Onion (Allium cepa L .) is one of the important vegetable crops commercially grown in India. India is the second largest producer of onion in the world, next only to China. In India, onion is being grown in an area of 0.83 million hectares with production of 13.57 million tonnes and the productivity is 16.30 t ha-1 which is low. Maharashtra is the leading onion producing state followed by Karnataka, Gujarat etc. In Maharashtra, onion is cultivated in an area of 415000 ha with production of 4905000 MT and the average productivity is 11.8 MT/ha. (Source FAO Website: March 2012 and for Indian Data Indian Horticulture Database 2011) which is low compared to world average. Depending upon the critical crop growth stages and soil types, 8 to 10 irrigations are usually given to onion in the Maharashtra state. Availability of irrigation water during rabi and summer seasons is the major limiting factor for onion production. During this season in command area farmers may get only 2 to 3 irrigation and could supplement it with well irrigation. Often the yields of wells are also low and in such cases farmers may not be able to provide 8 to 10 irrigations. If the water is stored in farm pond, only one or two irrigations are possible. Under such circumstances deficit irrigation is inevitable. For deficit irrigation it is necessary to study the response of onion to different water stresses during crop growth period. Therefore the present study was planned to study the response of onion yield to different water stress during important crop growth stages viz. vegetative stage, bulb development stage, bulb enlargement stage etc. Being a semi-perishable crop, it is subjected to deterioration during storage. Storage loss of onions is caused by rotting, sprouting, and physiological weight loss. Rabbani et al. [1] reported that storage losses in onion could be as high as 66%. Many factors, such as cultivars, bulb maturity, moisture content of the bulb, temperature, relative humidity, etc. are associated with spoilage of onion during storage. Thus, irrigation may have some effect on storability of onion as it helps increase moisture content of bulb. Many authors investigated the effect of irrigation on onion yield, but the literature revealed scanty information about the effect of irrigation on storage of onion. Soujala et al. [2] reported that irrigation had only a minor effect on the storage performance and shelf life of onion. A substantial increase of decomposition in onion during storage with increasing irrigation was reported by Shock et al. [3]. Nandi et al. (2002) reported that growth and yield of onion were significantly affected by irrigation, but not post harvest life. The study was, therefore, undertaken with a view to finding out the effect of irrigation on yield and storability of onion.

The field experiments to study the effect of deficit irrigation of different quantities in onion (Allium cepa L.) cv. N-2-4-1 were conducted during rabi season of 2012 and 2013 at Instructional Farm of the Department of Irrigation and Drainage Engineering, Dr. Annasaheb Shinde College of Agricultural Engineering, Mahatma Phule Krishi Vidyapeeth Rahuri. Climatically the region falls under the semi-arid and sub-tropical zone with average annual rainfall of 555.5 mm. The distribution of rain is uneven and is distributed over 15 to 37 rainy days. The annual mean maximum and minimum temperature ranges between 21.2°C to 41.8°C and 3.0°C to 24.6°C, respectively. The annual mean pan evaporation ranges from 2.3 to 14.9 mm/day. Experiment was carried out in Randomized Block Design (RBD) with 27 treatments and two replications based on different combinations of the quantity of water stress days (no stress- (0.00S), 20% stress- (0.20S) and 40% stress- (0.40S) during different crop growth stages vegetative Stage (VS) – up to 50 days, bulb development stage (BDS) - 50 to 75 days and bulb enlargement stage (BES) – 75 to 100.The different combinations of the treatments are:

The 27 treatments were replicated two times, making a total of 54 plots and two additional plots were worked for onion root study. The gross size of experimental site was 46 m × 40 m and net plot size was 4 m × 4 m. The blocks were separated by a distance of 2 m., while the basins in each block were separated by a distance of 1.5 m which serves as buffer to minimize lateral movement of water from one basin to another. The irrigations were scheduled at every growth stage of onion crop. The quantities of water were applied according to the treatments. There was no rainfall during period of experimentation. The depth of water to be applied during each irrigation was calculated according to the following formula.

Where, FC=field capacity,%

MC=moisture content at the time of irrigation,%

BD=bulk density of soil, g/cc

D=effective root zone depth, cm

Irrigations were scheduled at every growth stage of onion crop as per stress underlined in each treatment. The stress was estimated from the moisture content stress in the rootzone. The depths of irrigation water were applied according to the treatments. Irrigation was stopped before 25 days of harvesting (Doorenbos and Kassam, 1979).

Yield and storability of onion

The crop was harvested on 21 April 2013 when about 75-80% of the plants became matured. After harvest, bulbs were kept under a shade for 7 days for curing. Then, for storage studies, 10 kg bulbs from each treatment were taken, divided into 3 equal splits, each representing a replicate and stored on rack at room temperature for 165 days. The observations were done for sprouting, rotting, and total weight loss at 15 days intervals. The rotten bulbs from each treatment were sorted out at the time of recording the data. The temperature and relative humidity of the storage room were also recorded. The collected data on various parameters were statistically analyzed.

Number of irrigations and gross depth of irrigation water applied are given in Table 1. These values are shown graphically in Figure 1.

Figure 1: Depth of irrigation water applied in each irrigation treatment.

Table 1: Number of irrigations and gross depth of irrigation water applied in each treatment during 2012 and 2013.

Onion yield as influenced by water stress

The mean pooled onion yield for two seasons for all the treatments are given in Table 2. The yield data were analyzed statistically for randomized block design. The yields were statistically significant. The mean yields along with CD at 5% are presented in Table 2. These values are shown graphically in Figure 2.

Figure 2: Effect of deficit irrigation treatment of yield of onion crop.

Table 2: Oyield for different treatments during 2012 and 2013.

It is observed from above tables that during 2012 & 2013 the higher yields are observed in treatment T1 (0% water stress throughout growth period ) followed by T4, T2, T10, T3,T11, T5, T20, T16,T21, T12, T19, T7, T13, T8, T22, T15, T18, T9, T17, T23, T14, T24, T25, T26 and T27. The onion yield are observed lowest at T27 (40% stress at all stages). The yields of treatments T1, T2 and T4 are at par during 2012 and T1 and T4 are at par during 2013. However the total water applied at treatment T2 is less compared to treatment T1 and T4 in 2012 and the total water applied at treatment T4 is less compared to treatment T1 in 2013. Hence treatment T2 and T4 are the best treatment. But considering the grading of onion bulb the treatment T4 is best treatment which indicates that no water stress during vegetative stage, bulb development stage and 20% water stress at bulb enlargement stage results in statistically significant yield. The yields of treatments T10, T3, T11, T5, T20 and T16 are at par. The yields of treatments T21, T12, T19, T7, T13, T8 and T22 are at par. The yields of treatments T15, T18, T19, T17, T23, 14 and T24 are at par. The yields of treatments T25, T26 and T27 are at par.

The relationship between water stress and crop yield is important for scheduling deficit irrigation. Onion crop yield obtained from different treatments is presented in Table 2. The maximum yield was observed with full irrigation i.e. in treatment T1 (42.52 t/ha) in 2012 and (43.26 t/ha) in 2013. Lowest yield was observed in treatment T27 (21.35 t/ha) in 2012 and (19.78 t/ha) in 2013.i.e. maintaining the 40% water stress throughout the crop season. In treatment T14, i.e., deficit irrigation of 20% saved only 13.04% of water from full irrigation but reduced the yield significantly by 39.04% in 2012 whereas in 2013 deficit irrigation of 20% saved only 21.58% of water from full irrigation but reduced the yield significantly by 39.73%. Yield obtained in treatment T14, significantly different than T1.The best treatment T4 (20% water deficit during bulb development stage) shows significant yield i.e. 42.36 t/ha in 2012 and 40.91 t/ha in 2013 which is at par to treatment T1 and T2 and significantly different than treatment T14 and T127. The yields of treatmentsT9, T17, T23, and T24 are at par. The 20% water stress throughout growth period of onion crop reduces the yield up to 40% and 40% water stress throughout growth period of onion crop reduces the yield up to 50%.

Effect of irrigation on storage losses

Different kinds of losses of onion under deficit irrigation during storage period have been presented graphically in Figures 3-6. The data pertaining to rotting, sprouting, and total weight loss over 165 days have been presented in Table 3.

Figure 3: Rotten loss of onion during storage.

Figure 4: Sprouting loss of onion during storage.

Figure 5: Physiological weight loss loss of onion during storage.

Figure 6: Total weight loss of onion during storage.

Table 3: Percentage of rooted, sprouted and physiological weight loss in onion bulbs under deficit different irrigation treatments.

Rotting

The bulbs started rotting after 65 days of storage. Percentage of rotten bulbs of onion was significantly influenced by different levels of irrigation (Figure 3). Losses due to rotting of bulbs of treatments T27 were higher and identical during 165 days of storage period. The lowest rotting was found in treatment (T4). This result corresponds the earlier findings of Shock et al. [3] who reported substantial increase of decomposition of onion during storage with increasing irrigation threshold. The lowest rotting loss in the T4 treatment may be due to the fact that plots did not receive any irrigation that kept the bulbs less succulent and as a result less attacked by bacteria and fungi during storage.

Sprouting

Sprouting behavior of onion under different irrigation levels has been shown in Figure 4. The bulbs started sprouting after 80 days of storage. Sprouting was significantly higher after 165days of storage in the onions of treatment T23 and T24.The lowest sprouting was observed in non-irrigated treatment T10 during the entire storage period. The higher percentage of sprouting in the treatments that received higher number of irrigation may be due to slightly higher moisture content in the bulbs of these treatments. Thompson et al. [4] reported that the onion bulbs are naturally dormant at maturity and the length of this dormant period varies with the cultivar and conditions under which the bulbs were grown and stored. Salunkhe and Desai [5] described sprouting as a normal physiological change in stored bulbs that develops reproductive shoots in the second year. Irrigation, thus, has little effect on sprouting.

Physiological weight loss

Physiological loss in weight for onion bulb was measured during the storage period of 165 days in 2013 indicated that it increased with the increases in water stress from 0% stress to 40% stress up to 75 days of storage (Figure 6). Thereafter, PLW increased at faster rate in all treatment. But Physiological loss in weight in T27 (40% deficit) treatment was much higher than other treatment. The water stress treatment recorded higher physiological loss in weight during storage than no stress treatment. While in latter part of storage, i.e., 75 days onward, higher PLW was recorded in 40% stress (T27) and minimum at 0% stress. At 75 days of storage, PLW was recorded minimum 11.03% to maximum 19.50%, in all treatment, which increased to 38.21% and 49.09% after 165 days of storage for all treatments. The total Physiological loss in weight was minimum in treatment T4 and Physiological loss in weight was maximum in treatment T27. Results have shown that PLW (%) increased with the decrease in irrigation from 0% stress to 40% stress during the storage period of onion bulbs. Physiological loss in weight was maximum in treatment T27 due to spoilage of bulbs .The result indicated that crop grown under 40% stress has experienced water stress and hence it was forced to early maturity. Thus, it resulted into development of either immature or partial matured bulbs, which started rotting during storage at an early date in rainy season.

The onion yields and field water use efficiency are higher with less water stress and reduced with increase in water stress. A deficit irrigation strategy of supplying 20% deficit water during the bulb development stage did not reduce the onion yield significantly. The storage losses of onion were found to increase gradually with decrease in irrigation from 0% stress to 40% stress during the storage period of onion.