Growth, Dry Matter Partition and Radiation Interception in an Overwintered Bulb Onion (Allium cepa L.) Crop

Growth, bulb development, partition of dry weight between leafblades and bulbs, and the interception of solar radiation weremeasured in overwintered crops of five cultivars of bulb onionwith different maturity dates sown on successive dates in threeseasons. The onset of bulbing was later the later maturity ofthe cultivar. Later sowing did not delay the onset of bulbingbut it did delay maturity. There was little mean differencebetween cultivars in the duration of bulb growth defined asthe interval between onset of bulbing and maturity, but therewere considerable differences between cultivars within a season,and between seasons for a given cultivar. Duration of bulb growthranged from 11 to 46 days with a mean of 35 days. Increases in total shoot dry weight during bulb developmentand, in the absence of much bolting, bulb dry-matter yieldswere linearly related to the total radiation intercepted duringbulb growth. These relationships were similar to those reportedfor other crops in Britain. Radiation interception during thephase of bulb growth was low compared with other crops, witha mean value of 49 per cent and a maximum of 65 per cent. Thepercentage of solar radiation intercepted during bulb developmentwas higher from early sowings than from later ones, particularlyin early maturing cultivars. The harvest index was high, withtypically more than 80 per cent of the shoot dry weight in bulbsat maturity. Allium cepa L., onion, blub, growth, partition of dry matter, radiation interception     

The Influence of Seedling Size and Carbohydrate Status and of Photon Flux Density During Vernalization on Inflorescence Initiation in Onion (AIlium cepa L.)

Experiments were done in controlled conditions to investigatethe relationship of plant weight, leaf number and raising conditionsto inflorescence initiation in onion seedlings. Above a shootdry weight of 0.06 g the spring-sown cv. Rijnsburger could initiateinflorescences, whereas the autumn-sown cv. Senshyu semi-globeYellow needed to be heavier than 0.45 g. Plants raised at aphoton flux density of 200 µmol m^–2S^–1 anda temperature of 25 C required longer to initiate inflorescencesthan plants raised at 600 µmol m^–2S^–1 and17 C which had higher reserve carbohydrate content. The minimumleaf number for inflorescence initiation was larger for plantswith low reserve carbohydrate content. Photon flux densitiesof 50, 100, 200 and 400 µmol m^–2S^–1 duringvernalization at 9 C caused no differences in inflorescenceinitiation in plants previously raised at l7 C and 600 µmolm^–2S^–1 but the lowest photon flux density duringvernalization reduced initiation in plants previously raisedat 25 C and 200 µmol m^–2S^–1.     

Effects of Nitrogen Supply on Bulb Development in Onion Allium cepa L.

Onions were sub-irrigated with 0.6, 2.4, 4.8 and 12 mol m^–3nitrate while growing in constant photoperiods of 14, 15 or16 h. The lower the nitrogen, the later bulb scale initiationoccurred and in 14 h photoperiods bulb scales initiated onlyat the highest N level. In the early stages of the experiment,the ratio bulb width/neck width was higher in low N than inhigh N treatments, but the reverse was true at the later harvests,by which time bulb scales had formed. In a second experimentonions were sub-irrigated continuously with 0.6 mol m^–3or 12 mol m^–3 nitrate or transferred from the low to high,or from high to low N after 36 d of growth in 14, 15.25 and17 h photoperiods. An initial period of low N delayed bulb scaledevelopment and ripening relative to high N throughout, particularlyunder 14 h photoperiods. Late N deprivation did not preventor delay bulb scale initiation or ripening. In a field experiment,plants not fertilized before sowing but given the total quantityof N late in the growing season formed mature bulbs later thanplants fertilized with N before sowing, or given half the quantityof N before sowing and the second half late in the growing season. Key words: Onion, nitrogen, photoperiod, daylength, bulbing     

The rapid determination in controlled environments of parameters for predicting seedling growth rates in natural conditions

Parameters which characterise the response of seedling growth rate to temperature and mean daily radiant exposure were determined for cultivars of eight vegetable species and two ornamental bedding plant species using controlled environments. All species were grown for approximately four weeks in a series of four controlled environments consisting of factorial combinations of 20°C or 15°C with 7.78 or 0.98 MJ m^-2 radiant exposure per day to photosynthetically active radiation. Four of the vegetable cultivars had been used previously in a study of growth in the glasshouse at different seasons. The parameters derived from controlled environments were used, with temperature and light measurements from the glasshouse, to predict seedling shoot growth in the glasshouse. These predictions accounted for 92% of the observed variance in log shoot dry weight in a series of glasshouse experiments involving thirteen combinations of species and sowing date, each grown at four plant densities and sampled on four occasions. Equivalent parameters derived previously from the glasshouse data themselves accounted for 92% of the variance in the same predictive exercise. Differences in base temperature for growth, the responsiveness of relative growth rate to daily radiant exposure (light), and the potential relative growth rate were apparent between species and were quantitatively characterised by the parameters. To determine these parameters for a cultivar requires the equivalent of one square metre of controlled environment growing area for approximately sixteen days. Such parameters could be applied for scheduling and management in crop production, particularly in transplant production.     

Within-cultivar differences in relative leaf growth rates at low temperature of onion seedlings are not heritable

Onion seedlings of two open-pollinated cultivars were grown either at a constant 10°C or outdoors in spring and were selected for high, medium or low relative leaf growth rate, R_L. Flowering plants within each R_L group were crossed using controlled pollination for the 10°C selections and as a fly-pollinated polycross for the outdoor selections. The pollen donors of the 10°C selections were also selfed. Despite the wide variation of R_L in the parent populations, there was no indication of heritability of R_L. Mean R_L was significantly lower for the progeny from self-rather than cross-pollinations as was initial leaf area and rate of seedling emergence.     

The effect of temperature on the rate of sprout growth and development within stored onion bulbs

In order to understand better the effects of storage temperature on the time to visible sprouting in stored onions, sprout growth was measured by regularly dissecting samples from bulbs stored at 1, 10, 15 or 25°C for 243 days. The dry-weight of the shoot or sprout within stored onion bulbs increased exponentially with time. The rate of increase of sprout dry weight, as well as the rate of leaf initiation by the shoot apex was faster at 17° than at 10 or 25°C, and almost zero at 1°C. The rate of loss of dry weight from storage tissue was similar at 17°C and 25°C but slower at 10°C and slower still at 1°C.     

The response of growth rate to temperature in seedlings of several Allium crop species

Relative growth rates (R_w), leaf area ratios (LAR), net assimilation rates (NAR), and leaf weight ratios (LWR) were estimated for seedlings of onion, leek, Japanese bunching onion and chives grown at constant temperatures of 10, 12, 15, 19, 23, 27 and 31 C. R_w was greatest at 19 C in Japanese bunching onion and chives, and at 27 C in onions and leeks. R_w increased approximately linearly between 10 and 19 C in all species; between 19 and 27 C it either increased further, decreased or remained constant depending on species but at 31 C R_w was lower than the maximum in all cases. LAR's were greatest at 27 C in onions and leeks and at 23 C in the other two species. Between 10 and 19 C both LAR and NAR increased in all species. Between 19 and 27 C increases in LAR were compensated for by decreases in NAR with the result that R_w showed little change with temperature. Averaging over all temperatures the species were ranked in order of decreasing R_w, NAR and LWR as onion, leek, Japanese bunching onion and chives. For LAR this ordering was reversed with respect to onions and leeks. Plant to plant variability increased in the order onion, leek, Japanese bunching onion then chives. Although onions were the fastest growing of the Alliums tested their R_w was lower than that of several other vegetable species for which R_w was similarly measured.