甘藍節水灌溉之研究

Abstract

摘要 甘藍(Brassica oleracea L. var. capitata)為臺灣民生消費市場中不可或缺的蔬菜，平地甘藍產區之產季適逢降雨量較少月份，近年氣候變遷使缺水日益頻繁，農業灌溉用水因而受限，期能降低農業灌溉水量，並維持商品價值。本研究以甘藍‘初秋’為試驗材料，調查其於不同斷水日數、介質體積含水量(volumetric water content, VWC)範圍與作物需水量(crop water requirement, CWR)灌溉率處理下之外葉生理變化以及葉球鮮重與品質，探討節水灌溉指標。於甘藍葉球形成期進行斷水日數試驗，分別斷水4、8、12、16、20天，結束後復水；斷水4天時椰碳土介質VWC降至30%，之後復水24天採收，其葉球鮮重與正常供水組相同；而在斷水12天後，椰碳土VWC降至15%，外葉SPAD讀值顯著上升，其能反應作物面臨高強度水分逆境。在甘藍葉球充實期，於不同日期斷水不同天數；斷水4天組之泥炭土介質VWC皆降至30%以下，復水後葉球鮮重與正常供水組無顯著差異。自葉球形成期至採收，於不同發育時期均斷水僅4日，則葉球鮮重於各組間無顯著差異，而葉球硝酸鹽與抗壞血酸含量以正常供水組顯著低於所有處理。若在葉球充實期，每2天以灌溉率CWR 100%、90%、80%、70%、60%與50%處理，所有處理之泥炭土VWC在全期皆維持於50%以上，且葉球鮮重間皆無差異，但乾物率以CWR 50%組為顯著最高，葉球硝酸鹽與抗壞血酸含量以灌溉率CWR 100%組最低。故進一步在夏秋季葉球形成期，以灌溉率CWR 100%、60%、50%、40%、30%、20%處理；灌溉率等於及低於CWR 30%組之泥炭土VWC在第12天後小於50%，且葉球鮮重、球寬與硝酸鹽含量皆顯著較低，但官能品評之脆度、纖維度、甜味、辣味、苦味及菜味則無顯著差異，故灌溉率應不低於CWR 40%。於葉球形成期至採收，將泥炭土VWC維持在60%以上、60%~50%、50%~40%與40%~30%，以及在VWC連續4天以上於50%~40%與40%~30%時分別做復水處理；各組之SPAD讀值、Fv/Fm與NDVI分別在第20天、第6天、第10天開始出現顯著差異，其中以VWC 40%~30%組之葉球鮮重、球高、球寬、中心柱長為各處理中最低，而側邊葉球緊實度則以VWC 50%~40%之復水組與VWC 40%~30%之復水組皆為最低。自前兩試驗中分別選出最佳處理，於冬春季葉球形成期測試不同灌溉處理之差異，試驗期間VWC 50%~40%組之泥炭土VWC變化數值及葉球相關參數皆與CWR 60%、CWR 45%相近，而VWC 40%~30%組於處理期間之泥炭土VWC、葉球鮮重與葉球形態參數為所有處理中最低，但葉球內含抗壞血酸、總鹽與可溶性固形物含量較高。綜之，溫室泥炭土甘藍栽培，應以CWR 50%~40%為依據，進行節水灌溉，且避免泥炭土VWC低於40%以下超過連續4天，雖然水分利用效率未提高，但能節省約30%之灌溉水量，且維持葉球產量與品質。

Abstract Cabbage (Brassica oleracea L. var. capitata) is one of the important essential vegetable crops in Taiwan. Growth season in production area of lowland cabbage is relatively low in precipitation. Climate change causes precipitation reduction year by year and the agricultural irrigation water is limited. It is expected to lower the application of irrigation amount with good product quality and raise the crop water use efficiency (WUE). In this thesis, cabbage ‘KY cross’ was used as experimental material. The physiology of spread leaves and the yield and quality of leafy head under irrigation based on the range of medium volumetric water content (VWC) and crop water requirement (CWR) were investigated to select useful index for water saving irrigation. From initial head formation to harvest, VWC will drop to below 30% after only 4 days of water dis-supply. But after re-supply water for 24 days, fresh weight of leafy head would be insignificantly different with the control. On the 12th day of water dis-supply, VWC dropped to below 15% and SPAD value significantly raised up. SPAD value could reflect plant status under serious water deficit stress. From head formation to harvest, VWC dropped to below 30% after any 4 days of water dis-supply. After re-water, fresh weight of leafy head was not significantly different with the control. Four days of water dis-supply showed no effect on leafy head fresh weight. Nitrate and ascorbic acid contents in leafy head of control are significantly lower than any 4-days water dis-supply treatments from head formation to harvest. From head filling to harvest, irrigation were based on CWR 100%~50% per 2 days. In all treatments, VWC would maintain at higher than 50% and leafy head showed no significant difference with the control. Dry matter in CWR 50% treatment was the highest, while nitrate and ascorbic acid contents in CWR 100% the lowest. From head formation to harvest, irrigation was further based on CWR 100%, 60%, 50%, 40%, 30% and 20% per 2 days. In CWR 30% treatment, VWC dropped to below 50% after 12th day and fresh weight and equatorial diameter of leafy head were significantly lower. In organoleptic evaluation, crispy, firmness, sweetness, spice, bitterness and odd flavor showed no difference among all treatments. Hence, CWR 40% was suggested as the lowest irrigation rate. From initial head formation to harvest, with maintaining VWC in specific ranges of >60%, 60%~50%, 50%~40%, 40%~30%, re-water to field capacity after 4 days of 40%~30%, and re-water to field capacity after 4 days of 50%~40%. SPAD value, Fv/Fm and NDVI began to show significant difference among all treatments after 20th, 6th, 10th days, respectively. Leafy head fresh weight, polar diameter, equatorial diameter and core length were the lowest in VWC 30%~40% treatment. When water was re-supplied to the field capacity after 4 days of both VWC value 50%~40% and VWC 40%~30%, the head side firmness was the lowest. Hence, VWC 50%~40% was suggested as the lowest irrigation rate. The best treatments in previous trials, irrigation rates of CWR 45% and VWC 50%~40%, along with CWR 60%, CWR 35%, VWC 60%~50%, VWC 40%~30%, were applied to confirm the effect and efficiency. Medium VWC variation and leafy head shape index of VWC 50%~40% treatment had similar trends with those of CWR 60% and CWR 45%. VWC 40%~30% had the lowest VWC, fresh weight and shape index of leafy head among all treatments. In conclusion, cabbage cultivated with peat medium in the green house should irrigate based on CWR 50%~40% and avoid peat VWC below 40% continuously longer than 4 days. Although the water use efficiency of this practice did not increase, it could save about 30% irrigation water with maintaining fresh weight and quality of leafy head.