脱水速率和降温速率对葡萄柚种子超低温保存的影响

以新鲜葡萄柚(Citrus paradise)种子为材料，使用3种脱水速率处理至约20%含水量，然后用饱和盐溶液平衡至10种不同的含水量，再进行简化的二步法超低温保存；同时，以经过15 ℃、50%相对湿度(RH)条件脱水至含水量约12%的葡萄柚种子为材料，使用程序降温仪进行5个降温速率和3个预冷温度的传统二步法超低温保存。结果表明，葡萄柚种子对脱水敏感，种子含水量越低，成苗率越低。而且，脱水速率越快，对种子的损伤越大，成苗率呈现75% RH>50% RH>15% RH；冷冻处理对种子有进一步的伤害，含水量6%～8%的种子超低温保存效果最佳，但脱水速率对超低温保存的种子成苗率的影响不明显。用传统二步法超低温保存葡萄柚种子，与预冷对照相比，经液氮冷冻的种子成苗率都有明显下降。在预冷对照组中，-2.5 ℃·min^-1降温速率的种子成苗率最高，在冷冻保存中-0.5 ℃·min^-1和-0.25 ℃·min^-1的降温速率最有利于超低温保存。预冷温度从-40 ℃降至-60 ℃，未超低温处理种子的发芽率降低，但超低温处理种子的发芽率提高。

Effects of Dehydration Rates and Freezing Rates on Cryopreservation of Grapefruit Seeds

Fresh grapefruit seeds were firstly desiccated at 3 different dehydration rates to about 20% moisture content (MC), and equilibrated to 10 different MC by saturated salt solutions, then cryopreserved using a simplified two-step protocol; Meanwhile, another fresh grapefruit seeds was firstly dehydrated to about 12% MC under 15 ℃ and 50% relative humidity (RH) conditions, and then cool down at 5 cooling rates and 3 pre-cooling temperatures using a programmable cooler before frozen in liquid nitrogen. It was found that grapefruit seeds were sensitive to dehydration, and the emergence percentage fell with the decrease of MC. The damage to seeds increase with the acceleration of dehydration rate under slow dehydration. The germination percentage was expressed as 75% RH > 50% RH > 15% RH; Freezing treatment brought further damage to seeds, but there were no significant difference among germination percentage of frozen seeds dried at the three dehydration rates, and the best post thaw germination found in seeds at 6%-8% MC. Compared with the pre-cooling control, freezing in liquid nitrogen significantly reduced seed emergence. The seeds pre-cooled at rates of -0.5 ℃·min^-1 and -0.25 ℃·min^-1 had the best cryopreservation results, while the pre-cooling rate of -2.5 ℃·min^-1 was optimal for the pre-cooling control seed. The decrease in pre-cooling temperature from -40 ℃ to -60 ℃ reduced the emergence of non-cryopreservation seeds, but increased the emergence of cryopreserved seeds.