不同收获时期和控氮条件对水培小白菜硝酸盐含量的影响

在水培中采用收获前降低营养液浓度、完全停止供氮以及在不同时期采收等方法来研究小白菜硝酸盐含量的控制方式及其效果,结果表明:适时采收和收获前适当控氮可以降低小白菜的硝酸盐含量。在小白菜生长第26d左右硝酸盐含量最低,此后l~2d产量增长较快,此时采收品质较好;小白菜硝酸盐含量呈明显的日变化,并且与硝酸还原酶活性呈正相关,在下午16:00时左右采收硝酸盐含量最低,仅为894mg.kg-1;完全断氮7d后,小白菜硝酸盐含量显著降低,但产量也受到明显的影响。而采用低浓度营养液控氮4d左右采收既可控制硝酸盐含量又不会使产量过低。

Effects of harvesting time and controlled supply of nitrogen on nitrate content of Brassia campestris in hydroponics

The nitrate content in Pak-chai (Brassia campestris L.) grown in hydroponics was controlled by decreasing nutrient solu- tion concentration or adjusting N supply before harvest and by harvesting the vegetable at different time periods. The results showed that the nitrate content in Pak-chai reached the lowest level at 25 d after transplanting, after which its yield increased rapidly and was suitable for harvest. The nitrate content in Pak-chai changed during the day, reaching a lowest level (894 mg.kg-1) at 4:00 pm, and such a diurnal change was correlated with nitrate reductase activity. A complete withdraw of N supply for 7d could significantly re- duce nitrate content but markedly decreased the yield as well. A better alternative was to control N supply at low concentration 4 to 6 d before harvest with no adverse effects on yield.