Study on Screening of Drought Tolerance Index and Water Management of Bedding Plants

於氣候變遷、水資源匱乏等全球趨勢下，植物更易面臨乾旱逆境。且於庭園景觀上，維持景觀美質與節水間的平衡亦為重要課題。花壇植物常受乾旱影響生長及開花而降低觀賞價值，因此，本研究擬瞭解花壇植物適當灌溉時機，篩選能反映植物遭受乾旱程度之生理指標與較耐旱之花壇植物，並探討復水、馴化對花壇植物之影響，以期可供實際栽培管理上之參考。 斷水乾旱下日日春[Catharanthus roseus (L.) G. Don]的葉片厚度、葉片水勢及葉片相對含水量顯著下降，且與視覺評分具高度相關性。冷季草花金魚草‘舞蹈’ (Antirrhinum majus L. ‘Snapshot’)、矮牽牛‘豪放’ (Petunia ×hybrid Hort. ‘Eagle’)及一串紅‘紅霞’ (Salvia splendens Ker-Grawl. ‘Vista’)分別於介質體積含水量(volumetric water content, VWC)降至19.8%、20.4%及23.0%時葉片開始下垂；以上位葉水勢作為暫時萎凋點，三種植物分別為-3.61、-2.35及-3.16 MPa。冷季草花適當灌溉時機於耐旱能力中上之種類約為40-45% FC；耐旱能力稍弱之種類約為45-50% FC。 於斷水乾旱下篩選11種花壇植物之耐旱生理指標。葉綠素計讀值、相對電解質滲漏率與視覺評分具低度相關性(r＝-0.6985*、-0.7002*)；葉片水勢與視覺評分具中度相關性(r＝-0.7777**)；葉片厚度、葉片相對含水量與視覺評分具高度相關性(r＝0.8773***、0.8764***)。氣孔導度、葉溫與氣溫差和其中3種冷季草花之視覺評分具高度相關性(r＝0.8661***、0.8826***)。顯示葉片相對含水量、葉片水勢、葉片厚度及葉片溫度為較適合之生理指標。以視覺評分、葉片相對含水量及葉片水勢的變化，將供試植物分為耐旱能力相對較佳之鳥尾花[Crossandra infundibuliformis (L.) Nees]、千日紅(Gomphrena globosa L.)、繁星花[Pentas lanceolata (Forsk.) Schum]、三色菫(Viola tricolor L. ‘Delta’) (冷季)及粉萼鼠尾草(Salvia farinacea Benth. ‘Victoria Blue’) (冷季)；及耐旱能力相對較差之藍星花(Evolvulus nuttallianus J. A. Schultes)、黃帝菊[Melampodium paludosum (Pers) DC.]、夏菫(Torenia fournieri Lind.)、羽狀雞冠花(Celosia argentea var. plumosus)、小百日草(Zinnia angustifolia Kunth.)及金魚草(A. majus L. ‘Snapshot’) (冷季)。暖季草花適當灌溉時機於耐旱能力相對較佳者約為35-40% FC；相對較差者約為40-45% FC。 於VWC維持約40% FC之長期乾旱下，地上部生長以夏菫‘可愛酒紅’ (T. fournieri Lind. ‘Kauai Burgundy’)及黃帝菊‘小天星’ [M. paludosum (Pers) DC. ‘Little Sky Star’]受較顯著的抑制，地下部則不受影響。於生理測值部分，葉綠素計讀值與葉片厚度無顯著變化，葉片水勢及葉片相對含水量亦較不敏感；光合作用參數的變化較為顯著，葉片溫度及水分利用效率次之，其中以氣孔導度最為敏感，可作為良好之耐旱生理指標。長期乾旱下氣孔導度與對照組之比值維持於0.80以上的植物種類，能維持光合作用而可能具較佳的耐旱性。供試植物中以千日紅‘佛陀紫’ (G. globosa L. ‘Buddy Purple’)耐旱能力最佳、羽狀雞冠花‘和服’ (C. argentea var. plumosus ‘Kimono’)及黃帝菊‘小天星’次之，夏菫‘可愛酒紅’較差。 經乾旱-復水循環14天後，光合作用參數於千日紅‘小矮人紫紅’ (G. globosa L. ‘Gnome Purple’)無顯著差異；但百日草‘夢境紅’ (Z. elegans Jacq. ‘Dreamland Red’)除淨光合作用速率外均顯著下降，水分利用效率則顯著提升，顯示其對乾旱較為敏感。兩種植物之葉片水勢、葉片相對含水量、葉綠素計讀值、葉片厚度及生長量均於處理間無顯著差異。花壇植物於VWC維持40% FC或於乾旱至40% FC時復水，光合作用參數及地上部生長可能受抑制，但對植體水分關係影響較小，顯示40% FC為較溫和的逆境。將剛進入開花期之黃帝菊‘小天星’及夏菫‘可愛酒紅’以40% FC之乾旱-復水循環作為馴化，處理9天對生長及生理表現均無顯著影響，較適合作為馴化或節水的栽培模式；處理18天後，地上部生長受到抑制而地下部不受影響。黃帝菊‘小天星’之氣孔導度不受影響且根部生長較佳，可能具滲透調節能力以維持膨壓，耐旱能力較佳；夏菫‘可愛酒紅’ 之氣孔導度顯著降低，主要藉關閉氣孔以減少失水。馴化後植株以花箱栽培於戶外，各處理之外觀及光合作用參數皆無明顯差異。顯示適度降低灌溉頻度對生長及開花的影響並不劇烈，亦能減少水資源的消耗，且於後續戶外栽培時能具良好的觀賞品質及光合作用能力。Plants today are more likely to encounter drought stress as a result of climate change and water scarcity. Balancing ornamental beauty against water saving is an important task in gardening and landscaping. Drought frequently affects growth and flowering of bedding plants adversely, thus lowers their aesthetic value. In this study, we attempted to determine the appropriate timing of irrigation for bedding plants, select physiological indices that reflect the degree of drought stress the plants have suffered, discover drought-tolerant species, and discuss effects of rewatering and acclimation on bedding plants. Catharanthus roseus (L.) G. Don was subjected to a continuous substrate drying episode by withholding water from pots. Leaf thickness, leaf water potential, and relative water content declined significantly, and correlated highly with visual scores. Water was also withheld from cool-season bedding plants Antirrhinum majus L. ‘Snapshot’ (Snapdragon), Petunia ×hybrid Hort. ‘Eagle’, and Salvia splendens Ker-Grawl. ‘Vista’. All three plants began to show leaf drooping when substrate volumetric water content (VWC) had declined to 19.8%, 20.4%, and 23.0% respectively. Water potential of upper leaves, defined as the temporary wilting point, were -3.61, -2.35, and -3.16 MPa respectively. The appropriate timing of irrigation for cool-season plants occurs when VWC has dropped to 40-45% of field capacity for plants with higher tolerance for drought, and 45-50% for those with lower tolerance for drought. Physiological indices for drought tolerance were screened by withholding water from 11 species of bedding plants. The results showed that SPAD-502 value and relative electrolyte leakage had low correlations with visual scores (r＝-0.6985*, -0.7002*), leaf water potential had a moderate correlation with visual scores (r＝-0.7777**), and leaf thickness and relative water content had high correlations with visual scores (r＝0.8661***, 0.8826***). Stomatal conductance and difference of leaf temperature minus air temperature (Tleaf－Tair) also had high correlations with visual scores in three cool season bedding plants (r＝0.8661***, 0.8826***). These results indicate that relative water content, leaf water potential, leaf thickness, and leaf temperature would be suitable physiological indices of drought tolerance. Drought tolerance of the 11 tested species was assessed based on variations in visual scores, relative water content, and leaf water potential after drought treatment. Species which have relatively better drought tolerance include Crossandra infundibuliformis (L.) Nees, Gomphrena globosa L., Pentas lanceolata (Forsk.) Schum, Viola tricolor L. ‘Delta’(cool season), and Salvia farinacea Benth. ‘Victoria Blue’(cool season); those that have relatively lower drought tolerance include Evolvulus nuttallianus J. A. Schultes, Melampodium paludosum (Pers) DC., Torenia fournieri Lind., Celosia argentea var. plumosus, Zinnia angustifolia Kunth., and Snapdragon (cool season). The appropriate timing of irrigation for warm season plants occurs when VWC has dropped to 35-40% FC and 40-45% FC for species with higher and lower tolerance for drought respectively. Under long-term drought treatment by maintaining VWC near or a little below 40% FC, root growth had no significant change while shoot growth was inhibited significantly in T. fournieri Lind. ‘Kauai Burgundy’ (Torenia) and M. paludosum (Pers) DC. ‘Little Sky Star’ (Melampodium). SPAD-502 value and leaf thickness had no significant change during treatment period. Leaf water potential and relative water content were also less sensitive. Changes in photosynthetic parameters were most sensitive, leaf temperature and water use efficiency followed. Among these, stomatal conductance was the most sensitive index which could serve as a good index for drought tolerance. Plant species with stomatal conductance ratio above 0.80 under prolonged drought when compared with control were better able to maintain their photosynthetic capacity and were considered more drought-tolerant. Among tested plants, G. globosa L. ‘Buddy Purple’ had the best drought tolerance, followed by C. argentea ‘Kimono’ and Melampodium, while Torenia had poor drought tolerance. After 14-day dry-down cycles, photosynthetic parameters in G. globosa L. ‘Gnome Purple’ had no significant change. In Z. elegans Jacq. ‘Dreamland Red’, photosynthetic parameters declined except net photosynthetic rate, whereas its water use efficiency increased significantly indicating that Z. elegans Jacq. ‘Dreamland Red’ was relatively sensitive to drought. Leaf water potential, relative water content, SPAD-502 value, leaf thickness, and biomass had no significant differences between treatments in both species. Maintaining VWC near 40% FC may inhibit photosynthetic parameters and shoot growth, but has caused little effect on plant water relations, indicating 40% FC may just be a mild water stress in bedding plants. When Melampodium and Torenia just entering flowering period were given dry-down cycles as acclimation for 9 days and 18 days, no significant adverse effect on growth and physiology were observed in 9 days, suggesting that 9 days would be an advisable length for acclimation or water-saving cultural mode. When plants were subjected to 18-day acclimation, shoot growth was inhibited, but root growth showed no significant changes. In Melampodium, stomatal conductance had no significant change after acclimation, but its roots seemed to grow better. This result indicates that Melampodium may have better osmotic regulation to maintain turgor, and may have good drought tolerance. In Torenia, stomatal conductance declined significantly after acclimation indicating that it mainly prevents water loss by closing stomata. After acclimation, plants were transplanted in flower boxes outdoors. There have been no significant changes in appearance and photosynthetic parameters between different acclimation treatments. Modest reduction in irrigational frequency has no severe effects on plant growth and flowering while saving water, excellent ornamental value and photosynthetic ability can also be preserved with subsequent outdoor planting