2. Water Sorption Isotherm Characteristics of Seeds of Six Indigenous Forest Tree Species in Ghana

Abstract

The relationship between storage temperature, relative humidity and seed water content was investigated for six
indigenous forest tree seed species, namely Garcinia kola, Terminalia superba, Terminalia ivorensis, Mansonia
altissima, Entandrophragma angolense and Khaya anthotheca in Ghana. Seeds were equilibrated over a series of
lithium chloride solutions with relative humidities ranging from 12 to 93% and silica gel with relative humidity of
3% at 20 ºC. Seeds reached equilibrium with different days depending on seed size and structure, ranging from 13
days for E. angolense to 91 days for G. kola. When seeds equilibrated, moisture contents were determined
gravimetrically, and values of moisture contents were then plotted against relative humidity to construct moisture
sorption isotherms for the species. Seeds of T. superba, T. ivorensis, M. altissima, E. angolense and K. anthotheca,
exhibited a sigmoidal relationship between seed water content and relative humidity indicative of three regions of
water binding. Contrarily to other reports, the shape of the isotherm curve for G. kola – a desiccation sensitive
species – also showed the reversed sigmoid pattern similar to isotherm curves of orthodox species rather than the
monotonic shape. The isotherms showed that seed moisture content increased with increasing relative humidity.
Seed samples of G. kola, placed at all relative humidities chambers, lost water(desorption) as the initial water
content of 58% was very high and, therefore, possessed a higher water potential than the environments. Seeds of
the other species either lost water (desorption) to the relative humidity chambers, or absorbed water from the
chambers depending on the relative humidity of the environment they were placed.