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- Author or Editor: Gregory E. Welbaum x
It is unclear from previous reports whether muskmelon seeds require an afterripenig period to attain maximum germinability and vigor. In the current study, seeds ranging in age from 30 to 60 days after anthesis were stored at water contents ranging from 3 to 15% and at either 6 or 30°C to determine whether seed vigor increased during storage. Changes in vigor were assessed by conducting monthly germination tests on blotter papers saturated with water or polyethylene glycol solutions of known water potential. The germination percentages of immature seeds (30 and 35 DAA) were dramatically improved by 3 months of storage at low water content and temperature, while the mean time to germination and the variability of germination were reduced for all stages of development. Germination percentages in water decline after storage at high water content and temperature with immature seeds showing a greater rate of decline than mature seeds but at reduced water potentials, the same adverse storage conditions increased the germination percents es and rates of mature seeds. However prolonged storage under adverse conditions, resulted in a gradual decline in water stress tolerance. Afterripening occurred over a wide range of storage conditions and significantly improved seed vigor, particularly in immature seeds. Furthermore, the increases in vigor achieved from afterripening treatments were remarkably similar to the increases in vigor attained through priming. Priming may substitute for the afterripening requirement of muskmelon seeds.
Muskmelon (Cucumis melo L.) seed crops sometimes contain seeds with split coats that expand to twice their normal water content. These expanded seeds are often referred to as “fishmouth” seeds, because the split seed coat resembles an open fish's mouth when viewed longitudinally. “Fishmouth” seeds are dead seeds. However, little is known about why death occurs inside the fruit before harvest. Hermaphroditic flowers were tagged at anthesis and fruits were harvested at various intervals during the later stages of development and decay. Seeds were removed from the fruits and incubated in water on germination blotter paper for 14 days. The percentage of germinable, dead and “fishmouth” seeds were averaged for each Harvest date. Fruit pericarp samples were analyzed for pH, ethanol, and acetic acid content. At 50 days after anthesis (DAA), just past edible maturity, 100% of the seeds germinated. However, at 60 and 78 DAA germination dropped to 60 and 17%, respectively, while the occurrence of “fishmouth” seeds increased from 2 to 54% over the same period. The ethanol content of the tissue increased from 0.11 to 0.28%, the pH dropped from 6.2 to 5.1, and acetic acid concentration increased from 3.0 to 3.7 mM from 50 to 60 DAA, respectively. However, when dried seeds were incubated in the laboratory under conditions similar to those within the fruit, the formation of “fishmouth” seeds was related to the ageing effects of long term hydration and was not correlated with any chemical product within the fruit.
Seed production in the family Cucurbitaceae is more complicated than in dry-seeded grain crops because seeds mature within a moist fruit and are often held at high moisture content for several weeks before seed harvest. Muskmelon (Cucumis melo L.), a member of this family, was used as a model system to contrast seed development with crops that are dry at maturity. A detailed time course for `Top Mark' fruit and seed development is presented based on previous studies. In muskmelon fruit, precocious germination is inhibited osmotically by the low water potential of the surrounding fruit tissue. Muskmelon seeds exhibit primary dormancy that affects viability very early in development but has a greater effect on seed vigor and is removed by afterripening during dry storage. Osmotically distended or fish-mouth seeds are dead seeds that occur in cucurbit seed lots after aging kills the embryo without disrupting the semipermeable endosperm that completely surrounds and protects the embryo. Cucurbit seed crops should be harvested before the onset of fruit senescence to prevent aging of the seeds inside. Open-pollinated cucurbit seed crops are frequently once-over mechanically harvested. Mechanical harvesting combines seeds from many stages of development into a single seed lot, which may adversely affect quality and increase seed to seed variability. Hand harvesting cucurbit fruit at the optimal stage of development could improve seed quality in some instances but is more costly and time consuming and would increase production costs.
The globe artichoke (Cynara scolymus L.) is usually propagated vegetatively because plants grown from seed lack uniformity. Furthermore, in much of the United States, only a small percentage of plants grown from seed flower during the first season due to insufficient chilling for vernalization. Artichokes cannot be grown reliably as perennials without winter protection where temperatures are consistently below -10C. The new cultivars Imperial Star (IS) and Talpiot (TP) reportedly produce uniform plants from seed and a high percentage of flower heads (capitulum) the first year with minimal chilling. `Imperial Star' and TP were compared with the standard seed-propagated cultivars `Green Globe Improved' (GG) and `Grande Buerre' (GB). Plants of each cultivar were tested over a 3-year period in Blacksburg, Va., or for 1year in three other locations. Essentially all IS and GG plants flowered after receiving 1356 h of chilling at <10C. With 205 h of chilling, 83% of IS plants flowered compared to 25% for GG. No TP or GB plants flowered after receiving as much as 528 h of chilling. In the mountains of western Virginia, only IS plants established in the field in early May received sufficient chilling to produce flower heads during the late summer and early fall. June transplants did not flower because sufficient chilling was not obtained for vernalization. In warmer areas of central and eastern Virginia, fall establishment for spring harvest may yield a higher percentage of flowering plants compared to spring planting and summer harvest.
A distance learning homepage at: http://www.bsi.vt.edu/welbaum/hort4764/ was created to teach an introductory college-level course on vegetable crops to students at Virginia Tech. The course was created to serve students in the horticulture program at Virginia Beach, Va., students in the Commonwealth who cannot take classes on the Blacksburg campus, and students on the Blacksburg campus who could not schedule the classroom-based course. The course is not selfpaced, but directs students through 44 lessons on various topics including detailed descriptions of 28 different vegetables. The site is primarily in HTML format with archived student projects and old exams in PDF format. Audio clips are used to emphasis key information and to add a personal touch. There are >550 pictures and descriptions of vegetables and vegetable crop production linked to the website. Students can be examined using a computer testing system call Whizquiz that grades and corrects each exam. “Web Forum” software enables online discussion among students and the instructor. Discussion sessions have been successfully conducted between students and guests at distant locations. Links are provided to over 25 other websites with information on vegetable crops. The project was funded by a USDA/CSREES Higher Education Challenge Grant.
Maturity at harvest determines seed viability and vigor. However, separating seeds from different stages of development can be difficult using existing seed sorting technologies. New technology non-destructively sorts seeds based on their chlorophyll fluorescence (CF), so seeds with the same dry weight but with different physiological maturates can be separated. We determined whether chlorophyll content of muskmelon (Cucumis melo L. cv. Top Mark) seeds changes during development and whether those changes were related to viability and vigor. Seed viability and vigor were determined using an Association of Official Seed Analysts wet paper towel germination test. `Top Mark' seeds from nine stages of development were run through the SeedMaster Analyzer (Satake USA Inc., Houston, Texas), which calculated the chlorophyll content of each seed. The CF signal was fed into a computer to obtain a frequency histogram. Forty, 45, and 55 days after anthesis (DAA) seeds had germination percentages of 96%, 98%, and 100%, respectively, the highest in the study. Fifty-five DAA had greater seed vigor and viability and contained the lowest CF values; 207 on the 1000-value scale. The less-mature seeds contained higher chlorophyll content and had the lowest seed vigor and viability. Seed vigor and chlorophyll content were negatively correlated in this study. All seeds with high CF values had low vigor, but not all seeds with low CF values have high vigor. Seed aging during storage can reduce viability and vigor independent of chlorophyll content. Based on chlorophyll content, the SeedMaster Analyzer can non-destructively remove immature, low-vigor seeds that have the same physical characteristics and weight as more mature seeds. Chlorophyll fluorescence technology may allow the seed industry to further improve seed quality and maximize vigor.
Various osmotic solutions and solid carriers have been used to prime (controlled hydration followed by drying) seeds. Broccoli (Brassica oleracea L., var. italica, cvs. Brigadier and Earlidawn) seeds were primed in solutions of polyethylene glycol, potassium nitrate (KNO3), and mannitol or calcium silicate (Micro-ccl E) and vermiculite to determine which treatment provided the greatest enhancement of germination. Germination percentage and rate as well as head yield were determined in a series of laboratory, glasshouse, and field experiments over a three year period. Water potentials (Y) ranged from -0.8 to -2.6 MPa at 20°C for both osmotic solutions and carriers. The duration of the priming treatments were 3, 7, or 10 days. All priming treatments, except KNO3 and mannitol, consistently reduced the mean time to germination in the laboratory, reduced the mean time to field emergence, increased final stands, but did not affect the mean time to harvest. However, solid phase was more effective than osmotic priming at each Ψ tested. The best solid priming mixture was 1.0:0.8:1.8 (seed:carrier:water) for 7 days at 20°C using Micro-cel E. This treatment was measured to have a Y = -1.2 MPa, and other types of priming at the same Y were less effective. The Y threshold below which no priming effect was obtained was Ψ= -2.4 MPa. Priming response was closely associated with, but not entirely dependent on, the Ψ of the treatment.
Priming (controlled hydration followed by drying) has been shown to decrease seed storage life in some species The germinablity of primed (0.3 M KNO3, 6 d, 25°C) and unprimed muskmelon (Cucumis melo L., cv. PMR 45) seeds were compared after storage for 9 yrs at less than 20°C and 6% moisture content (MC) (dwt basis). Germination performance was compared at 30°C in water and polyethylene glycol solutions of -0.2, -0.4, -0.6, -0.8, and -1.0 MPa water potential or in water at 15, 20, and 25°C. Seeds were also germinated in field soils at 17, 19, and 21% (dwt. basis) MC in a greenhouse, Some seeds were subjected to controlled deterioration at 20% MC and 45°C for 72 hrs prior to testing. The germination percentage and rate of stored, primed seeds at 30°C and all water potentials was less than stored, unprimed seeds. At 30°C, stored, unprimed seeds germinated more rapidly and to higher percentages at -0.2 MPa than in water, while germination percentages and rates of stored, primed seeds were essentially the same. At 15, 24, and 25°C, stored, primed seeds outperformed unprimed seeds in all germination tests. In saturated soils at 21% MC, there was no germination of either stored, primed or unprimed seed. At 17% soil MC, stored, primed seeds germinated 73% compared to only 56% for unprimed seeds. The enhancement due to priming was retained after 9 yrs of storage at germination temperatures <30°C. At higher temperatures, the germination of unprimed seeds was superior to primed.
Intensive, deep-batch, hydroponic systems that use float beds (FBs) are used extensively by the tobacco industry to produce transplants. FBs and a modified FB system with separate drying and flooding stages called ebb-and-flood (EF) beds were used to grow 12 diverse horticultural crops to maturity. Beds were filled with 570 L of water with 114 mg·L−1 N and 143 mg·L−1 K or 66 mg·L−1 N and 83 mg·L−1 K in 1994 and 1995, respectively. The EF beds were flooded for 6 hours, then drained for a 6-hour dry stage each 12 hours in 1994, and flooded for 1 hour and dried for 5 hours each 6-hour period in 1995 from May through August. Although both systems were suitable for producing Chinese water spinach (Ipomoea aquatica Forssk.—see footnote in Table 1), vegetable amaranth (Amaranthus tricolor L.), zinnia (Zinnia elegans Jacq.), and sweet basil (Ocimum basilicum L.), the EF system provided greater control over water availability and higher oxygen concentration in the root zone.