Ultrastructural and biophysical changes in developing embryos of Aesculus hippocastanum in relation to the acquisition of tolerance to drying

Changes in ultrastructural, biochemical and biophysical characteristics of embryonic axes of Aesculus hippocastanum during development are related to changing levels of desiccation tolerance. Histodifferentiation was complete 30 days after flowering (DAF) and fruits were shed about 120 DAF. During t his period, the dry mass of embryonic axes increased from about 0.5 to 4 mg and the water content decreased from 10.2 to 2.0 g H2O g(-1) dry mass (g g (-1)). In spite of the large changes in water content, water potentials of freshly harvested material declined slightly during development from -0.65 to -2.0 MPa. Tolerance of desiccation increased as embryos matured. If eval uated on the basis of critical water contents for survival; tolerance appea red to increase continuously, maximum tolerance being achieved at 120 DAF w hen embryos survived water contents as low as 0.30 g . g(-1). When evaluate d from critical water potentials, three distinct levels of desiccation tole rance could be recognized at - 1.8 MPa (30-40 DAF), -4 MPa (48-90 DAF) and -12 MPa (100-120 DAF). During development, total dry matter increased while sugar content (g g(-1) dry mass) and osmotically active material (mmol g(- 1) dry mass) decreased. The subcellular organisation of axes was always typ ical of metabolically active tissues. Maximum tolerance (-12 MPa) was assoc iated with a reduced amount of monosaccharides and the appearance of water with unusual calorimetric behaviour. Our data are consistent with several o f the current hypotheses regarding the mechanisms of desiccation tolerance. Accumulation of dry matter reserves, reduced levels of monosaccharides, pr esence of dehydrin-like proteins and ability to form glasses appear to be a ssociated with the changes in desiccation tolerance. However, none of these factors allow A. hippocastanum embryos to achieve the extreme level of des iccation tolerance typical of orthodox seeds. This may be because A. hippoc astanum embryos do not reach physiological maturity and remain metabolicall y active even after they are shed from the parent plant. Also, embryos may acquire incompetent protectants or lack as yet unidentified protective mech anisms.