°C.
Higher temperature changes:
- respiration
- water demand
- enzyme activity
- heat stress
- leaf temperature
Therefore:
PAR without thermal context is incomplete.
50. Cultivar Choice Is an Environmental-Management Decision
The 2024 heat study shows large genetic differences among bok choy cultivars.
The 2025 DLI study also showed substantial cultivar differences in:
- light-use efficiency
- tipburn sensitivity
- metabolite response
Therefore, cultivar is not merely a seed-selection issue.
It affects how the entire greenhouse environment should be interpreted.
51. Tipburn Is a Good Example of Why One “Perfect Environment” Does Not Exist
Higher DLI:
increased biomass
and:
increased vitamin C and glucosinolates.
But it also:
increased tipburn incidence.
So the production objective is not simply:
maximum photosynthesis.
It is:
maximum marketable yield.
Those are different quantities.
52. Preharvest DLI Did Not Determine Shelf Life in the 2025 Study
This point should replace the old claim that changing VPD before harvest improved shelf life.
The direct study found:
higher DLI increased growth and nutrition
but:
did not significantly change postharvest shelf life.
Therefore, there is no justification for an AquaHorti statement such as:
“VPD 1.0–1.4 kPa near harvest improves shelf life.”
53. Postharvest Hydration Matters Much More Directly
Bok choy loses market quality when:
- leaves wilt
- petioles lose water
- color fades
- decay develops
Cold-chain management and minimizing water loss are therefore critical after harvest.
The production environment matters.
But preharvest PAR/VPD should not be portrayed as a substitute for postharvest management.
54. A Practical Greenhouse Measurement Workflow
Step 1 — Identify the Cultivar
Record:
- cultivar
- green or red leaf
- baby or full-size type
- known heat sensitivity where available
Step 2 — Measure PAR at the Actual Canopy
Measure where leaves receive light.
Do not use lamp output specifications alone.
Step 3 — Record DLI
In a greenhouse, log PAR through the complete day.
Compare:
- sunny vs cloudy days
- seasons
- different benches
- supplemental-light treatments
Step 4 — Record Photoperiod
Two crops can receive the same DLI through very different combinations of:
PPFD × hours.
The 2025 study shows that this matters for bok choy.
Step 5 — Monitor CO₂
Record crop-zone CO₂ during active photosynthesis.
Compare it directly with the PAR timeline.
Step 6 — Track Air Temperature
Pay particular attention to:
- midday heat
- prolonged hot periods
- cultivar response
Step 7 — Track Humidity and VPD
Use VPD to understand atmospheric water demand.
Do not treat it as a direct texture target.
Step 8 — Monitor Root-Zone Water
Record:
- irrigation frequency
- substrate moisture
- nutrient-solution status
Step 9 — Watch the Growing Point
Monitor new leaves for:
tipburn.
The youngest tissue can reveal calcium-transport problems before overall yield is obviously affected.
Step 10 — Measure the Commercial Outcome
Track:
- fresh weight
- leaf number
- petiole quality
- tipburn incidence
- marketable percentage
- harvest days
If nutritional quality matters, consider:
- vitamin C
- nitrate
- glucosinolates
55. Practical Research-Based Light References
Direct modern pak choi work supports approximately:
10.8–18.9 mol/m²/day
as a particularly strong DLI research region.
The same study used approximately:
167–292 µmol/m²/s
during 18-hour photoperiod treatments.
It also tested:
250 µmol/m²/s
across:
12–21 h photoperiods.
These values are much more defensible than the old stage-specific:
80–450 PPFD
table because they come from a direct controlled comparison.
56. Should We Call 18.9 DLI the Optimum?
No.
18.9 DLI produced strong growth and nutritional responses.
But higher DLI also increased:
tipburn risk.
The economically best point therefore depends on:
- cultivar
- marketability
- lighting cost
- tipburn control
- nutritional target
A universal optimum cannot be extracted from biomass alone.
57. Practical CO₂ Reference
Direct ‘Wuyueman’ research provides:
~400 vs 800 ± 50 ppm
as a strong bok-choy-specific CO₂ comparison.
At elevated CO₂:
- growth increased
- photosynthesis increased
- chlorophyll increased
- vitamin C increased
- soluble sugar increased
Therefore:
around 800 ppm is a defensible research reference.
It is not a universal optimum.
58. Practical VPD Reference
There is currently no sufficiently validated bok-choy-specific stage VPD optimum.
Therefore, the most scientifically defensible recommendation is:
Do not publish a rigid VPD target table as if it were proven.
Use VPD to interpret:
- temperature
- humidity
- irrigation demand
- tipburn risk context
- canopy water status
59. Practical Water Reference
The strongest direct greenhouse evidence supports:
managing actual root-zone water status.
The two-season greenhouse irrigation study showed that water-management strategy could change yield from approximately:
4–5 kg/m²
to:
nearly 7 kg/m²
depending on season and treatment.
That is real crop-specific evidence.
60. A Better Way to Think About Bok Choy Measurements
Instead of asking:
What PPFD gives bok choy the best texture?
ask:
How much daily light is the crop receiving, and how efficiently is it using those photons?
Instead of:
What is the ideal VPD for firm petioles?
ask:
How strong is atmospheric water demand, and are the root zone and growing point maintaining appropriate water and calcium supply?
Instead of:
What CO₂ level should I use?
ask:
Does CO₂ become limiting while PAR is high, and does enrichment improve marketable yield in this cultivar?
Instead of:
How do I maximize growth?
ask:
How do I maximize marketable yield without increasing tipburn or production cost?
That is a much stronger production question.
Final Takeaway
Greenhouse bok choy does not have one scientifically established PAR, CO₂ and VPD recipe for fast growth, firm petioles, tender leaves and long shelf life.
But current pak-choi-specific research gives us several unusually strong conclusions.
DLI matters.
A 2025 direct study tested:
10.8, 13.5, 16.2 and 18.9 mol/m²/day
in three pak choi cultivars.
Increasing DLI increased:
- fresh biomass
- dry biomass
- vitamin C
- soluble sugars
- several glucosinolates
But the way DLI was delivered mattered.
Increasing DLI through:
longer photoperiod
produced approximately:
67% fresh-weight gain
across the tested range,
while increasing DLI primarily through:
higher PPFD
produced approximately:
26% gain.
At the same:
18.9 DLI
the highest-intensity:
350 PPFD
treatment produced the lowest fresh weight among equal-DLI treatments.
That means:
more PPFD is not automatically more productive.
The same experiment also found:
higher DLI increased tipburn incidence.
So maximizing biological growth is not identical to maximizing marketable yield.
Spectrum matters.
Greenhouse supplemental blue-light research found:
50 µmol/m²/s supplemental blue
favored yield,
while higher blue intensity favored several nutritional-quality traits and reduced nitrate.
CO₂ matters.
Direct pak choi research comparing approximately:
400 vs 800 ± 50 ppm
found elevated CO₂ increased plant growth and photosynthesis.
Leaf vitamin C increased by approximately:
7.7%
chlorophyll by:
33.5%
and soluble sugars by:
21.9%.
But this still does not establish 800 ppm as a universal commercial optimum.
Water matters.
Two-season greenhouse research showed that improved root-zone water management could increase pak choi yield substantially while reducing evaporation and improving irrigation water-use efficiency.
Heat matters.
Direct 2024 bok choy research found that 35/25°C high-temperature conditions reduced fresh shoot weight by approximately:
23% in a heat-tolerant cultivar
and:
35% in a heat-sensitive cultivar.
So cultivar must be part of environmental interpretation.
For VPD, current bok-choy-specific evidence does not justify one rigid stage-by-stage kPa table.
The stronger greenhouse strategy is therefore:
Measure PAR at the actual canopy.
Record DLI across the full day.
Record photoperiod as well as PPFD.
Monitor CO₂ during active photosynthesis.
Track temperature, humidity and VPD together.
Measure root-zone water status.
Watch the young leaves for tipburn.
Then compare all of those measurements with the crop outcome that actually matters:
marketable yield, tipburn incidence, leaf quality, nutritional composition and production efficiency.
That provides a much stronger technical basis for greenhouse bok choy than unsupported stage-by-stage PAR / CO₂ / VPD recipes.
References
Susilo, K.R., Eu, A., Besemer, B., Heuvelink, E., de Vos, R.C.H. & Marcelis, L.F.M. Extended Photoperiod Improves Growth and Nutritional Quality of Pak Choi Under Constant Daily Light Integral. Frontiers in Plant Science, 2025.
Zheng, Y., Zhang, Y., Liu, H., Li, Y., Liu, Y., Hao, Y. & Lei, B. Supplemental Blue Light Increases Growth and Quality of Greenhouse Pak Choi Depending on Cultivar and Supplemental Light Intensity. Journal of Integrative Agriculture, 2018.
Hou, L., Shang, M., Chen, Y., Zhang, J., Xu, X., Song, H., Zheng, S., Li, M. & Xing, G. Physiological and Molecular Mechanisms of Elevated CO₂ in Promoting the Growth of Pak Choi (Brassica rapa ssp. chinensis). Scientia Horticulturae, 2021.
Park, J.-E., Kim, J., Purevdorj, E., Son, Y.-J., Nho, C.W. & Yoo, G. Effects of Long Light Exposure and Drought Stress on Plant Growth and Glucosinolate Production in Pak Choi (Brassica rapa subsp. chinensis). Food Chemistry, 2021.
Xie, Y., Wang, S., Luo, C., Sun, M., Wang, Y., Yang, J. & Wang, Q. Using Plastic Mulching Improves Greenhouse-Grown Pakchoi Growth and Water Use Efficiency Under Irrigation Scheduling Based on Soil Water Content. Agronomy, 2020.
Dong, C. et al. Physiological and Transcriptomic Responses of Bok Choy to Heat Stress. Plants, 2024.
Frede, K., Schreiner, M., Zrenner, R., Graefe, J. & Baldermann, S. Carotenoid Biosynthesis of Pak Choi Sprouts Grown Under Different Light-Emitting Diodes During the Diurnal Course. Photochemical & Photobiological Sciences, 2018.
Energy-Efficient Light Spectrum Modulation for Optimising Biochemical Composition, Resource Utilisation and Physiological Responses of Pak Choi in Multi-Tier Production System. Journal of Agriculture and Food Research, 2026.
UC Davis Postharvest Research and Extension Center. Bok Choy (Pak Choi, Pok Choi): Produce Facts.
Related AquaHorti Tools
For instantaneous PAR / PPFD measurements and checking greenhouse light distribution, see AquaHorti AH-Quantuv.
For recording changing greenhouse PAR throughout the day and measuring DLI, see AquaHorti AH-PARDLI.
For greenhouse monitoring where PAR, DLI, CO₂, temperature, humidity and VPD need to be reviewed together over time, see AquaHorti AH-200.