| Summary: | 博士 === 國立臺灣大學 === 園藝學研究所 === 103 === High temperature is a critical factor limiting crop cultivation. It results abnormality changes plant morphological development as well as physilogocal metabolism then affects a reduction in economic yield. Leafy radish (Raphanus sativus L. var. oleiformis Pers.) with short growth period and numerous nutritional value had gradually became a new and alternative leafy vegetable. However, most current cultivars are heat-sensitive.
Therefore, the present study was to evaluate the genetic diversity of collected germplasm, establish cell membrane thermostability (CMT) heat selection index then breed for heat tolerant leafy radish. It also investigated the physiology, biochemical, and anatomy changes of leafy radish with different heat tolerance and evaluated the effect of calcium chloride (CaCl2) pretreatment on inducing heat tolerance. It respected to understand the heat tolerant mechanism and the role of Ca2+ under high temperature stress of leafy radish for facilting its heat tolerant breeding.
Twenty one collected leafy radish germplasm had significant variance in vegetative traits, vernalization, and heat tolerance. The genetic diversity was analyzed by inter simple sequence repeat (ISSR) molecular marker. 292 bands were amplified form 21 ISSR primers, 76.7% of them were reproducible and polymorphic that showed high diversity of leaf radish accessions. The grouping result according to clustering analysis had closely relationship with plant vegetative, reproductive traits and their origins. Besides, relative injury (RI) of cotyledon tissues under 50oC、30 mins water bath treatment had closely relationship with yield reduction caused by high temperature RI value lower than 50% was suggested as a heat toleranct selecting index for leafy radish genotypes during seedling stage then facilitate heat tolerant breeding efficiency of leafy radish.
For breeding heat tolerant leafy radish, germplasm were collected, evaluated then self-crossed, purified and segregated. The horticultural trait such as leaf characteristics, yield, disease incidence in field as well as CMT were taken for screening hybrid progeny according to maternal line selection followed by comparative and regional trails then developed new heat tolerant line F-11-09. Its yield, plant height, shoot fresh weight root length, and total soluble sugar (TSS) content were not affected by 35/30oC high temperature. The incidence for damping-off, and downy mildew of F-11-09 were also better than its parent S-07-01 and commercial cultivar ‘Taichung No.1 ’ Its leaf texture is relatively soft and flexible without trichome on both abaxial and adaxial sides, and the flavor is sweet. The yield is about 2.9-3.1 ton．ha-1. Therefore, new heat tolerant line F-11-09 can be suggested for leafy radish extension and cultivation.
A comparative study of new heat tolerant line F-11-09, its female parent S-07-01 and heat sensitive ‘Taichung No. 1’ under 40/35oC and 25/20oC was conducted to elucidate the heat tolerant mechanism of leafy radish. After 28 days of treatment, the maintance of net photosynthesis rate (Pn) and chlorophyll fluorescence (Fv/Fm) accompanying double increase of stomatal conductance (gs) and transpiration rate (E) in line F-11-09 revealed that the heat tolerant line was physiologically adjusted to be more efficient in heat dispersing. Anatomically, line F-11-09 developed more xylem vessels, enlarged 5 folds of stomatal aperature, and enhanced stomata opening percentage to 93% which contributed to 6 folds of increase in occupancy of total stomata apertures on leaf under high temperature stress. Furthermore, the occupancy was correlated between gs and E with coefficient of 0.87 and 0.98 confirmed that stomatal factors played an important role in heat tolerance in leafy radish. Line F-11-09 was able to maintain its cellular membrane stability by inducing superoxide dismutase (SOD) and catalase (CAT) activity to detoxified the unbalance of activated oxygen species (AOS) production and scavenging within the first 24 hours and other antioxidant enzyme activities within extended treatment of high temperature stress. Such result indicated that line F-11-09 was able to lower the non-stomatal limitation effect on Pn.
Besides, exogenous application of 200 or 500 mg．L-1 CaCl2, twice significantly improved the heat tolerance of leafy radish. It decreased the decline of net photosynthetic rate Pn, Fv/Fm, total chlorophyll content, and increased gs as well as E for removing accumulated heat. Plants with lower H2O2, malondialdehyde (MDA) concentration had the slighter oxidized damages by increasingg the activities of antioxidant enzymes SOD, CAT, ascorbate peroxidase (APX), and glutathione reductase (GR). In addition, pretreating Ca2+ chelator, ethylene glycol bis(2-aminoethyl) tetraacetic acid (EGTA) remarkly inhibited the activities of antioxidant enzymes that inversely proved the mechanism of Ca2+ under heat response in leafy radish.
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