Effects of Accumulated Growing Degree Days (GDDs) on Sesame (Sesamum indicum) Growth and Yield under Greenhouse Condition in the Mekong Delta, Vietnam

Sesame has high nutritional value, sesame seeds contain oil, protein, sugar, water, ash, and two kinds of oleic acid and linoleic acid; sesame is commonly used in the diet or as a vegetable oil. Sesame is grown in many tropical countries, and the optimum temperature for sesame growth is 25-30 °C. Due to climate change, the temperature is increasing, in the Mekong Delta of Vietnam, the temperature in summer can reach nearly 35 °C. The research question is whether high air temperature affects sesame growth and yield. To answer the above question, the study was conducted in greenhouse conditions in An Giang province with different temperature conditions, including 3 extreme chambers from 32.7, 32.9, and 34°C; automatic fan and a misting system for to control temperature. Black sesame plants were grown in 42 pot (34 x 28 x 28cm), each chamber with 4 plants per pot. All pots were filled with the same amount of alluvial soil (Fluvi Mollic Gleysols). Results showed that accumulated growing degree days in chambers 1, 2, and 3 were 2115, 2019, and 1994 GDDs, respectively. At 2115 GDDs gave the highest value of fresh biomass (1417g/pot), dry biomass (276 g/pot), dry weight capsule (109 g/pot), and dry seeds (51 g/pot). Regarding harvest index, chamber 3 with 1994 GDDs gave the highest harvest index of 0.22; the other chambers were in the range of harvest index of 0.18-0.20. In conclusion, under greenhouse conditions with 2115 GDDs is ideal for the growth and yield performance of black sesame.


Introduction
The Vietnamese Mekong River Delta (VMD), the largest aquaculture and agriculture production area of Vietnam, has been identified as particularly susceptible to the impacts of climate change [1,2].In agricultural cultivation, the growth and yield of crops are affected by many factors, such as varieties, soil, water, and climate.Climate change is also one of the challenges facing the agricultural production sector.Climate change causes weather and climate to follow clear rules and becomes more unpredictable no longer.Such as floods, droughts, natural disasters, and saltwater intrusion occur more frequently.These directly impact crop growth and development -the conditions to ensure crop yield [2].
Sesame (Sesamum indicum L.) belongs to the family Pedaliaceae, an oilseed crop.The optimum temperature during its life cycle is 25-35 o C, and sesame is tolerant to drought and adapted to different kinds of soils [3].In VMD, sesame is grown on dry land or in rotation with rice.The most planted sesame in An Giang is Long Xuyen city: 798.2 ha in 2016, and 573.0 ha in 2017.Chau Phu district with 817.3 ha in 2015 but only 5 ha in 2017.Cho Moi in 2015 and 2016 has an area of over 500 ha of sesame cultivation.Tri Ton district has an area of 131.2 hectares of sesame in 2017.The remaining districts do not have much sesame area [4].
Growing degree days (GDD) are a calculation of the development of plants or insects and use to estimate crop phenology.The GDD is calculated using daily minimum and maximum air temperatures and the base temperature [5].GDD use to assess effect of temperature and describe the biological processes [6,7].This research was carried out to determine the phenological development of sesame under different temperatures.

Materials and Methods
The study was carried out in the greenhouse at An Giang province, Mekong Delta, Vietnam with different temperature conditions, including 3 extreme chambers from 32.7, 32.9 and 34 o C (Table 1).Air temperature in each chamber was monitored and recorded hourly by Tinytag data loggers.An automatic fan and misting system were used for very high temperature (>45 o C).
Sesame plants were grown on 42 pots (34 x 28 x 28cm) for each chamber with 4 plants per pot.All pots were filled with the same amount of alluvial soil (Fluvi Mollic Gleysols).Plants were watered through a drip system.Fertilizer application was 120N -46P2O5 -60 K2O kg/ha.

Results and Discussion
The longest growing days was found in thermal chamber 1 at 91 days, and the input of GDD was 2115 degree -days.The lowest growing days were found in TC 3 as 86 days, and the input of GDD was 1994 degree -days (Table 2 and 5).Similarly, the soonest flowering was in TC 3 (35 DAP) and followed in TC2 (40 DAP) and TC1 (45 DAP) (table 2).Sesame plant height increased slowly from 10 to 30 DAP but increased rapidly from 40 to 70 DAP (Table 3).Sesame plant height was 208 to 242 cm at harvest.Those results were different with some results.For example, according to the high trait reported of this sesame variety, the plant height is about 100 -120 cm [10].Other study reported plant height was 114 cm [11].Fresh biomass from seedling to 86 DAP were not significantly different among chambers (table 4).Fresh biomass grew gradually from germination to 40 DAP but increased quickly from 50-80 DAP.At 40 DAP, fresh biomass was 102 -136 g/pot but after 50 DAP, fresh biomass increased about five times (562 -770 g/pot).
Data of fresh and dry of capsule/pot, seed, and biological yield of sesame were given in Table 5.The sesame sown in TC 3 and TC 1 produced higher seed yield (52.6 and 51.7) and followed by TC 2 (48.8 g/pot).The biological yield (fresh biomass at harvest) was produced significantly higher in TC 1 (1417 g/pot), TC 2 (1383 g/pot) than that in TC 3 (798 g/pot).
Regarding harvest index, chamber 3 gave the highest harvest index of 0.22.The other chambers were in the range of harvest index of 0.18 and 0.20.GDD for sesame crops under different thermal chambers from sowing to maturity were presented in Table 6.The total heat load was maximum under TC 1 condition (2115 o C -days) followed by TC 2 (2019 o C -days) then TC 3 (1993 o C -days).The accumulated GDD of 750, 841, and 900 o C -days required for flowering was received from 35, 40 and 45 DAP under TC3, TC2 and TC 1, respectively.The study of [12] on the effect of thermal indices of Sesame in the New Alluvial Zone of West Bengal found that GDD for sesame from germination to maturity was 1584.87 o C -day.The GDD requirement varied for different phenophases and depended upon the duration and occurred due to different crop duration [13].
Sesame grows well in hot to warm to produce maximum yields.The optimum temperature for development and yield is about 25 to 37 o C during its growth period.High temperatures, over 40 °C, seriously affect flowering and the number of capsules [14].

Conclusion
When grown under high temperatures (chamber 3), sesame growing days are shorter, and biomass is lowest.The sesame grown in chamber 1 with 2115 GDDs is ideal for growth with the highest fresh biomass and yield performance (51.7 g/pot).

Table 1 .
Average temperature range during the experiment in chambers.

Table 2 .
Effect of temperature on flowering and harvesting days.

Table 3 .
Effect of temperature on plant height (cm).

Table 4 .
Effect of temperature on fresh biomass (g/pot).

Table 5 .
Effect of temperature on fresh and dry weight capsule, fresh and dry biomass, seed yield and HI of sesame.

Table 6 .
Accumulated GDD ( 0 C-day) for sesame crop from planting to maturity.