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Cultivar and temperature effects on starch in the primary thickening meristem of onion bulbs
The Journal of Horticultural Science and Biotechnology
Abstract
The effect of cultivar and temperature on the temporal pattern of starch occurrence in the primary thickening meristem (PTM) of onion (Allium cepa L. var. cepa) bulbs was studied. Bulbs of four cultivars differing in their sprouting behaviour in dry storage were analysed during bulb development for the concentration of starch in the PTM and inner sprout growth by histochemical analysis and determination of leaf ratio, respectively. Generally, in all cultivars the concentration of starch in the PTM was high during active growth, decreased to a low value during sprout dormancy, and increased before or concomitant with inner sprouting. The duration of the low starch period varied between cultivars. If onions were stored at low (1°C), intermediate (15°C), or high (30°C) temperature, inner sprouting was delayed at low and high temperature compared with the intermediate temperature, whereas starch increased before sprouting at low and intermediate temperature, but became undetectable at high temperature. Moreover, if benzyladenine was injected into onions stored at high temperature, sprouting was readily induced, but starch in the PTM remained undetectable and no root initiation occurred. Since the PTM is not only responsible for the stem thickening but also for root initiation we conclude (i) that low starch in the PTM primarily indicates root dormancy and only indirectly sprout dormancy, and (ii) that high temperature-imposed sprout dormancy is actually based on root dormancy.
... In our experiment colour, texture, and flavour of stored onion bulbs were not affected negatively by low oxygen concentrations (data not shown). Sprouting, monitored by determination of leaf ratio (for method see: ERNST et al. 1999), was reduced in onion bulbs stored at low oxygen levels compared to the control (21 % O 2 ) (data not shown). That means, during low oxygen storage there weren't observed any negative effects, concerning the above mentioned parameters. ...
... Fructan series with exclusively (2 -> 1) fructosyl-fructose linkages are present in Allium cepa L., i.e. neokestose-based series with chain elongation only at the glucose end of the original sucrose molecule and with chain elongation from both sides of the sucrose molecule, an inulin series and an inulo-n-ose series without terminal glucose (ERNST et al. 1998). Starch is far less abundant than the other carbohydrates (ERNST and BUFLER 1994;ERNST et al. 1999). ...
Onion (Allium cepa var. cepa; cv. 'Sherpa', type Rijnsburger) bulbs were stored for 36 weeks at 2°C under different oxygen concentrations (0.5, 1.0 and 21%). Samples were collected after 9, 18, 27 and 36 weeks of storage. Water-soluble carbohydrates, respectively fructans were analysed using anion exchange chromatography combined with pulsed amperometric detection. Concentrations of total water-soluble carbohydrates in onion bulbs were different only for sampling dates after 27 and 36 weeks of storage. The reduction in water-soluble carbohydrates was highest during storage at 21% oxygen and lowest during storage at 0.5% oxygen. The reason for this effect was a reduced breakdown of fructans during low oxygen storage indicated by the fructan concentration and the degree of polymerisation. Our experiments' results suggest that vegetable crops containing higher concentrations of fructans are principally suitable for low oxygen storage.
... According to a method described by TUCKER (1989) and ERNST et al. (1999) initial sprouting can be determined by measuring the ratio between the leaf blade length and the leaf sheath length because the leaf blade elongates earlier than the leaf sheath. The leaf ratio was measured in ten onion bulbs for leaves with a sheath length of approximately 3 mm after each CA-storage period (except Jan. 18) and three times during shelf-life period. ...
The effect of ultra low oxygen concentration on the storability of onions (Allium cepa var. cepa) was studied. Onions of the cultivar 'Sherpa' (type Rijnsburger) were kept for 9, 18, 27 and 36 weeks at 2°C in atmospheres with 0.5, 1 and 21% O2. Quality assessment was performed after each storage time and the following period of 3 weeks of shelf-life. After storage in 1% O2, quality maintenance of the onion bulbs was much better than after storage in 21% O2, which was characterized by lower weight loss, reduced respiration intensity and inhibited sprouting in spring. Lowering the O2 concentration to 0.5% in the storage atmosphere led to similar results concerning respiration intensity and sprout inhibition, but weight loss was accelerated during the last storage period between March and May. Some onion bulbs were covered by a slight mould layer after storage until March or May, which was presumably due to the high humidity in some of the storage containers. None of the bulbs showed internal injury or off flavour due to fermentation metabolism.
... The lower sprouting incidence of light red onion cvs N-2-4-1 and Bhima Kiran, have been attributed to the lower respiration rate (Tripathi and Lawande,12). It was revealed that the temperature has profound impact on sprouting and low and high temperature prohibit sprouting and encouraged at intermediate temperatures (Ernst et al.,5). Thus, the biochemical changes that are taking place during the storage period are affected by storage environment, respiration rate and other factors and the nutrients required for the dormant sprout to grow are dependent on the biochemical processes linked to the sprouting (Chope et al.,2). ...
Enhancement of shelf-life is imperative to stabilize the supply chain of onion when the price fluctuations are due to short of supply amid huge postharvest losses. Sprouting has been a major hurdle to alleviate postharvest losses. The screening of available onion cultivars for abscisic acid (ABA), a growth hormone associated with sprouting could be helpful to categories them for storability potential. Twenty two different varieties were analyzed for initial ABA contents and their sprouting up to five months. Results revealed that the highest ABA content was recorded in Bima Red (80.61 ng/g) followed by Phule Safed (60.63 ng/g). Whereas, the lowest ABA contents were observed in Palam Lohit (9.84 ng/g), followed by Agrifound Rose (11.39 ng/g). Despite, comparatively lower ABA contents, sprouting was not observed in Bhima Kiran, N-2-4-1 and Pusa Madhvi up to five months. It was observed that, the dormancy of onion during storage could not be attributed to the initial ABA content.
... Starch has been found in A. cepa in the primary thickening meristem (PTM) during sprouting, but not during dormancy; absence of starch may therefore be useful as a marker for dormancy (Ernst and Bufler, 1994). Ernst et al. (1999) studied four cultivars stored at 0, 15 and 30°C. Low starch in the PTM indicated primarily root dormancy, and only indirectly sprout dormancy. ...
The experiment was conducted at the farmer’s okra field in Randomized Block Design with 8 treatments replicated thrice. The bio-efficacy of different novel insecticides against white fly revealed that among the treatments, seed treatment of imidacloprid 5 g/kg seed + neem Azal @ 5000 ppm spray + indoxacarb 75 g a.i/ha spray + Bacillus thuringiensis (Biolep) @1 kg/ha spray was significantly most effective treatment and reduced whitefly count by 63.01 - 52.13 and 52.45 - 42.34% at one and seven days after first and second spraying, respectively. However, imidacloprid 5 g/kg seed + neem Azal @ 5000 ppm + indoxacarb 75 g a.i/ha + Bt @ 1 kg/ ha combination was not so much effective treatment after 3rd spray as that was after 1st and 2nd spray of the insecticides and biopesticides.
Onion bulbs (Allium cepa L.) of cultivars with long-, medium- and short-storage lives, viz. Renate, Ailsa Craig and SS1, respectively, were stored in controlled atmosphere (CA) conditions (3.03 kPa CO2; 5.05 kPa O2; 2 °C). Bulb abscisic acid (ABA) concentration, pyruvate, fructans, total soluble solids (TSS) and firmness were measured throughout storage.In all cultivars, bulb ABA concentration declined exponentially during storage. The greatest decrease in ABA concentration occurred during the first 80 days of storage. Although the pattern of decline was similar for the long-, medium- and short-storing onion bulbs, onion cv. SS1 bulbs had the lowest initial ABA concentration. Onion bulb ABA concentration at harvest (measured on a fresh weight basis) may prove to be a better indicator of storage life. The ABA concentration at harvest (DW) may be indicative of a greater difference in sprouting during storage between cv. SS1 and the other cultivars than between cvs. Renate and Ailsa Craig.It is hypothesised that the storage potential of bulbs of different onion cultivars is inversely related to the time at which they reach a minimal ABA content. Thus, the storage life of short-storing cultivars (e.g. cv. SS1) might be prolonged by slowing the decline in ABA concentration. This could help extend the period for supplying these onions from temperate regions. Onion bulbs of cvs. Renate, Ailsa Craig and SS1 were characterised by high, intermediate and low concentrations of pyruvate, fructan and total soluble solids, respectively.
This fully revised, expanded and updated edition of the successful text, Onions and Other Vegetable Alliums, relates the production and utilization of these familiar and important vegetable crops to the many aspects of plant science underpinning their production and storage technologies. Chapters cover species and crop types, plant structure, genetics and breeding, physiology of growth and development as well as pests and diseases, production agronomy, storage after harvest and the biochemistry of flavour, storage carbohydrates and colour and how this relates to nutritional and health benefits. From this wide perspective it is possible to see many examples where underlying scientific knowledge illuminates, explains and can improve agronomic practice. The reader will get an insight into how molecular methods are revolutionizing the study of taxonomy, genetics, pathology and physiology and how these methods are being applied in the breeding of improved crops.
summaryStems of Allium cepa L. were prepared for histochemical analysis and stained by periodic acid-Schiff reaction and Lugol's iodine for detecting starch grains (amyloplasts). Starch was found in the root cap of the root initials, around the central cylinder vascular tissues of the stem and in the primary thickening meristem during sprouting but not during dormancy.
In onion (Allium cepa L.) postponement of sprouting is necessary to achieve long term storage. We studied the factors determining sprouting during dry storage at 16°C. The period to visible sprouting depends on the length of the dormancy period, if present, and on the growth rate of the sprout. In the three cultivars tested, sprouts were initiated within 2 weeks after harvest indicating the absence of a real dormancy period. Sprout length increased linearly during storage. The mitotic activity of the apex decreased before harvest, was low at the transition from scale to leaf formation, and increased again when the sprout was initiated. From a few weeks before harvest, the initially high fructan content of the scales decreased, leading to a large increase in fructose. The sprout always contained enough carbohydrates for growth (between 50 and 60 mg g−1 dry weight, of which 30% was fructan). The activity of sucrose synthase (EC 2.4.1.13) increased as the sprout grew, indicating an increase in sink strength. Invertase (EC 3.2.1.26) was absent in all bulb organs, during the various developmental stages. Although carbohydrates and enzymes were available for fast sprouting, sprout growth was still linear instead of exponential during dry storage at temperatures favorable for growth (16°C). The relative importance of factors determining sprouting are discussed.
Sprouting of onion bulbs is inhibited at high storage temperatures. To investigate the role of cytokinins in this phenomenon small bulbs of the ‘Rijnsburger’ cv. Augusta were stored from four weeks after harvest at 5, 15 and 30°C for a period of 18 weeks. Bulb samples were investigated for time to rooting and subsequent sprouting in moist vermiculite at 15°C and assayed for endogenous cytokinins at six-week intervals. Root dormancy gradually decreased during storage, the fastest at exposure to 30°C; rooting was followed by sprouting. After 18 weeks of temperature exposure, no visible sprouting had occurred in dry storage; internal sprouts had relative lengths of 0.29, 0.72 and 0.31 at 5, 15 and 30°C, respectively. Cytokinin activity was low during the first six weeks of temperature exposure. At 5 and 15°C a sudden rise in cytokinin level was found after 12 weeks and a further increase after 18 weeks. At 30°C, however, cytokinin levels increased very little during the storage period. Injection of plant growth regulators into large bulbs of the ‘Rijnsburger’ cv. Hyton that had been stored for 28 weeks at 25°C showed that benzyladenine strongly promoted sprouting. The data indicate that inhibition of sprouting at high temperature is caused by low levels of endogenous cytokinins.
The role of the root system in sprouting of bulbs of various onion cultivars and one shallot cultivar was investigated. Sprouts emerged two to three weeks after the rooting of bulbs planted in moist vermiculite. Removal of the roots retarded sprouting by 2–8 weeks, depending on cultivar. Experiments with bulbs in water culture showed that in some cultivars the effect of root removal on sprouting was reversed by inclusion of 10 µmol l–1 benzyladenine (BA) in the water. The data suggest that in these cultivars cytokinin is the main factor limiting sprout growth and that the root system supplies cytokinin to the bulbs. Derooted bulbs of other cultivars, however, showed little sprouting with BA application. In these cultivars a combination of BA and wounding the base plate was required to compensate for root removal. Wounding of the base plate also enhanced sprouting of derooted bulbs on water without BA which suggests that, besides cytokinin, a second factor present limits sprout growth. The data are discussed in relation to cultivar differences in time to sprouting in dry storage.
Bulb development in the onion plant, which occurs in response to the stimulus of long days, has two main morphological features: swelling of the base of the pseudostem and the formation of scales from leaf initials produced at the stem apex. The swelling may be expressed as the ‘bulbing ratio’, i.e. the ratio of the maximum diameter at the base to the minimum at the neck, but this is a somewhat insensitive measure wider adverse light conditions and no ratio can be specified at which bulbing is first apparent. In the absence of bulbing, leaf initials have at all stages of development a blade longer than the leaf sheath, giving a ‘leaf ratio’ of blade/sheath at all times greater than unity. In scale formation, when bulb development begins, the sheath of a small initial starts to grow sooner than in foliage leaf formation and blade growth is suppressed; the leaf ratio therefore falls rapidly below unity and this is diagnostic of bulbing. Characteristic graphs of leaf ratio plotted against leaf number from the apex demonstrate the stage of bulbing reached and a fall in leaf ratio from near the apex in such a graph can indicate the beginning of bulbing before any actual scales (with ratios less than unity) have been formed. In normally grown plants, differentiation into foliage leaves or scales appears to take place when the initials reach about 1 mm in length. When the incidence of bulbing within a treatment is very variable, difficulties arise in averaging the leaf ratio curves for the different plants. Mean number of scales per plant, or mean minimum leaf ratio, may then be used as measures of bulbing. Heavy shading in winter was found to delay but not prevent the bulbing of those plants which survived. ‘Buds’, consisting of a portion of stem with the apex surrounded by leaf initials, but no scales, were dissected out from onion sets and cultured aseptically in tubes. Limited growth was obtained and rooting occurred in some cases. In the presence of sucrose, swelling of leaf sheaths and reduction of the leaf ratios took place; these responses increased with sucrose concentration. Glucose similarly stimulated bulbing but high osmotic pressure caused by mannitol or mineral nutrients had no effect. Rooting appeared to reduce the degree of bulbing, possibly by competing for carbohydrate supply. The bulb development in these sterile cultures was most unusual in occurring in short photoperiods and the possibility of a residue of ‘bulbing hormone’ being stored in sets is discussed.
In solution culture the effect of optimal (20 °C) and supra-optimal (30 °C) root-zone temperatures on root growth and root morphology of six potato clones (Solanum tuberosum L. spp.) was studied. Growth was compared with sweet potato (Ipomoea batatas L.) and cassava (Manihot esculenta Crantz). Significant genotypical differences in the responses of potato roots to supra-optimal temperatures were observed, indicating the potential for selecting heat tolerant potato clones. In both heat tolerant and heat sensitive clones, the size of the root system was reduced by supra-optimal root-zone temperature. This was principally a result of decreased number and length of lateral roots. The first symptom of heat damage was a reduction in the rate of cell division, followed by cessation of root elongation. Bending of the apical root-zone together with the formation of root hairs on the inner (concave) and lateral roots on the outer (convex) side were other symptoms; these coincided with the loss of starch granules in the root-cap.
The validity of a test system in morphometric histopathology depends on the variation sources involved. Biological variation is one of the variation sources, but is also the object to be studied with morphometry. We studied the variation sources in interactive computerized morphometry in two test systems involving two different commercially available image analyzers. The measurements were made on nuclei in a microscopic field of 5 transitional cell tumors (papilloma and WHO grade I-III carcinomas) of the urinary bladder. It turned out that different observers selected a variable number of nuclei for measurements, the coefficient of variation (CV) in the number of nuclei being 11-11.5%. Mean CV in nuclear perimeter measurements was 4.4-4.8%, and in nuclear area measurements 8.2-8.6%. The variation in measurements by 1 observer was smaller, the CV values being 2.8% for the number of nuclei, 1.2% for nuclear perimeter, and 2.4% for nuclear area. The results showed that interobserver variation can be considerable in these systems. It is suggested that special sampling rules should be tested with the idea of finding the relevant approach with the smallest interobserver variation.
Practical histochemistry
- Chayen
- L Bitensky