Eco-physiological adaptation of black sesame in tropical Australia

Global demand and Australian import of spice commodities continue to increase due to the growing consumption of spices in flavouring, seasoning, health, and pharmaceutical applications worldwide. This thesis research undertook scoping studies of three spice crops (cumin, kalonji and black sesame) by evaluating their response to temperature for germination as a proxy for adaptation and subsequent eco-physiological evaluation of black sesame for tropical adaptions. Black sesame, one of the widely used spice crops for food flavouring and seasoning purposes, also receives significant demand for health applications as the sesame lignans compounds, e.g. sesamin and sesamolin, possess anticarcinogenic, antioxidative and antihypertensive properties. Sesame is a highly drought-tolerant crop that can be farmed in moisture-limited conditions of the tropical Australian farming system. The study was conducted to assess the suitability of black sesame farming in the tropics and subtropics of northern Australia. Several controlled environment and field trials were carried out as a progressive approach starting from the screening of temperature-dependent germination response of selected spice crops; suitability of black sesame cultivation across different seasons in the subtropics of Australia; screening of drought-tolerant black sesame genotypes; stability analysis of black sesame genotypes in moisture limited environments; use of gamma radiation in black sesame for the suppression of seed germination to facilitate the protection of intellectual property rights in the sesame food supply chain, and finally, preliminary economic analysis of black sesame cultivation. Temperature-dependent germination response of three selected spice crops: black sesame, kalonji and cumin was carried out to assess the effect of temperature on the seedling establishment to explore the potential field planting schedule of these crops in northern Australia. Seed germination trials carried out in temperature-controlled incubators at 15, 25, and 35 °C along with a control at an ambient temperature of 19-21 °C revealed that black sesame seed germination was completely inhibited at the lower temperature of 15 °C. Germination ranged from 84-89% when grown under 20 -35 °C, recording an increase in germination speed with increasing temperature till 35 °C. The germination of kalonji seeds, however, was fully inhibited with the higher temperature at 35 °C; reduced by a lower temperature of 15 °C and recorded the highest germination and speed indices at 20 – 25 °C. Germination of cumin seeds was fully inhibited at all higher temperature regimes (20-35 °C) and showed high germination (85%) at a lower temperature of 15 °C. The findings imply potential adaptation of black sesame for seedling establishment all-around the year in the tropics and subtropics except during the winter in sub-tropics. Kalonji cultivation could be inhibited during summer in the tropics and subtropics due to a lack of seedling establishment. Cumin seedling establishment could only be feasible during the winter months in subtropic and temperate regions. Due to the superior adaptation of black sesame establishment under a wide range of planting window in northern Australia, it was chosen as a crop of choice for further study. Black sesame genotype Moriah was planted at four different planting dates in central Queensland to assess the suitability of cultivation under four seasons in sub-tropics of northern Australia. The trial crop was sown in eight replicated raised beds (3.1 × 0.82 m) on 27th Sep 2019, 26th Dec 2019, 27th Mar 2020, and 26th June 2020 targeting spring, summer, autumn, and winter planting, respectively. The results showed that black sesame yield was highest under spring and summer sowing while winter sowing recorded the least seed yield. The crop reached physiological maturity at 105, 103, 134, and 138 days after sowing after attainment of 1193, 1245, 826 and 887 °C growing degree days (GDD), respectively, calculated at the base temperature of 15.7 °C. Winter planting was significantly limited due to low seedling establishment, asynchrony in plant growth, chilling injury and flower and pod abortion. Chilling injury and flower abortion were also recorded during the autumn sowing as the minimum temperature plummeted below 9°C by the end of May. The findings imply limitation in winter sowing of black sesame in the subtropics, with the crop favouring spring and summer windows. Assessment of drought tolerance of fifteen black sesame genotypes was carried out by evaluating their performance under two irrigation regimes of 100% (full) and 50% (deficit) evapotranspiration demand (Etc). Mean seed yield of 2191 kg/ha and 1573 kg/ha was recorded under full and deficit irrigation with net water input of 598 mm and 335 mm respectively, till 80 days after sowing. Genotypes AVTBS#2, AVTBS#4, AVTBS#16, AVTBS#13, and AVTBS#18 recorded the lowest yield penalty under moisture deficit, with the genotypes AVTBS#4, AVTBS#13 and AVTBS#18 being statistically high yielding genotypes under both irrigation regimes. The results indicate these genotypes were highly drought-tolerant, recording yield penalty below 19% when the water input was reduced by 44%. These genotypes with greater water use efficiency could be utilized for growing black sesame under water-limited environment and rainfed farming system. The stability of the black sesame genotypes across multiple environments under moisture deficit was assessed using two years of yield data from strategic deficit irrigation in the years 2019 and 2020. The mean seed yield increased from 855 kg/ha in 2019 to 1573 kg/ha in 2020 when net water input was increased from 266 mm to 335 mm respectively. Seed yield among the genotypes was not significant under the severe moisture deficit conditions applied in 2019. GGE biplot revealed genotype AVTBS#1 as the most stable across all the environments and genotypes AVTBS#4, AVTBS#13 and AVTBS#18 best suited for strategic irrigation conditions of 2020. The potential use of gamma radiation for suppression of seed viability and germination in black sesame was studied to facilitate the marketing of unhybridized black sesame lines in the international food market. Black sesame seeds treated with 0.0, 0.1, 0.3, 0.4, 0.5, 1, 2, 4, 8, and 16 kilograys of gamma radiation were kept for germination test at ambient temperature. The results showed that the germination of the seeds was completely suppressed with gamma radiation dose of as low as 0.5 kilograys. Low dose treatment of sesame seeds with gamma rays is unlikely to alter the nutrition properties of the sesame seeds. Preliminary economic analysis of black sesame production conducted in a case study from a single farm in central Queensland showed the gross margin of AUD 146.9 per hectare at seed yield of 320 kg/ha and farmgate price of AUD 2000 per ton. Sensitivity analysis on gross margin showed minimum seed yield requirement of 0.27 tons/ha and 0.25 tons/ha to reach a breakeven point at the farmgate price of AUD 1800 and AUD 2000 respectively. Yield and seed quality improvements are required to make black sesame commercially competitive for best chances of integrating sesame in the broadacre farming systems in northern Australia. Keywords: sesame, Sesamum, northern Australia, drought tolerance, stability, gamma radiation, gross margin