Abstract

Soil with low available Zn in the topsoil and subsoil are widespread in Australia and other countries, and constitute one of the limiting factors for the sustainable production of crops. The effects of soil and foliar Zn application was studied to correct subsoil deficiency in three genotypes of oilseed rape (Brassica napus L.) grown in pots (100 cm long and 10.5 cm in diameter) in a glasshouse experiment. The top soil (upper 10 cm soil in pots) was supplied with zinc in all treatments whereas Zn was either omitted from subsoil (-Zn: no added Zn in subsoil) or Zn supplied in subsoil as soil application (+Znsoil: Zn applied @1 mg kg-1 soil in subsoil) or foliar spray of Zn only (no added Zn in subsoil but foliar spray of Zn at four and six weeks after sowing of oilseed rape). Soil application of Zn was found to be more effective in correcting Zn deficiency for shoot dry matter production at 60 days after sowing in Zn-inefficient genotype 92-13 compared with foliar application of Zn, whereas foliar spray was equally effective in Narendra and Zhongyou 821. Zinc-efficient genotype Narendra had significantly higher shoot dry matter compared with 92-13 and Zhongyou 821 under -Zn treatment. Narendra had the lowest Zn content in roots but highest Zn content in shoots suggesting superior Zn transport mechanism from roots to shoots. Results indicate that subsoil Zn has a profound effect on vegetative growth and Zn content in shoots and roots of oilseed rape. Oilseed rape genotypes differ in their ability to tolerate low Zn in subsoil. Where subsoils are Zn-deficient, fertilization of the topsoil may not be adequate for optimal Zn nutrition of oilseed rape. Soil application of Zn in subsoil may be more effective for Zn-inefficient genotypes, while foliar spray of Zn may be equally effective for Zn-efficient genotypes of oilseed rape.

Introduction

Among the micronutrients, Zinc deficiency is most widespread on a wide range of soils under both cold and warm climates (Cakmak et al., 1996; Graham et al., 1992, Grewal et al., 1997). Soils with low available Zn in the topsoil and subsoil are also widespread in Australia, and constitute one of the limiting factors for the sustainable production of crops (Graham and Ascher, 1993; Grewal et al., 1997). Incorporation of Zn with other fertilizers into the normal cultivation zone of the soil (top 10 cm) is a common practice to overcome the Zn deficiency problem. Because Zn is readily fixed in soil, little of this Zn moves down the profile below the cultivation zone (Brennan and McGrath, 1988). During periods of dry weather, roots may not absorb Zn and other nutrients adequately from dry topsoil (Graham et al., 1992). Thus, the application of Zn fertilizer in the cultivation zone may not be meeting the crop requirement for root growth and nutrient use where subsoils are low in Zn.

The role of zinc in subsoil nutrition is of particular interest because of its importance in maintaining membrane integrity of root cells (Cakmak and Marschner, 1988; Welch, 1995) and the possible role of zinc in reducing the toxic effects of excessive boron (Graham et al., 1987; Singh et al., 1990); excessive boron in subsoil is a problem in a large proportion of South Australian soils. Graham and Ascher (1993) demonstrated large yield and root growth responses in wheat and barley to subsoil fertilizer application.

In oilseed rape, root growth was impaired and seed yield was severely depressed when Zn was omitted in subsoil (Grewal et al., 1997). Application of Zn in subsoil may not be feasible from the practical point of view. Growing Zn-efficient genotypes may help under low Zn subsoils. Zinc-efficient genotypes had been found to be more tolerant to subsoil Zn deficiency compared with Zn-inefficient genotypes (Grewal et al., 1997). Foliar spray of Zn may also help to correct the subsoil Zn deficiency. Studies were therefore undertaken to evaluate the comparison between soil and foliar Zn application to correct subsoil Zn deficiency in oilseed rape genotypes differing in Zn efficiency.

Materials and Methods

Zinc-deficient siliceous sandy soil (Laffer Sand) was washed three times with deionized water, and dried in a glasshouse. DTPA-extractable Zn of this soil after washing was 0.13 mg kg^-1 soil. Polyethylene-lined cylindrical PVC pots (100 cm long, 10.5 cm diameter pots) were used for growing plants. The 11 kg of soil per pot was divided into 1.25 kg for topsoil (top 10 cm: top 7-17 cm zone in pots) and 9.75 kg for subsoil (lower 10-93 cm: 17-100 cm zone in pots). Upper 0-7 cm zone of pots was kept unfilled. Calcium carbonate powder (0.3% w/w) was mixed in dry soil to lower the availability of native soil Zn, before the following nutrients were separately applied in solution (in mg kg^-1dry soil) to both topsoil and subsoil: Ca(NO₃)₂. H₂O, 918; KH₂PO₄, 71.8; K₂SO₄, 113.6; MgSO₄.7H₂0, 140; NaCl, 3.2; CuSO₄.5H₂O, 2.3; CoSO₄.5H₂O, 0.23; H₂MoO₄.H₂O, 0.23; MnSO₄.4H₂O, 3.69; H₃BO₃, 1.4; FeSO₄.7H₂O, 0.7 and NiSO₄.6H₂O, 0.15. These basal nutrient solutions were mixed in the soil before potting. Solution of ZnSO₄ 7H₂O was added to the topsoil to supply 0.5 mg Zn kg^-1 in all the treatments and mixed in the soil before potting. The rate of Zn added to topsoil was similar to that of farmers usage in South Australia. Zinc treatments were applied to the subsoil as no added Zn (-Zn) or 1 mg added Zn kg-¹ dry soil (+Zn (soil)) and mixed in soil before potting. In foliar spray treatment of Zn: no Zn added in subsoil but 0.5% ZnSO4 solution was applied as foliar spray twice at four and six weeks after sowing (called as +Zn (foliar)).

Soil for both zones (topsoil and subsoil) was watered to 12% by weight with Double deionised water (DD water). A polypropylene tube (28 cm long, 3 cm internal diameter) was inserted into the upper 28 cm zone of each pot for watering the subsoil and referred to here as a subsoil watering tube. Germinated seeds of oilseed rape genotypes were sown at a depth of 1 cm. The three genotypes tested were 92-13 (traditional Brassica napus genotype from China found to be most Zn-inefficient in our previous experiments), Zhongyou 821 (a traditional Brassica napus genotype from China found to be Zn-inefficient in our previous experiments), Narendra (Zn-efficient B. napus genotype from Australia; this is a canola type because it is low in both erucic acid and glucosinolate)). There was little variation in seed Zn concentration (44±4 mg kg-1 DM) and seed Zn content (130±8 ηg per seed) of these three genotypes.

Plants were grown in a glasshouse at the Waite Institute (Adelaide, South Australia) from mid September to mid November. Temperature ranged from 10 to 32^°C during the day and 4 to 12^°C during the night. The relative humidity would drop down to 30% during the day and increase up to 90-95% during the night.

During the early vegetative phase (3 weeks), plants were watered with double deionised water from the top. As the roots established, watering was done both to the surface and through the subsoil tubing to avoid any downward movement of Zn from topsoil to subsoil. Efforts were made to maintain the 12% moisture in the soil by weighing the pots every alternative day.

Plants (two plants per pot) were harvested 60 days after sowing. Roots were washed with deionized water before drying. The oven dried (70^°C) samples were then digested in concentrated nitric acid at 130^°C and analysed for Zn by inductively coupled plasma spectrometry.

The experiment was set up in a completely randomized design with a factorial arrangement of treatments (three genotypes × three Zn treatments × three replicates).

All data were subjected to an analysis of variance. ANOVA include separating main effects of oilseed rape genotypes and Zn treatments, and their interaction effect (genotypes × Zn treatments). Least Significant Difference (LSD at P = 0.05) was used to assess the differences among pairs of treatment means.

Results and Discussion

The interaction effect between genotypes and method of Zn application was significant on shoot dry matter accumulation (Figure 1). Soil application of Zn was significantly superior to foliar application of Zn for shoot dry matter accumulation in 92-13, whereas foliar spray of Zn was equally effective as soil application of Zn in Narendra and Zhongyou 821. Narendra was significantly superior to 92-13 and Zhongyou 821 under -Zn subsoil treatment.

Figure 1. Shoot dry matter accumulation (g/plant) of oilseed rape genotypes at 60 days growth. Interaction between genotypes and method of Zn supply was significant (LSD_0.05= 0.5).

Root dry matter was significantly improved by soil application of Zn in 92-13 and Zhongyou 821 (Table 1). Foliar spray of Zn increased root dry matter significantly only in Zhongyou 821. Increase in root dry matter due to soil or foliar application of Zn was not significant in Narendra. Narendra and Zhongyou 821 produced significantly higher root dry matter than 92-13 under -Zn subsoil treatment.

Table1. Root dry matter accumulation (g/plant) of oilseed rape genotypes at 60 days growth

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Genotypes -Zn subsoil +Zn (soil) +Zn (foliar)

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92-13 1.64 2.36 1.88

Zhongyou 821 2.32 2.80 2.70

Narendra 2.26 2.50 2.32

LSD_0.05

Genotypes × subsoil Zn 0.30

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Zn concentration of shoots was significantly improved by soil application of Zn, while the effect of foliar application of Zn was not significant (data not given)). Zn content in shoots was also significantly improved by soil and foliar application of Zn (Figure 2). Soil application of Zn was significantly superior to foliar spray of Zn for increasing Zn content in shoots of all the three genotypes. Narendra had significantly lower Zn content in roots (Data not given) but had higher Zn content in shoots compared with 92-13 and Zhongyou 821. Narendra had the lowest Zn content in roots but highest Zn content in shoots suggesting superior Zn transport mechanism from roots to shoots.

Figure 2. Zinc content of shoots (ug/plant) in oilseed rape genotypes at 60 days growth. Interaction between genotypes and method of Zn supply was significant (LSD_0.05= 29).

The results of the current study reveal that low-Zn in subsoil had a significant effect on the root and shoot growth, and Zn content of roots and shoots of oilseed rape inspite of the presence of added Zn in topsoil. Genotypes had a differential response to low Zn in subsoil. Zn-efficient genotype (Narendra) was more tolerant of low-Zn subsoil than the Zn-inefficient genotypes 92-13 and Zhongyou 821. This means that Zn efficiency is a trait which confers tolerance to low-Zn subsoils which are difficult for broad-acre farmers to treat economically. Soil application of Zn was found to be more effective in correcting Zn deficiency, due to low Zn in subsoil, in Zn-inefficient genotype 92-13 compared with foliar application of Zn, whereas foliar spray was equally effective in Narendra and Zhongyou 821. The results suggest that foliar spray of Zn could be used effectively to overcome the problem of Zn deficiency due to low-Zn in subsoil in Zn-efficient genotypes.

Acknowledgements

This research was supported by the Australian Centre for International Agricultural Research (ACIAR Project 9120). Technical assistance with ICP analysis by Mrs T O Fowles and Mr N H Robinson is gratefully acknowledged.

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