Axes and cotyledons of recalcitrant seeds of Castanea sativa Mill. exhibit contrasting 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 integrity in isolated axes and cotyledons of the recalcitrant seeds of Castanea 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. Drying rates had no effect on the rise in viscosity in axes and cotyledons, In both tissues, the cytoplasmic viscosity during drying remained constant at 0.2 Poise until 1.6 g water/g DW (g/g), thereafter it increased exponentially, 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 membranes in cotyledons lost their integrity below 0.6 g/g, regardless of the drying rate. Drying axes and cotyledons exhibited contrasting responses of their metabolism to drying. At the onset of drying, the rates of O(2) uptake declined rapidly in drying axes. However, respiration in drying cotyledons sequentially increased to c. 1.4-fold at 1.2 g/g then decreased concomitantly with the loss of membrane integrity. The respiratory quotients (CO(2) output/O(2) input) remained constant around 0.9 until the loss of membrane integrity, then rose to 2.8. As a symptom of mitochondrial injury, the levels of reduction of cytochromes were assessed in situ in fresh and dried cotyledons using light spectroscopy. The levels of reduced cytochrome c and aa(3) were lower in dried C. sativa cotyledons than in dried orthodox cotyledons of cowpea, indicating that a disruption in the electron transport chains may have occurred during drying. Desiccation sensitivity in recalcitrant seeds may be due to the inability to actively depress their metabolism during drying, thereby increasing the chances of initiating peroxidative damage during drying.