| Summary: | 碩士 === 國立臺灣大學 === 園藝學系研究所 === 86 === High nitrate content in the foodstuff has been proven to be hazardous to t
he human health. Among the horticultural crops, leafy vegetables tend to accum
ulate high nitrate content in the tissues. Experiments have been con-ducted to
produce vegetables of both reasonable yield and high quality, i.e.low nitrate
, high vitamin C and high mineral contents. The growth medium prepared from c
ultivated soil mixed with 5-20% v/v bark compost contained 8.7-13.4% organic
matter has been used in the experiments. On 5th (when Pak-choi seeds germinati
on rate exceeds 90%)and 20th day after sowing, each 31.11kg N/ha in urea form
was applied to the soil. Pak-choi har-vested on 30th day contained less than 1
000mg/kg f.w.of nitrate.Those har-vested between 35-40th days contained less t
han 500mg/kg f.w., while vitamin C content in all samples exceeded 1000mg/kg f
.w.. Pak-choi grown in a medium containing 12.4-13.4%organic matter and harve
sted on 30th day after sowing resulted in a plant weight of 20 gram up and a n
itratecontent of less than 550 mg/kg f.w.. The result seems to be an ideal way
of production. The analyses of Pak-choi depicted that the nitrate content
in outer leaveswas significantly higher than those of inner leaves, while this
tendency couldnot be observed in leaf lettuce. The average nitrate content ra
nged around 1000mg/kg f.w. and vitamin C 1600mg/kg f.w.in middle leaves. In g
arland chrysanthemum , an increased rate of nitrogen application corre-lated p
ositively with an increase in nitrate concentration. An application of187.65kg
N/ha in the treatment resulted in as much as nitrate content 4000mg/kgf.w.. D
rought treatment of the medium resulted in a high nitrate content in Pak-chei
leaves when harvested in the early morning. 40 day-old Pak-choi derooted, ba
re rooted or with intact roots and attached soil could be cold stored at 21℃
up to 21 days. The former 2 treatments leadto a nitrate content less than 250
mg/kg f.w. and vitamin C content higher than1200 mg/kg f.w. in the leaves, whi
le the latter (bare rooted ones) resulted inan inferior quality. Cold storag
e of lettuce seemed not to change much both in nitrate and vitaminC contents,
while an acropetal transport of N and K seemed to occur from rootsto shoots du
ring storage.
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