Leafy vegetables cultivated in greenhouses during the winter are sometimes exposed to cold air from outside the greenhouse to enhance sugar and nutrient content. To analyze the possible involvement of photosynthetic and respiratory activities in this process, we evaluated the gas-exchange activity of spinach (Spinacia oleracea L.) plants cultivated in an unheated greenhouse in mid-winter in Sapporo, where the daily mean air and soil temperatures are approximately -5 and 0 °C, respectively. Shoot fresh weight showed little increase, whereas the net leaf photosynthetic rate (P_n) attained 20 µmol m^-2 s^-1 and the CO₂ concentration in the greenhouse ([CO₂]) was sometimes lower than 200 µmol mol^-1, which was suggestive of active photosynthetic CO₂ uptake. After its peak in the morning, P_n decreased in the afternoon, presumably owing to ‘midday depression’ caused by suppressed water uptake in the root zone. Observed diurnal [CO₂] change was consistent with a significant CO₂ uptake during the daytime. The change also suggested that respiration was active immediately after sunset and suppressed at night. In addition, we calculated the whole-greenhouse CO₂ emission rate (R) as a measure of night respiration in the plants, taking into account the air ventilation of the greenhouse. The R value was positive under sub-zero air temperatures in the greenhouse and was positively correlated with the nighttime air and soil temperatures. These experimental data suggest active photosynthesis and respiration of winter-sweetened spinach in the greenhouse, despite the low air and soil temperatures and growth retardation, and implies their involvement in the sweetening process.

中文翻译：

---

日本札幌冬季未加热温室中菠菜的光合和呼吸活动

冬季在温室中种植的带叶蔬菜有时会从温室外部暴露于冷空气中，以增加糖和营养成分。为了分析光合作用和呼吸活动在此过程中可能的参与，我们评估了札幌冬季中空未加热温室中种植的菠菜（菠菜（Spinacia oleracea L.））的气体交换活性，那里的日平均气温和土壤温度分别约为-5和0°C。枝条鲜重几乎没有增加，而净叶片光合速率（P _n）达到20 µmol m ^-2 s ^-1，温室中的CO ₂浓度为[[CO ₂]）有时低于200 µmol mol ^-1，这表明活性光合作用吸收了CO ₂。在早晨达到峰值之后，P _n在下午下降，这大概是由于根部区域水分吸收受到抑制而导致的“午间低压”。白天观察到的[CO ₂ ]日变化与白天的大量CO ₂吸收一致。这种变化还表明，日落后立即呼吸活跃，晚上则受到抑制。此外，我们还考虑了温室的空气流通，计算了整个温室的CO ₂排放率（R），作为植物夜间呼吸的量度。在[R在温室中低于零的气温下，该值是正的，并且与夜间的空气和土壤温度呈正相关。这些实验数据表明，尽管空气和土壤温度低且生长发育迟缓，但温室中冬季加糖的菠菜仍具有活跃的光合作用和呼吸作用，并暗示它们参与了甜味过程。