Gebhard Bufler’s research while affiliated with University of Hohenheim and other places

Low-temperature sweetening of parsnip (Pastinaca sativa L.) roots, which may occur both pre-harvest and post-harvest, is a well-known phenomenon, but is poorly understood. 'White King' parsnip roots were analysed over two seasons (2008 and 2009) during field growth and over six months cold storage at 1°C for their dry matter content, as well as their starch, sucrose, glucose, fructose, and malto-oligosaccharide (MOS) concentrations and for their changes in amylolytic enzyme activities (2009 only). In 2008, when the air temperature fell below 5°C for 12 d in the last 8 weeks before harvest, the concentration of starch decreased significantly (P ≤ 0.05) and the concentration of total sugar increased significantly. In 2009, when temperatures did not fall below 5°C during the last 8 weeks before harvest, the concentrations of starch (339 + 17 mg g-1 DW) and total sugars (168 + 12 mg g-1 DW) did not change significantly. However, in both years, during 24 weeks in cold storage at 1°C, starch levels were almost completely depleted, whereas sucrose concentrations and, to a lesser extent, MOS concentrations increased at least two-fold. Glucose and fructose concentrations also increased significantly during 24 weeks in cold storage, but to a much lower level than sucrose or MOS. Total amylolytic activity and a-amylase activity doubled during the first 8 weeks in cold storage, whereas /3-amylase activity did not change significantly. As products of a-amylase activity, changes in MOS concentration paralleled the changes in α-amylase activity. The accumulation of sucrose and MOS may raise the culinary quality of cold-stored parsnip.

Carrot (Daucus carota L.) roots are traditionally considered as containing negligible amounts of starch. Depending on the type of extraction medium used to remove interfering sugars from root tissue however, the yield of starch content varies. To re-examine the role of starch in carrots, high-starch cultivars were identified and their starch and sugar content monitored during root development in the field and during storage. Starch up to 150 or even 180 mg g−1 dry weight accumulated in some of these cultivars. Low-temperature storage (2 °C) decreased the starch content dramatically and concomitantly increased the sucrose content. Sensory triangle tests and ranking tests of sweetness performed with juices produced from carrot roots revealed significant differences between freshly harvested and stored roots, indicating a gain in sweetness achieved by starch mobilization. Post-harvest sweetening of carrots was accompanied by a transient increase in total amylase activity and malto-oligosaccharides (MOS) content. In contrast, if roots were stored at room temperature (19 °C), there was also a decrease in starch content and an increase in sucrose content, but total amylase activity and MOS content declined, suggesting different metabolic pathways at the two temperatures. The possible role of starch in sugar accumulation and in stress adaptation of carrot is discussed. It is concluded that starch is a key reserve carbohydrate in carrot.

Polyacetylenes are a class of highly bioactive compounds which may negatively affect the sensory properties of Apiaceous plants such as carrots and parsnips. In the present study, the effects of organic nitrogen fertilisation, water supply, and cold storage on the contents of falcarinol (FaOH), falcarindiol (FaDOH), and falcarindiol-3-acetate (FaDOAc) were investigated in four carrot (Daucus carota L.) and two parsnip (Pastinaca sativa L.) cultivars. Relatively high contents of polyacetylenes were observed at an early developmental stage of the roots of both carrot and parsnip, decreasing to a transient minimum approx. 1 month prior to harvest maturity. The supply of super-optimal levels of nitrogen (50 or 100 kg N ha -1) using organic fertiliser did not significantly affect the polyacetylene contents of carrot or parsnip at harvest (except FaDOH contents in 'Rodelika' carrot). Different levels of water supply to pot-grown carrots in climate chambers induced complex, cultivar-dependent, and apparently development-dependent patterns of change in polyacetylene contents and profiles. Cold storage of field-grown 'Bolero F 1' and 'Rodelika' carrot significantly increased the levels of individual polyacetylenes in both cultivars. However, carrots previously supplied with super-optimal amounts of nitrogen fertiliser (50 or 100 kg N ha -1) either showed a transient increase ('Bolero F 1') or a transient decline ('Rodelika') in polyacetylene contents during cold storage. The results suggest active pre- and post-harvest synthesis and metabolism of polyacetylenes in carrot and parsnip. This may affect the sensory and bio-functional properties of these crops.

Exogenous ethylene has recently gained commercial interest as a sprouting inhibitor of onion bulbs. The role of ethylene in dormancy and sprouting of onions, however, is not known. A cultivar (Allium cepa 'Copra') with a true period of dormancy was used. Dormant and sprouting states of onion bulbs were treated with supposedly saturating doses of ethylene or with the ethylene-action inhibitor 1-methylcyclopropene (1-MCP). Initial sprouting was determined during storage at 18 degrees C by monitoring leaf blade elongation in a specific size class of leaf sheaths. Changes in ATP content and sucrose synthase activity in the sprout leaves, indicators of the sprouting state, were determined. CO(2) and ethylene production of onion bulbs during storage were recorded. Exogenous ethylene suppressed sprout growth of both dormant and already sprouting onion bulbs by inhibiting leaf blade elongation. In contrast to this growth-inhibiting effect, ethylene stimulated CO(2) production by the bulbs about 2-fold. The duration of dormancy was not significantly affected by exogenous ethylene. However, treatment of dormant bulbs with 1-MCP caused premature sprouting. Exogenous ethylene proved to be a powerful inhibitor of sprout growth in onion bulbs. The dormancy breaking effect of 1-MCP indicates a regulatory role of endogenous ethylene in onion bulb dormancy.

Dormancy and sprouting were studied in two onion cultivars with a clear period of dormancy using a large number of replicates, and single-bulb sampling. Dormancy was evaluated on the basis of sprout leaf elongation. Changes in respiratory activity, ATP and ADP content, sucrose synthase activity, dry matter (DM) content, and the contents of glucose, fructose, sucrose, fructan, and water-soluble carbohydrates (WSC), were determined in sprout leaves, stem plate and bulb scales at weekly intervals. During dormancy, and before sprouting, changes in DM contents and various WSC in different bulb parts suggested mobilisation and translocation of WSC. After a period of dormancy, the sucrose synthase activity and respiratory activity in the centre of the bulb increased, concomitant with initial sprout growth. The demonstrated separation between dormancy and the sprouting period in these cultivars should provide a good starting point for comprehensive studies on dormancy regulation in onion bulbs.

Experiments were designed to relate features of S metabolism to the dormant and/or sprouting states of onion bulbs (Allium cepa L.) during storage at 18°C. For this purpose, onion bulbs were dissected at 2-week or 4-week intervals into sprout leaves, stem plate, inner scale, and fleshy leaf to measure total sulphur content, sulphate content, γ-glutamyl transpeptidase (GGT) activity and enzymatically-formed pyruvate (EPY). The determination of initial sprout growth was based on measurements of sprout leaf elongation. Two long-day cultivars ('Golden Bear F1 and 'Copra F1). with a clear period of dormancy, were chosen for these experiments. Changes in total sulphur content showed no clear relationship to either the dormant or the sprouting phase, whereas the sulphate content of each bulb part studied tended to be lowest at the transition from dormancy to sprouting. EPY, as an equivalent to flavour precursor content and pungency, increased in the inner scale of both cultivars throughout dormancy, and initial sprout growth. In contrast, EPY was lowest in the sprout leaves and stem plate at the end of dormancy in 'Copra F1, but subsequently increased during initial sprout growth. GGT activity started to increase in the fleshy leaf and in the inner scale at the start of sprouting in both cultivars. In general, changes in S metabolism at the transition from the dormant to the sprouting state suggest sprouting-enhanced sulphur metabolism, depending on the cultivar. In addition, the results indicated remobilisation and reallocation of sulphur compounds within the onion bulb during dormancy and initial sprouting.

Effects of various agronomic factors on onion (Allium cepa L.) raw material for a combined recovery of onion juice concentrate and onion oil were studied in respect to their impact on crucial onion quality traits such as pungency - estimated as total pyruvic acid (PA) - and total soluble solids (TSS). Cultivar differences, year-to-year variability and effects of planting method, bulb size, S and N fertilization, as well as the impact of the type of preceding crop within a crop rotation were investigated in field trials. In a cultivar comparison of 16 cultivars, a weak correlation between TSS and PA was observed (r=0.64, P=0.008). In a year-to-year variability study PA ranking among six cultivars remained fairly constant over three years. The method of planting had a significant (P<0.05) effect on PA. Bulbs directly sown (cvs. 'Sturon' and 'Stuttgarter Riesen') showed increased pungency compared to bulbs grown from sets. Within a batch of onion bulbs (cv. 'Sturon'), grading for bulbs size exhibited no significant difference in PA among three calibres. In a crop rotation onions grown after legumes as preceding crop had significantly higher pungency (P<0.001) compared to onions grown after cereals. This effect is assumed to be a result of enhanced mineralisation of organic N and S sources. TSS contents were not significantly affected by any treatment. In fertilization experiments, a combined N (0 and 130 kg N ha-1) and S (0, 50 and 100 kg S ha-1) application increased pungency and showed a significant NxS interaction for PA (P=0.032), but not for bulb-S and bulb-N. No effect on TSS was observed. S application of 100 kg S ha-1 vs. 0 kg S ha -1 had no qualitative impact in terms of relative composition of major onion oil compounds and ratio of the biosynthetic aroma precursors methyl- and 1-propenyl-L-cysteine sulfoxide, but caused a remarkable increase of absolute amounts of volatiles (steam distillate), aroma precursors and industrially produced onion oil.

SUMMARY In the framework of a study of avocado germplasm resources, special attention was paid to Costa Rica, as it was found that its avocado tree population differs from those known to be from nort- hern countries of origin. The wild avocado of Monteverde described in this article is a unique type of avocado, not known from any other habitat, but only from the wild forest of Monteverde, and it could not be recognized botanically, as any other known species. Its description shows that it is closer to Persea nubigena than to any known avocado, but its fruit is much bigger. The origin of the avocado that was discovered in Moteverde and measures for its future conservation and eva- luation are discussed.

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.