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Wageningen Universiteit (2006)

Conservation of tree seeds from tropical dry-lands

Neya, O.

Titre : Conservation of tree seeds from tropical dry-lands

Auteur : Neya, O.

Université de soutenance : Wageningen Universiteit

Grade : PhD thesis 2006

Résumé partiel
The tropical trees, Azadirachta indica (neem), Lannea microcarpa, Sclerocarya birrea and Khaya senegalensis, are important multipurpose species. Unfortunately, difficult seed storage behaviour limits the utilization of these species in reforestation programs and agroforestry systems. This thesis presents the results of investigations aimed at a better understanding of the seed biology, particularly focussed on the improvement of seed survival after drying and subsequent dry storage. Seeds collected from several stands in Burkina Faso were studied in an attempt to elucidate the causes of viability loss and improve life span. Initially it was thought that the difficulty in storing seeds of these species is due to intolerance of desiccation and sensitivity to low (subzero 0C) storage temperatures. However, the results of the current study indicated that other factors are responsible for the difficult storage behaviour of these seeds. When the moisture content (MC) was reduced from 27 to 5%, neem seeds became sensitive to rehydration temperatures below 25°C, resulting in a noticeable decrease in germination percentage. Rehydration of these dried seeds at elevated temperatures (25-400C) for one hour prior to further incubation at 300C substantially improved germination to a level comparable to that of fresh seeds (27% MC). We concluded that neem seed is desiccation tolerant, but that during drying its germination requirements change, which is the main cause of its difficult storage behaviour.

Germination capability of S. birrea seeds was low in all seed lots at harvest, but improved substantially during drying and/or dry storage. Improvement of germination with drying is completely in contrast with what would happen in a desiccation-sensitive seed. We interpreted the observed increase in germination during drying and dry storage as resulting from the slow disappearance of an initially present physiological dormancy. Seeds of S. birrea thus behave as orthodox seeds, but the irregularity in their germination behaviour has obviously contributed to discrepancies between reports on the species seed biology.

Seeds of L microcarpa harvested in three successive years were extremely variable as to their germinability and storability. Some seed accessions had a generally low germination, while others had an initially high germination. Drying did not affect germination capability of the poorly germinating seed accessions, but did so in the well germinating accessions. These observations would suggest that seeds can be either tolerant or sensitive to desiccation. However, when mechanically scarified prior to germination tests, seeds from all accessions germinated at high percentage (>80%), regardless of MC and age. This indicates that seeds experienced germination constraints upon drying rather than being sensitive to desiccation. Imbibed, intact seeds that failed to germinate displayed a moderate rate of respiration with RQ values< 1, which indicates that the seed coat is permeable to water and respiratory gases. On scarification, water uptake and respiration steeply rose in narrow association with radicle emergence. Non-scarified seeds lost germination ability and cellular viability after approx 15d of moist incubation at 300C. Apparently, coat-imposed inhibition of germination does not prolong viability of seeds in the hydrated state. These results suggest that radicle emergence is inhibited mechanically and not by germination inhibitors or physiological dormancy. We conclude that L microcarpa seeds display orthodox storage behaviour, but that physical dormancy explains the difficult seed storage behaviour reported for this species

Mots clés : seeds / trees / plant physiology / azadirachta indica / lannea / sclerocarya birrea / khaya senegalensis / seed germination / storage / burkina faso / germinability

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Page publiée le 23 mars 2007, mise à jour le 3 juin 2022