| Summary: | 碩士 === 國立臺灣大學 === 園藝暨景觀學系 === 103 === Poinsettia is an important pot flower in the world. It is the second largest pot flower with annual sales over 120 million pots valued at about NT$60 million in Taiwan. Poinsettia is native to the mountains of Mexico and requires heavy fertilization. The global climate change has resulted in extreme summer heat and uneven distribution of rainfall in Taiwan. Moreover, agricultural productions worldwide have been moving towards sensible irrigation and fertilization with an emphasis on sustainability. In response to the extreme climate as well as the changes in production methods, this study aims to exa mine these four subject areas:
1) Water requirement and establishing indicators for irrigation for poinsettia
2) Fertilizer requirement
3) Selection of heat-tolerant species
4) Chemical treatments to improve heat tolerance
In terms of water requirement and establishing indicators for irrigation in the production of pot poinsettia, the plants were cultivated with 20%, 30%, and 40% VWC for eight weeks. The result showed that 20%VWC had caused deterioration in appearance quality where plant height, stem diameter, leaf area, number of leaves and shoot dry weight were all significantly lower than those with 30% or 40% VWC treatment. Significant difference in ornamental quality was found between 30% and 40% VWC treatments. It is recommended that the media water content for the production of poinsettia should be at least 30%VWC to prevent defoliation and the associated problems in the decline of quality.
Leaf as well as plant temperatures, net photosynthesis, rate of transpiration, and media VWC are all suitable indicators for irrigation that help to identify potential drought stress prior to any stress-related changes in plant appearance become visible.
Initial application of fertilizers at 14 days after cutting has yielded the best result showing optimal performance in leaf area, plant height, stem diameter, shoot dry weight, chlorophyll meter readings and normalized difference vegetation index (NDVI). Varying the proportions of N-P-K in liquid fertilizer for different growth stages showed that higher nitrogen given during seedling stage (30N-4.6P-8.3K) resulted in the best performance in plant height, stem diameter, and shoot dry weight for all subsequent growth stages.
The selection of heat-tolerant species was done through cell membrane thermostability (CMT). The following nine species of 15 cm pot poinsettia turned out to be heat-tolerant: ‘Noel’, ‘Jacobson Peterstar’, ‘Jubilee Red’, ‘Winter Rose’, ‘Novia’, ‘Marbella’, ‘Luv U Pink’, ‘Premier Red’, and ‘Shimmer Surprise’. When under high temperatures, heat-tolerant species ‘Noel’, ‘Jacobson Peterstar’, ‘Novia’ and ‘Marbella’ showed smaller declines in net photosynthesis, stomatal conductance, shoot dry weight and leaf area than those for the non-heat-tolerant ‘Red Velvet’ and ‘Ice Punch’.
Calcium chloride and salicylic acid effectively improved the heat tolerance for poinsettia ‘Noel’, ‘Red Velveteen’, and ‘Pepride’ where the measurements in leaf temperature and CMR were significantly better than the control group.
Optimal concentrations in chemicals varied among the cultivars. The best application concentration for calcium chloride was 10 mM whereas the best application concentrations in salicylic acid differed for the species. Salicylic acid concentrations of 400 μM for ‘Noel’, 200 μM for ‘Red Velveteen’, 100 and 200 μM for ‘Pepride’ resulted in superior appearance quality.
In conclusion, plant temperatures, media VWC and measurements for photosynthesis all can be used to assess the water status of the plant accurately. Plant temperatures and VWC measurements are inexpensive, fast, and accurate; thus have practical applications for the poinsettia industry. Applying fertilizer 14 days after cutting and heavy nitrogen given during the seedling stage will enhance the growth potential for poinsettia and shorten the production cycle. Cell membrane thermostability has proved to be a viable method for the selection of heat-tolerant poinsettia species. Foliar spray of calcium chloride or salicylic acid would improve heat tolerance in poinsettia. The recommended application concentration for calcium chloride is 10 mM, and 200 μM for salicylic acid.
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