Abstract

Media summary

Key Words

Growth, Leaf water potential, Light intensity, Wheat, Water supply.

Introduction

The objective of this experiment was to study the interaction of water supply and light intensity on the growth of spring wheat under carefully controlled water conditions. To accurately and reproduceably control the water supply to individual plants, we used a water control system (Tamaki et a1. 1999) suggested by Wookey et al. (1991) and originally described by Snow and Tingey (1985).

Methods

The water control system (Tamaki et a1. 1999) consists of a sleeve in which plants are grown in a rooting medium, which fits into a device containing a column of floral foam through which the water flow passes toward the rooting medium. The height differential between the rooting medium and the water table, and the hydraulic conductivity of the pathway through the floral foam are controlled. The roots are prevented from growing into the floral foam column by a nylon screen.

Four seeds of spring wheat (cv. Penawawa) were sown in each plant sleeve, using a rooting medium of a 2:1:1 mixture (by volume) of pumice, sand and peat moss. The plants were grown at 25/15º C day/night temperature and a photosynthetic photon flux density (PPFD) of 100, 200, 300 or 400 μmol/m²/s (I₁₀₀, I₂₀₀, I₃₀₀, I₄₀₀) with a 14-h photoperiod. Plants were illuminated under VHO ‘cool white’ 110-W fluorescent lamps and 60-W tungsten-filament lamps at different bench heights. The PPFD was measured 4 cm above the top of the canopy using LI-188B quantum sensor (LI-COR, Lincoln, NB, USA).

Table 1. Fertile tiller numbers of primary, secondary, tertiary and quaternary tillers, and of subtillers from T0-T4 primary tillers. Values followed by different letters are significantly different at p<0.05.of water supply

^AW_H, W_M and W_L represent high, medium and low water supplies; I₁₀₀, I₂₀₀, I₃₀₀ and I₄₀₀ represent 100, 200, 300 400 μmol/m²/s PPFD with a 14-h photoperiod, respectively.

After germination, the plants were thinned to one per sleeve to eliminate population density effects. The plants were irrigated as needed with a modified Long Ashton complete nutrient solution (Hewitt, 1966) until the second leaf was mature. Thereafter, three water supply treatments were imposed:

W_H (high water supply): The water table was set at 12 cm below the root screen, with no ceramic disc in the floral foam column.

W_M (medium water supply): The water table was set at 12 cm below the root screen, and a ceramic disc of flow rate of 50 mL/h/cm²/atm (Soil Moisture Equipment, Santa Barbara, CA, USA) was installed in the floral foam column.

W_L (low water supply): The water table was set at 12 cm below the root screen, and a ceramic disc of flow rate of l mL/h/cm²/atm was installed in the floral foam column.

The LWP was measured every 7 d on the second youngest main stem leaf on each plant, using SC10A multiple chamber thernmocouple psychrometer (Decagon Devices, Pullman,WA, USA), from 5 until 40 d after the initiation of water supply treatments (DAIT) (data not shown). Individual tillers were tagged and labeled as they emerged from the leaf sheaths, using the nomenclature suggested by Klepper et al. (1982). Primary tillers were designated T0 to T5. At anthesis, all plants were harvested and fertile tillers and plant height were recorded. Leaf area was measured using LI-3100 leaf area meter (LI-COR Biosciences, Lincoln, NE, USA). The plants were then separated into shoots and roots and oven-dried for 48 h at 75ºC and weighed.

Results

Fertile tiller numbers at anthesis are shown in Table 1. Total surviving tiller number increased as light intensity and water supply increased. Differences in the total tiller numbers were mainly due to differences in the number of subtillers from T0, T1 and T2 primary tillers. There were no significant effects of water supply on tillering at I₁₀₀. Tillers were initiated primarily from the main stem and there were no subtillers except at W_H, where a subtiller was produced at T2 on one of eight plants.

At I₂₀₀, there were fewer tillers produced at W_L. At W_L, there were no subtillers from T0 and T5 (data not shown), and only the primary tillers and the secondary tillers T1 and T2 were produced.

Table 2. Phyllochron and leaf numbers on the main stem.Values followed by different letters are significantly different at p<0.05.

^ASymbols are the same as in Table 1.

At I₃₀₀ and I₄₀₀ there were similar effects of water stress. Total tiller numbers decreased significantly at W_L. At W_L plants initiated significantly fewer primary and secondary tillers and no subtillers from T3 and T4. Furthermore, the PPFD had no significant effects on total tiller numbers at W_L. At W_H and W_M, however, higher PPFD increased tillering.

Plants responded to I₁₀₀ by failing to produce particular tillers. No T0 tillers were produced at I₁₀₀ (data not shown). Subtillers from T0 were not found in any treatments of I₁₀₀ and I₂₀₀, and in W_L at I₃₀₀. The T0 tillers were produced on only 1 of 24 plants among all treatments at I₂₀₀, on only 2 of 24 plants in all treatments at I₃₀₀, and 8, 7 and 5 of 8 plants at W_H W_M and W_L, respectively at I₄₀₀ (data not shown). All T1 tillers survived in all treatments at I₃₀₀ and I₄₀₀ (data not shown). However, T1 survived on only 7 of 24 plants in all treatments at I₂₀₀, and T1 was absent at I₁₀₀ (data not shown).

Increasing PPFD and water supply resulted in an increase in the rate of leaf emergence on the main stem and, therefore, a decrease in phyllochron value (Table 2). Phyllochron was significantly higher in W_L at I₃₀₀ and I₄₀₀ but not at I₁₀₀ and I₂₀₀. Leaf numbers were not affected significantly by water supply rate, but were affected significantly by PPFD (Table 2).

Water supply had no effect on plant growth under low PPFD (Table 3). Plant height was not significantly affected by water deficiency at I₁₀₀ and I₂₀₀, but at I₃₀₀ and I₄₀₀ plants were significantly shorter at W_L. Both increased water supply and PPFD increased shoot and root weights. At I₁₀₀ and I₂₀₀, shoot and root weights were not affected by water supply, but at I₃₀₀ and I₄₀₀, they were affected significantly. The dry weight of plants grown at W_L was not affected significantly by PPFD, but W_H increased it significantly as PPFD increased. With the decrease in water supply, the shoot/root ratio decreased at I₂₀₀, I₃₀₀ and I₄₀₀, but increased at I₁₀₀. Leaves grown at I₁₀₀ had larger areas but lower specific leaf weights than leaves grown at higher PPFDs. Wheat plants grown at I₁₀₀ allocated relatively more dry weight to the production of leaf area and less to the roots than did plants grown at higher PPFDs. Although the plants grown at I₁₀₀ had low dry matter, they had a higher shoot/root ratio and higher leaf area than plants grown at higher PPFD. This indicates a morphological adjustment to compensate for low PPFD. The larger leaf size at I₁₀₀ was reflected by increased duration of leaf elongation. However, the with the longer duration of leaf expansion there was a reduction in the number of leaves produced and, consequently, fewer available sites for tiller production

Table 3. Plant growth responses under different water supply rates and PPFD. Values followed by different letters are significantly different at p<0.05.

^ASymbols are the same as in Table 1.

Conclusion

The present experiment showed a significant interaction of water supply and PPFD on development, tillering and dry matter production of spring wheat. At low PPFD few tillers were produced, the dry matter production was small, and the water supply rate had little impact on plants. As the PPFD increased, the water supply rate affected the production of tiller and dry matter significantly. In other words, as the water supply increased, the growth was largely affected by PPFD. Since higher PPFDs increased transpiration, the plant growth was promoted under increased water supply. Further studies under much higher PPFDs would be worthy to suite the field conditions.

References

Hewitt EJ (I966). Sand and water culture methods in plant nutrition. Commonwealth Agric. Bureaux, Buckinghamshire, pp 547

Klepper B, Rickman RW and Peterson CM (1982). Quantitative characterization of vegetative development in small cereal grains. Agonomy Journal 74, 789-792.

Snow MD and Tingey DT (1985). Evaluation of a system for the imposition of plant water stress. Plant Physiolology 77, 602-607.

Tamaki M, Ashraf M, Imai K and Moss DN (1999). Water and nitrogen stresses on the growth and yield of spring wheat. Environment Control in Biology. 37, 143-151.

Wookey PA, Atkinson CJ, Mansfield TA and Wilkinson JR (1991). Control of plant water deficits using the ’Snow and Tingey system' and their influence on the water relations and growth of sunflower. Journal of Experimental Botany 42, 589-595.