Study on Screening of Drought Tolerance Index and Water Management of Bedding Plants
Date Issued
2015
Date
2015
Author(s)
Lee, Han-Yin
Abstract
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.
Subjects
drought
bedding plants
timing of irrigation
screening index
acclimation
Type
thesis
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