Abstract
During drying and freezing, protective mechanisms act to maintain seed physiological quality. Some of these mechanisms contribute to the integrity of cell membranes. The damage caused to cell membranes due to cell stress can be seen in ultrastructural studies. The aim of this study was to investigate ultrastructural changes in endosperm cells of coffee seeds brought about by drying and by exposure to low temperatures. Seeds of Coffea arabica were dried in silica gel to moisture contents of 40, 20, and 5 % (wb) and brought to equilibrium at temperatures of 10, -20, and -86oC. Germination, vigor, and tetrazolium tests were performed for evaluation of seed physiological quality. Ultrastructural damage was analyzed by scanning electron microscopy. Coffee seeds with 40% moisture content have whole, swollen, and expanded cells, with a filled lumen and without signs of damage. The physiological and ultrastructural quality of seeds exposed to below zero temperatures with 40% moisture content is compromised. They have null germination and empty cells, indicating leakage of cell content. Drying of coffee seeds leads to uniform contraction of inner cell content. Drying of coffee seeds to 5% moisture content leads to intense contraction of cell volume, with physiological and ultrastructural damage. Key words: Scanning electron microscopy; desiccation tolerance; Coffea arabica L.
Highlights
Conservation of coffee seeds for long periods continues to be a challenge for producers and plant breeding programs due to coffee seed sensitivity to desiccation
According to the results of the germination test (Figure 2A), the moist seeds, i.e., those with 40% moisture content, only survived under the storage temperature of 10 oC. When these seeds were exposed to -20 and -86 oC, there was no germination percentage. This may have occurred due to damage caused by ice crystals since the seeds were exposed to ultra-low temperatures in a hydrated state
The seeds dried to 20% moisture content exhibited high germination after storage at temperatures of 10 and -20 oC (Figure 2A), with normal seedling percentages near 80%
Summary
Conservation of coffee seeds for long periods continues to be a challenge for producers and plant breeding programs due to coffee seed sensitivity to desiccation. Seeds are stored to preserve genetic resources; the purpose is to maintain the integrity and viability of plant structures over long periods. The conservation of orthodox seeds in germplasm banks consists of storing them at low moisture contents, around 5±2%, at a temperature of -20 °C (Umarani; Aadhavan; Faisal, 2015). The temperature of -20 °C provides for preservation of seed quality for a longer period of time, there is a critical and ideal moisture content indispensable for preserving quality for each different storage temperature (Walters, 2015). For seeds with recalcitrance characteristics, such as coffee, conservation is done through germplasm banks in situ (Dussert et al, 2012)
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