Axes and cotyledons of recalcitrant seeds of Castanea sativa Mill. exhibitcontrasting responses of respiration to drying in relation to desiccation sensitivity

Oxidative damage originating from uncontrolled metabolism is thought to be responsible for the sensitivity to drying in recalcitrant seeds. This study compares the responses of respiration to drying and the loss of membrane i ntegrity in isolated axes and cotyledons of the recalcitrant seeds of Casta nea sativa Mill. Electron spin resonance spectroscopy of two nitroxide spin probes introduced into the seed tissues was used to assess the cytoplasmic viscosity and the membrane permeability during fast and slow drying. Dryin g rates had no effect on the rise in viscosity in axes and cotyledons, In b oth tissues, the cytoplasmic viscosity during drying remained constant at 0 .2 Poise until 1.6 g water/g DW (g/g), thereafter it increased exponentiall y, Axes were found to be more tolerant to drying than cotyledons: membranes showed minor changes in their permeability during drying and 50% viability was retained in dried axes containing 0.12 g/g, In contrast, plasma membra nes in cotyledons lost their integrity below 0.6 g/g, regardless of the dry ing rate. Drying axes and cotyledons exhibited contrasting responses of the ir metabolism to drying. At the onset of drying, the rates of O-2 uptake de clined rapidly in drying axes. However, respiration in drying cotyledons se quentially increased to c. 1.4-fold at 1.2 g/g then decreased concomitantly with the loss of membrane integrity. The respiratory quotients (CO2 output /O-2 input) remained constant around 0.9 until the loss of membrane integri ty, then rose to 2.8. As a symptom of mitochondrial injury, the levels of r eduction of cytochromes were assessed in situ in fresh and dried cotyledons using light spectroscopy. The levels of reduced cytochrome c and aa(3) wer e lower in dried C. sativa cotyledons than in dried orthodox cotyledons of cowpea, indicating that a disruption in the electron transport chains may h ave occurred during drying. Desiccation sensitivity in recalcitrant seeds m ay be due to the inability to actively depress their metabolism during dryi ng, thereby increasing the chances of initiating peroxidative damage during drying.