Abstract

Media summary

Key words

Barley, waterlogging, foliar fertilizer spray.

Introduction

Plants may suffer from nutrient deficiencies due to reduced uptake of nutrients, denitrification and leaching of mobile nutrients, and the dilution of ions in waterlogged soil (Drew 1991). Waterlogging damage may be attributed more to the changes in the concentration of solutes in the soil water than to the direct effect of O₂ deficiency (Drew and Lynch 1980). Ionic homeostasis is probably a key component of the cellular adaptive pathway under waterlogging stress (Subbaiah and Sachs 2003). Soil nutrient supply can modify plant responses to waterlogging. Nitrogen can alleviate the adverse effects of waterlogging on shoot growth, but nitrogen alone would not improve shoot growth if the supply of other ions was limiting (Drew 1991).

Understanding the response of different barley varieties to waterlogging and nutrient supply is important in the selection of waterlogging tolerant varieties and the improvement of plant productivity in waterlogged soil. The response of barley to waterlogging and applied foliar nutrients was investigated in this paper.

Material and Methods

Plant growth and treatment

Six barley cultivars (Naso Nijo, a cultivar of Japanese origin; Franklin and Gairdner - two Australian cultivars; ZP, TX9425 and DYSYH – cultivars of Chinese origin) were grown in a glasshouse with open slatted sides, providing substantial wind protection but maintaining temperature similar to that of the outside environment, during the summer season in 2003-2004. Plants were grown in 2L pots (4 seedlings in each pot) filled with dark grey vertisol soil collected from Cressy Research Station in Tasmania. Average daily/nightly temperatures varied between 21°C/14°C in December and 18°C/13°C in March.

Two pots of each variety were placed in black plastic tanks (12 pots with 48 seedlings in each tank), with a total of 24 tanks being set up. Once the seedlings were 2 weeks old, 18 tanks were waterlogged, with the water level kept at the soil surface. Six of these tanks were sprayed daily with ¼-strength Hoagland nutrient solution (shown as 1/4FHWL), and another 6 tanks sprayed with full strength Hoagland solution (FHWL), 10ml daily for each plant. The full-strength nutrient solutions contained (mol m^-3): MgSO₄, 2.0; Ca(NO₃)₂, 5.0; KNO₃, 5.0; NH₄H₂PO₄, 1.0, together with micronutrients and iron-EDTA. The treatment was maintained for 3 weeks.

Chlorophyll fluorescence was measured with a pulse-amplitude modulation portable fluorometer (Mini-PAM, Heinz Walz GmbH, Effeltrich, Germany) at midnight for dark-adapted leaves.

Statistical Analysis

The experiment was carried out as a randomized split plot design with each tank as a main plot, and the varieties as subplots. Data for all growth and physiological parameters were analyzed by analysis of variance (ANOVA) with General Linear Model (GLM) using the Minitab Statistical Program (Minitab Release 13.2, Minitab Inc., USA). Differences among treatments were compared using the LSD at the 0.05 level of probability.

Results and Discussion

Yellow leaf percentage

After 3 weeks waterlogging treatment, the percentage of yellow leaves to total leaves significantly increased compared with the control plants, in which no yellow leaves were observed in all six barley varieties. Among these six varieties, TX9425 and DYSYH showed the lowest yellow leaf percentage, followed by ZP, while Naso Nijo, Franklin and Gairdner showed much higher values (Fig. 1). In waterlogged plants sprayed with either ¼ strength Hoaglands solution (1/4FHWL) or with full strength Hoaglands solution (FHWL), the yellow leaf percentage still showed a significant increase compared with the control plants. However, compared with the waterlogged plants without extra nutrients, yellowing was significantly reduced for all FHWL plants while no significant difference was found for 1/4FHWL plants in all varieties except in Franklin (Fig. 1). Drew et al. (1979) reported that supply of nitrate to plants in waterlogged soil alleviated waterlogging injury to shoots as follows: first, nitrate provides a substrate for the nitrogen metabolism of growing shoot tissues; secondly, there is evidence that in sunflower and rice a continuous supply of nitrate to roots is necessary for the synthesis of cytokinins in roots, and their transport to shoots, and the delay of premature leaf senescence.

Figure. 1 Percentage of yellow leaves to total leaves after 3 weeks waterlogging treatment in six barley varieties. No yellow leaves were observed in the control plants. LSD_0.05 shows differences among treatments at the 0.05 level of probability.

Dry weight

The shoot and root dry weight of treated plants was reduced in all six varieties. In waterlogged plants, the shoot weight as a percentage of the control ranged from 44% to 69%, and the root weights ranged from 30% to 62% (Fig. 2A and B), with ZP and TX9425 showing larger values than the other four varieties. Compared with the waterlogged plants without extra nutrients, ¼ FHWL plants did not show significant improvement in shoot growth except in DYSYH. The root growth of TX9425 and ZP improved significantly, but not in others. The full strength Hoaglands solution significantly improved the growth of shoots and roots in all varieties, except for the roots of DYSYH. The foliar spray of Hoagland nutrient solution therefore substantially alleviated waterlogging stress in barley.

Figure. 2 Percentage of A) shoot and B) root dry weight for treated plants compared to the control plants. LSD_0.05 shows differences among treatments at the 0.05 level of probability.

Length of roots

The length of the longest root in waterlogged, 1/4FHWL and FHWL plants was significantly reduced compared with the control. Waterlogged plants produced mainly adventitious roots, as most seminal roots died after 3 weeks stress. Among waterlogged, 1/4FHWL and FHWL plants, no significant difference in the longest root length was found (Fig. 3A), which meant the improvement in root dry weight in FHWL plants was not attributed to the increase of root length, but was due to the production of more adventitious roots. A typical root morphological change under waterlogging is shown in Fig. 3B. In these adventitious roots, cortical cells break down and air channels (aerenchyma) were formed to facilitate gas diffusion from the shoot to root (Pang et al. 2004, in press).

Fig. 3 A) The length of the longest root after 3 weeks treatment in six barley varieties; B) A typical morphological change of root after waterlogging stress. LSD_0.05 shows differences among treatments at the 0.05 level of probability. CK = control, WL = waterlogged.

Chlorophyll fluorescence

The variable to maximum fluorescence ratio (Fv/Fm) is considered to be a measure of Photosystem II (PSII) effectiveness in primary photochemical reactions. After 3 weeks treatment, Fv/Fm for waterlogged plants was significantly reduced in all varieties compared with the control plants, however, the decrease in TX, ZP and Gairdner was smaller than in the other three varieties. Compared with the waterlogged plants, ¼FHWL treatment showed no significant alleviation in Fv/Fm for all varieties, while it was significantly improved for FHWL plants in all varieties except Naso Nijo. For TX and Gairdner, there was no significant difference when compared with the control plants.

Fig. 4 Chlorophyll fluorescence after 3 weeks treatment in six barley varieties. LSD_0.05 shows differences among treatments at the 0.05 level of probability.

Conclusion

Waterlogging adversely affected the growth of barley plants, but foliar spray with full strength Hoagland solution alleviated these effects by reducing the senescence of leaves, increasing the production of adventitious roots, and increasing the effectiveness of PSII. This means that selection of varieties for waterlogging tolerance must be conducted under strictly controlled nutritional conditions.

References

Drew MC, Sisworo EJ, Saker LR (1979) Alleviation of waterlogging damage to young barley plants by application of nitrate and a synthetic cytokinins, and comparison between the effects of waterlogging, nitrogen deficiency and root excision. New Phytologist 82, 315-329

Drew MC, Lynch JM (1980) Soil anaerobiosis, micro-organisms and root function. Annual Review of Phytopathology 18, 37-66.

Drew MC (1991) Oxygen deficiency in the root environment and plant mineral nutrition. In 'Plant life under oxygen deprivation'. (Eds MB Jackson, DD Davies and H Lambers) pp. 301-316. (Academic Publishing: The Hague).

Pang JY, Zhou MX, Mendham N, Shabala S (2004) Growth and physiological responses of six barley genotypes to waterlogging and subsequent recovery. Australian Journal of Agricultural Research, in press.

Subbaiah CC, and Sachs MM (2003) Molecular and cellular adaptations of maize to flooding stress. Annals of Botany 91, 119-127.