The target of vegetable allium production is a high yield of high-quality produce. The attainment of such a crop is the end result of many processes of growth and development. Well-established principles determine the yield of all crops, and some of these have been investigated for onions and leeks. The yield of a crop is determined by: (i) the quantity of light absorbed by its leaves while harvestable dry matter is being produced; (ii) the efficiency with which the absorbed light is converted by photosynthesis into sucrose; (iii) the conversion coefficient between photosynthetic sucrose and the biochemical constituents of the harvested material; (iv) the proportion of photosynthetic output transferred to the harvested fraction of the plant; and (v) the weight losses due to respiration and decay after the above photosynthetic and biosynthetic
processes have occurred. Each of these aspects will now be considered in more detail.
The quantity of light absorbed (i) depends on the quantity of light radiating on the field, the percentage absorbed by the leaves and on the duration of growth of the yield-bearing components. Clearly then, a high-yielding crop must produce a leaf canopy with a high percentage absorption of incident light during a season of high solar irradiation. Furthermore, the longer the duration over which such a leaf canopy is transferring photosynthate to harvestable material, the higher the yield. Much of this chapter is concerned with explain-ing how the season of, and magnitude of, leaf canopy development is controlled by physiological responses to environmental conditions. For the bulb-forming species the situation is complex, because bulb development ultimately leads to a cessation of foliage leaf production and this prevents the renewal and maintenance of the crop leaf canopy. There is no such finite limit to the crop leaf canopy for non-bulbing species like leek and Japanese bunching onion.
Nevertheless, the growth of allium crops can also be curtailed by the onset of unfavourable weather conditions or by flower stalk development (bolting).
The proportion of the total incident light intercepted by leaves depends on the area of leaf surface per unit area of ground, the Leaf Area Index (LAI).
Figure 4.1 shows the relationship between LAI and percentage incident solar radiation intercepted by the leaf canopy (I%) for onions in summer in central England. This is well summarized by the equation:
I% = 85.4(1 e–0.377.LAI) (Eqn 4.1)
Fig. 4.1. The relationship between percentage light (photosynthetically active radiation, PAR) interception by leaf canopies of onion cv. ‘Robusta’ and Leaf Area Index (LAI) determined non-destructively from leaf surface areas calculated as leaf length x maximum width x 2.2 (from Mondal, 1985).
Using this equation we can calculate the LAI needed to achieve a high light interception. For example, 60% interception requires an LAI of 3.2.
The edible alliums have narrow, upright leaves, termed ‘erectophile’
foliage, resulting in a relatively low proportion of incident light interception per unit of leaf area compared with crops with broader, more horizontal leaves, particularly when the sun is high in the sky (i.e. at a large solar angle).
Interception of photosynthetically active radiation (PAR) by crop leaf canopies frequently conforms to the equation:
ln(T/I) = –k.LAI or, equivalently, T/I = e–k.LAI (Eqn 4.2) T = PAR transmitted through the leaves, I = PAR incident on the crop and k is a constant, termed the extinction coefficient.
Several authors have measured k values for onions, yielding values ranging from 0.25 (Daymond et al., 1997), 0.34 for the data in Fig. 4.1 for LAIs up to 4 and, assuming the ratio of extinction coefficients of PAR to that for total solar radiation is 1.21 to 0.47 (Tei et al., 1996). In the latter study the values of k for lettuce and red-beet were 0.66 and 0.68, respectively. The values for onion are in the range typical of crops with leaves at a steep angle to the horizontal (Squire, 1990).
A small fraction of the intercepted PAR is reflected or transmitted, and not absorbed. Tei et al. (1996) found that the PAR absorbed by an onion crop averaged about 93% of the PAR intercepted after allowing for PAR reflected and PAR reflected from the ground and then absorbed. The erectophile leaf habit results in a high proportion of the leaf surface being directly sunlit compared with crops having more horizontal leaves. This promotes efficient utilization of the incident PAR when the crop is large with high LAI, but spaced seedlings capture only a small fraction of the incident PAR and are easily suppressed by light competition from overgrowing weeds with more horizontal leaves. As the onion leaf canopy grows there is a tendency for the larger leaves to fold under their own weight, resulting in the leaves becoming more horizontal in the later stages of growth (Tei et al., 1996).
The efficiency with which absorbed light is converted to primary photo-synthetic products (ii) can be affected by the temperature and water status of the leaves. Clearly, if temperatures are above or below the optimum for photo-synthesis, efficiency will be reduced. Similarly, if leaves are water-stressed to the extent that stomata are closed and the diffusive resistance to CO2 entry is increased, then this too will reduce photosynthetic efficiency. Therefore, for photosynthesis and growth the crop must have adequate supplies of water and mineral nutrients, and temperatures must be suitable.
The conversion coefficient between the weight of sucrose produced by photosynthesis and the weight of dry matter stored in the structural and storage tissues of the plant (iii) depends primarily on biochemical composition. A lower weight of lipid, protein or lignin is produced per unit of sucrose utilized in bio-synthesis than is the case for structural or storage carbohydrates (Penning de
Vries et al., 1974). The edible parts of alliums contain primarily simple sugars and fructan storage carbohydrates (see Chapter 8), plus the structural carbo-hydrates and protoplasmic constituents typical of most plant cells. They comprise, therefore, carbohydrate-rich tissues and should have a high ratio of final dry weight to photosynthate utilized.
As regards (iv), the extent of partition to harvestable material, in bulb onions at the optimum time for harvesting, when 80% of plants have ‘soft necks’, about 80% of the shoot weight is in the bulb. Bulb weight will continue to increase after this stage and the percentage of total weight in the bulbs will increase. For example, 2 weeks after the 80% ‘soft necks’ stage, an average of 89% of shoot dry weight comprised bulb in one series of experiments (Brewster et al., 1986). Bulb onions therefore have a high ‘harvest index’ (the proportion of total yield in harvested material). In non-bulbing alliums the harvest index will vary according to how much green leaf is harvested in addition to the pseudostems.
In salad onions and chives nearly 100% of the shoot may be harvested. The fraction of the leek crop that is harvested varies with marketing traditions and demands. For example, in southern England the market requires the pseudostem only, whereas in northern UK leeks are sold with an additional ‘flag’
of green leaf above the pseudostem. Trimmed leek pseudostems comprise 33–50% of the total shoot weight, with a mean of about 40%. Leeks trimmed with a green ‘flag’ would typically comprise 55–60% of the total shoot weight.
Clearly, the harvest index of leek crops is lower than for bulb onions.
Concerning crop respiration and decay (v), even in the absence of bolting and in favourable weather conditions, leeks can reach a maximum or ‘ceiling’
yield (e.g. Fig. 4.50). Theoretically, such a ceiling will be reached when dry matter gains from photosynthesis are equalled by dry matter losses due to respiration and tissue senescence. Such a situation has not been observed in onion crops, because bulb ripening and leaf senescence terminate crop development before a ceiling is reached (see Fig. 4.2).
Respiration results from two broad components – first, ‘growth respiration’, the CO2 output associated with the synthesis of new tissue and, secondly,
‘maintenance respiration’, the CO2output needed to maintain existing tissues alive (McCree, 1970). The ratio of growth respiration to dry matter growth determines the photosynthate conversion efficiency (iii above) and varies with the biochemical make-up of the new growth, as indicated. Maintenance respiration varies with tissue and temperature. Leafy shoots have a much higher maintenance respiration than storage organs like bulbs (de Visser, 1994a; Tei et al., 1996). Onion and garlic bulbs in store have very low respiration rates (see Chapter 7, ‘Bulb Respiration Rates’).
Onion bulbs taken directly from plants still actively growing in the field had maintenance respiration rates of 0.0010 and 0.0011 g CO2/g/day at 9 and 19°C, respectively – about 58% the rate of red-beet storage roots in the same conditions and less than 5% of the maintenance respiration rates typical of leaves and stems (Tei et al., 1996). Respiration rates per unit of crop dry weight
will be low as harvest approaches in onions, since they have a large proportion of the total weight in the bulb (see (iv) above). The overall maintenance respiration rate of bulb onions close to the normal harvest time (i.e. with 81%
of the shoot dry in bulbs) was estimated as 0.0035 g CO2/g dry matter/day, only 60% of the value of a beet crop at a comparable stage (Tei et al., 1996).
The net result of these processes, total dry biomass production, has been investigated in onions. Well-irrigated onion crops produce, during bulbing, an average of about 1.6 g of shoot dry matter per MJ of solar radiation intercepted by the leaf canopy. Similar conversion efficiencies have been reported for potatoes, sugarbeet and in cereals before anthesis. However, values of mean conversion efficiency vary, and are low in conditions of high irradiance (high light levels) and high temperature, and high in irradiance, lower-temperature conditions (Brewster et al., 1986).
As might be expected, lack of irrigation in dry weather decreases conversion efficiency. In well-irrigated crops a range of conversion factors between 1.2 and 2.0 g/MJ has been observed. Tei et al. (1996) reported an efficiency of conversion of absorbed PAR to dry-matter of 5.08 g/MJ in a well-irrigated and -fertilized bulb onion crop during the bulbing phase. Efficiencies of red-beet and lettuce in the same experiment were 71 and 48% of the onion value, respectively.
Fig. 4.2. The change of total shoot (solid lines) and bulb (broken lines) dry weight with time of irrigated, well-fertilized onion crops when grown at 400 (upper graph) and 50 plants/m2(lower graph). A, autumn-sown, S, spring-sown (from Brewster et al., 1986. Courtesy of Annals of Botany).
About 50% of total solar radiation energy is PAR (Squire, 1990), so these efficiencies in terms of PAR should be halved for comparison with results based on total solar irradiance.