Comprehensive summary of temperature requirements for each growth stage of eggplant (eggplant)

Overview

Eggplants are known as high-temperature crops, and by strictly controlling the optimal temperature and critical temperature range at each growth stage, stable growth and high-quality yields can be achieved from germination, healthy seedlings, fruit set, fruit enlargement, and harvest. The following is a systematic arrangement of temperature parameters (optimal_min, optimal_max, base_temperature, low_stress_threshold, high_stress_threshold, max_temperature, frost_threshold, and flowering stage sterility_risk_threshold) for each main growth stage of eggplant based on Japanese academic and technical materials and various extension guidance materials. It is also noted that each value or term has variations in breadth or definition in the literature.

Main growth stages and Japanese notation

1. Sowing (hashu, sowing)
2. Germination (hatsuga, germination)
3. Seedling/nursery
4. Transplanting
5. Vegetative growth
6. Pinching/pruning
7. Flowering
8. Fruit set)
9. Fruit enlargement/growth
10. Harvest

This table focuses on the stages of ``germination'', ``seedling raising'', ``vegetative growth'', ``flowering'', ``fruit set'', and ``fruit enlargement (including harvesting)'', which are particularly important in management in Japan.

List of temperature requirements/parameters by growth stage

Table: List of temperature parameters for each growth stage of eggplant

Around 3°C

Growth stage	optimal_min	optimal_max	base_temperature	low_stress_threshold	high_stress_threshold	max_temperature	frost_threshold	sterility_risk_threshold (Flowering period only)	References
Germination	20-25°C	30-35°C	11-15°C	11-13°C	35-37°C	35-40°C	Rapidly reduced below 10°C	—	[1][2][3][4][5][6][7][8]
Seedling raising	22-25°C	28-30°C	12-13°C	12-13°C	30-32°C	33-35°C	0-3°C	—	[2][3][4][6][8][9][10][11]
Vegetative growth	20-22°C	28-30°C	10-12°C	13°C	32~35°C	35~37°C	0~3°C	—	[2][4][6][8][11][12][13]
Flowering (reproductive growth)	18-20°C	28-30°C	10-12°C	13-15°C (Especially night temperature)	35°C (be careful of night temperature of 25°C)	35-40°C	around 3°C	less than 15°C or more than 35°C (pollination・Poor fruit set) >	28~30°C	10~12°C	13~15°C (night temperature)	35~36°C	35~40°C	—	[4][6][8][11][14][15][16]
Fruit enlargement/harvesting	12～ 15°C	28~30°C	10~12°C	12~13°C (night temperature)	35°C	35~40°C	Around 3°C	—	[2][4][6][8][11][14][15][16]

Notes on parameter definitions

• optimal_min/optimal_max: Range where growth is most stable and fastest (with day and night differences). Mainly describes daytime temperatures (nighttime temperatures are especially important during the flowering and fruiting stages).
• base_temperature: The theoretical lower limit at which physiological growth is observed. Practically, focus on optimal_min.
• low_stress_threshold: Temperature at which obvious growth slowdown and poor fruiting begin to appear. Be especially careful about night temperatures.
• high_stress_threshold: Temperature range at which high temperature damage begins.
• max_temperature: The temperature above which damage or death occurs rapidly.
• frost_threshold: Temperature at which frost damage and tissue death occur. Around 3°C at many stages. Seedlings are lethal at around 0°C.
• sterility_risk_threshold: only during flowering period. Substantial sterility risk range at flowering, pollination, and fruit setting stages. If the night temperature is less than 15°C/daytime is more than 35°C, pollen production and germination ability will decrease, and the fruit set rate will decrease.

Detailed analysis and scientific background by growth stage

Germination period

• Optimal temperature (25-30°C or 20-30°C): Germination occurs at the fastest speed and all at once.
• Germination base temperature (11-15°C): Germination is possible at the very limit, but the germination speed and rate will be significantly lower.
• Stress threshold: Germination almost stops below 10°C or above 35°C. No light required (photophobic seeds).
• Maximum temperature: Poor germination above 35-40°C. According to popularization materials from Nagano Prefecture, etc., the upper limit for germination is 35°C, and it is impossible to germinate at temperatures above 40°C.
• Frost damage threshold: It is practically of little importance at the germination stage, but below 10°C the germination rate deteriorates extremely.
• Data: [1][2][3][4][5][6][7][8]

Seedling period

• Optimal temperature: Popularization and literature indicate that 25-30°C during the day, 15-20°C at night, and 22-30°C are desirable.
• Base (lower growth limit) temperature: 12-13°C. If it falls below this, there is a risk of growth stopping or overgrown seedlings.
• Stress threshold: It slows down below 12-13°C, and even above 30°C, growth is poor and seedling elongation (weak elongation) increases.
• Maximum temperature: Above 30-32°C, the risk of deformity and damage increases.
• Frost damage threshold: Seedlings die at 0-3°C. Risk of injury below 10°C.
• Documents: [2][3][4][6][8][9][10][11]

Vegetative growth phase

• Optimal temperature: 20-28°C, healthy growth at 12-15°C or higher at night. Avoid sudden temperature changes.
• Lower limit: Complete stop below 10-12°C.
• High temperature stress: Growth is inhibited at temperatures above 32-35°C.
• Frost threshold: Damage at 0-3°C.
• Data: [2][4][6][8][11][12][13]

Flowering period (Flowering → Pollination / Fruit setting)

• Optimal temperature: 18-20°C (night) to 28-30°C (day) is the most stable.
• Base temperature: 10-12°C, however, 15°C or higher (at night) is required for normal development of flower buds and reproductive organs.
• Low temperature stress threshold: When the night temperature falls below 13-15°C, sterility, flower drop, and poor fruit set are noticeable. Japan's facility cultivation manual also strongly recommends "maintaining a night temperature of 15°C during the flowering and fruiting stages."
• High temperature stress threshold: At temperatures above 35°C during the day, defective flowers and pollination problems (poor pollen tube elongation, flower drop, etc.) occur frequently.
• Maximum temperature: Almost stops functioning above 40°C.
• Frost damage threshold: around 3°C or less.
• Sterility risk threshold: When the night temperature is below 15°C or the daytime temperature is above 35°C, the fruit set rate decreases rapidly (decreased pollen germination rate and fertilization rate) [4][6][8][11][13][14][15][16][17].
• Data: [4][6][8][11][13][14][15][16][17]

Fruit setting period/Fruit enlargement ~ Harvest period

• Optimal temperature: 28-30°C during the day, 12-15°C at night (stable fruit enlargement).
• Lower limit: 10 to 12°C (stops growth) / Poor hypertrophy and increased malformation at night temperatures below 12°C.
• High temperature stress: At temperatures above 35°C (during the day), enlargement stops and quality deteriorates, such as discoloration and dull fruit.
• Maximum temperature: Severe damage at 35-40°C (no fruit set or enlargement).
• Frost damage threshold: Fruit damage occurs at around 3°C or below, and low temperature damage occurs at below 10°C in post-harvest storage management.
• Documents: [2][4][6][8][11][14][15][16]

About the ambiguity of terms and numbers and regional differences

• There are many publications that set the "base temperature" at 10 to 12°C as the lower limit for growth, but in practice there are many instructions to raise the standard to a night temperature of 15°C.
• The frost threshold is often described as the “tissue death/lethal level” at 0–3°C, and cold damage becomes a problem at temperatures below 10°C when storing fruits after harvest.
• “High-temperature injury” tends to occur at temperatures above 35°C, but there are some differences depending on the region, variety, and cultivation type (e.g., greenhouse vs. open-field farming).
• In particular, there are many reports that the "sterility risk threshold" during the flowering stage is that the pollen germination rate and fruit set rate drop sharply when the nighttime temperature is less than 15°C and the daytime temperature is more than 35°C. Fluctuations and short-term low temperatures increase the risk, so stable temperature management is necessary.

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Overview

Eggplants basically prefer high temperatures both day and night, and it is extremely important to maintain stable night temperatures of 15°C or higher and daytime temperatures of 15°C or higher and less than 35°C during the day, especially during the reproductive growth stage (flowering and fruiting). It is recommended to maintain a stable temperature of 20 to 30°C during the germination and seedling stages, and the same temperature range during the vegetative growth and fruit enlargement stages. Neglecting temperature control can cause a variety of problems such as elongated seedlings, sterility, flower drop, deformed fruit, and poor quality, so management must be clearly aware of the "appropriate temperature" and "risk threshold" at each growth stage. Japanese technical guidance materials and academic papers are all based on the above temperature parameters.

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References

[1] [Illustrated] Comprehensive summary of eggplant cultivation [Basic lectures you need to understand if you want to grow]: https://www.youtube.com/watch?v=U0hcJpgTKNk
[2] Eggplant germination temperature - How to grow eggplant.com: https://nasu-sodatekata.com/Eggplant cultivation qa/Eggplant - Germination temperature.html
[3] Causes and remedies for eggplant not germinating: https://kateisaiennkotu.com/faq/nasu_hatsugasinai.html
[4] Long-term cultivation technology for facility eggplant (AgriKnowledge): https://agriknowledge.affrc.go.jp/RN/2010845975.pdf
[5] Guidelines for the optimum germination temperature and seedling-raising temperature (suitable growth temperature) for major fruit vegetables - Nagano Prefecture: https://www.pref.nagano.lg.jp/sakuchi/nosei-aec/joho/gijutsu/documents/kasai-ondo.pdf
[6] Heat retention management to stabilize fruit set through pollination in unheated, semi-prompted eggplant cultivation: https://agriknowledge.affrc.go.jp/RN/2010730717.pdf
[7] Relationship between parthenocarpy and fruit enlargement under low night temperature in eggplant: https://agriknowledge.affrc.go.jp/RN/2010752825.pdf
[8] Relay and purchasing of onions, green onions, eggplants, cabbage, and spinach...: https://www.cool-c.com/column/52
[9] [Eggplant raising] What is the basic seedling-raising period and cultivation method? Temperature control to prevent growth... https://minorasu.basf.co.jp/80562
[10] Ⅱ Environmental control guidelines for eggplant - Aichi Prefecture: https://www.pref.aichi.jp/uploaded/attachment/412058.pdf
[11] Cultivation of eggplant | Natural Agriculture International Research and Development Center: https://www.infrc.or.jp/breeding/point/point-eggplant/
[12] Eggplant - BSI Institute of Biological Sciences: https://bsikagaku.jp/cultivation/eggplant.pdf
[13] 1 Cultivation characteristics (MAFF): https://www.maff.go.jp/j/seisan/kankyo/hozen_type/h_sehi_kizyun/pdf/miyagi_yasai18_10.pdf
[14] Change management of harvested fruit weight according to the number of fruits set promotes eggplant cultivation ... - Aichi Prefecture: https://www.pref.aichi.jp/uploaded/attachment/374683.pdf
[15] Effect of light and temperature on the phenology and maturation of the fruit of eggplant (Solanum melongena) grown in greenhouses: https://www.researchgate.net/publication/248911654_Effect_of_light_and_temperature_on_the_phenology_and_maturation_of_the_fruit_of_eggplant_Solanum_melongena_grown_in_greenhouses
[16] Fruit Set in Solanaceous Vegetable Crops as Affected by Floral and Environmental Factors: http://www.globalsciencebooks.info/Online/GSBOnline/images/0812/EJPSB_2(SI1)/EJPSB_2(SI1)88-105o.pdf
[17] Protect eggplants from high temperature damage | Column - Seiko Ecology: https://ecologia.100nen-kankyo.jp/column/single284.html