A microcalorimetric analysis of quinoa seeds with different initial water content during germination at 25°C
Introduction
Quinoa (Chenopodium quinoa Willd.) is an ancestral crop from the Andes of South America. The high nutritional value and energetic content of this crop makes it very suitable as-food [1]. Much research is carried out world-wide about agricultural aspects of this crop [2], but little is done on the physiological level. Quinoa seeds have the advantage of fast germination in vitro, although they germinate very poorly in soil. The reasons for this are still a matter of debate [3].
Physiologists nowadays are deeply concerned with storage conditions of seeds in which water content and temperature play a fundamental role [4]. Seeds normally goes through a period of desiccation and maturation as the final step of their development [5], [6]. Desiccation of developing seeds inhibits metabolic pathways that lead to germination and growth but promotes germination on subsequent imbibition. However, mature dry seeds always contain certain amount of water which can be tightly bound to macromolecules (type 1), semi-bound having glassy characteristics (type 2) or less defined producing changes in the phase behaviour of membrane lipids (type 3) [5]. Seeds with water involved in type 1 or 2 bonding scarcely support metabolic reactions during storage, but some metabolism has been detected in seeds with water of the type 3.
Microcalorimetry has proven to be a suitable technique to monitor quinoa seed germination [1]. In the present work, we report the behaviour of quinoa seeds stored at different relative humidities during germination at 25°C, by using titration microcalorimetry. The optimum moisture content for storage of quinoa seeds at 25°C is also evaluated.
Section snippets
Plant material
Seeds of quinoa (Chenopodium quinoa Willd.) obtained from the market in Salta Province, Argentina were stored at 33% relative humidity (RH) and 5°C until used. Seeds in all experiments were pre-sorted by hand and excessively small, large and damaged seeds were discarded.
Moisture adjustment
Seeds were suspended in small aluminium foil baskets from the lids of 500 ml jars over saturated salt solutions for 8 days at 25°C. Moisture content was determined by drying seeds for 48 h at 75°C in a forced draft drying oven.
Results and Discussion
Water content values (WC) of quinoa seeds, after equilibration at different relative vapour pressures (p/p0) between 0.113 and 0.845 at 25°C was plotted against the corresponding p/p0. Fig. 1 shows the water sorption isotherm of quinoa seeds which is described with 95% confidence by the expression
Fig. 2 shows specific thermal power (p)–time (t) curves of germination during 240 min measured using quinoa seeds with an iWC of 0.054 g g−1. Subsequent p–t
Acknowledgements
E.S. thanks Ingemar and Lars Wadsö and Gerd Olofsson for providing facilities to perform part of this work at the Thermochemistry Division, Chemical Centre, Lund University, Sweden and for helpful discussions. This work was supported by grants to E.S. from Swedish Institute, Research Council from National University of Tucuman and CONICET.
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