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Determination of the most sensitive developmental period of wheat (Triticum aestivum) to salt stress to optimize saline water utilization

B. Kamkar1, M. Kafi2 and M. Nassiri Mahallati3

1 PhD student, Ferdowsi University of Mashad, Iran (b.kamkar@wali.um.ac.ir)
2 Associate Professor of Crop Physiology, Ferdowsi University of Mashad, Iran
3Assistant Professor of Crop Physiology, Ferdowsi University of Mashad, Iran

Abstract

A green house experiment was conducted to determine the effects of different salinity levels(control,100,200,and 300 mol/M3) and different periods of salinity exposure(from 2-leaf stage to first node , 2-leaf stage to the end of pollination, and from 2-leaf stage to the end of growth season )was studied on wheat(Triticum aestivum,cv.Falat) yield components by path coefficient analysis method. Path coefficients indicated that grain yield/spike was a function of grain number/spike primarily and mean grain weight in second instance. The main effect on level of salinity was on floret abortion, and subsequently grain/spike and grain weight. Grain yield/spike reduced by increasing of salinity level and salt exposure duration. Also, the effect of grain number reduction on yield was not reduced by compensatory effect on mean grain weight. Reduction of total photosynthesis production by some factors as leaf number, area and area duration reduction, and reduction of photosynthesis rate resulted in source limitations and reduction in grain/spike occurred in response to source limitation. Therefore, it seems that if salinity stress can be avoided in special developmental period of wheat (that potential seed number and fertile florets will be determined), damage to reproductive sinks will be decreased. This can help growers to use some approaches such as the use of high and low quality irrigation water either in combination or as separate applications, with any considerable reduction of yield.

Media Summary

The most sensitive yield component of wheat to salinity and its related phenological stage determined by path coefficients to obtain methods to reduce yield reductions.

Key words:

Wheat, salinity, developmental stage, path analysis

Introduction

The salinity of soils and water is one of the major problems inhibiting their effective utilization in agriculture, especially in arid and semiarid regions. About 25% of world's total area (including 15% of Iran’s land) is saline. This area contains about 33% of the world’s and 50% of Iran’s irrigated lands, respectively. The level of salinity in the water supply can be highly variable in some regions of Iran, even over short distances, and can range between semi desirable through nondesireable. Previous studies on different crops, for instance corn (Khaddah and Ghowail 1963), cowpea (Maas and Poss 1989), and triticale (Francois et al. 1988) indicated that sensitivity to salinity changes during growing season. Lunin et al (1963), also, showed that the sensitivity of vegetables changes during growth stages. By characterising the sensitivity to salt stress of different developmental periods of wheat (Triticum aestivum) the quality of the water could be better matched to the stage of development, which may allow improved management of irrigated crops. For example, high quality water would be used during the most sensitive developmental period or high and low quality water could be combined in that period to reduce the salinity levels in the water and minimize the yield reduction caused by salinity. In this study, we used path coefficient analysis to determine the most sensitive developmental period of wheat to salinity stress to optimize saline water utilization in agriculture.

Material and methods

This experiment was conducted in a greenhouse in 24 sand-filled boxes (0.6 x 0.4 x 0.4 m deep). During the experiment the mean day and night time temperatures were 18C and 24C, respectively and the mean relative humidity was 482%. Plants were grown under a light period 16 hours. Wheat cultivar Falat was used in this study, because it is one of the most common cultivars in semiarid regions of Iran, especially in stressed environments. The plants were irrigated with a modified Hoagland solution added to local tap water. Salinity treatments were used in combination with Hoagland solution with an automatic pump system connected to a drip irrigation system that was arranged above the boxes. Every irrigation cycle continued until the sand was saturated. The nutrient solution was allowed to drain from the saturated sand into 60 L reservoirs after each irrigation for recycling in the next irrigation. Water lost by evapotranspiration was replenished every day to maintain constant osmotic potentials in the solution. The solution pH was maintained between 6 and 6.5 by adding HCl or NaOH as required. The experimental design was a factorial experiment with two completely randomized replications. The factors were salinity in 4 levels (control, 100, 200, 300 mol/m3)and three periods of exposure to salinity (from 2-leaf stage to first node , 2-leaf stage to the end of pollination, and from 2-leaf stage to the end of growth season ). To avoid osmotic shock in the seedlings, salinity was increased gradually over three consecutive days. The three periods of salt stress were selected so that the effects of the severity of salinity stress and the duration of salinity on the most important yield components could be studied. Correlation coefficients between the different measured characteristics (Fig. 1) but using individual pot values (n=24), in an ontogenic diagram and path analysis coefficients were calculated by CORR &REG (STB) procedures in SAS program (SAS, 1985). Data were analysed by path coefficient analysis, using a method similar to that described by Gebeyehou et al. (1982), to partitioning the correlation coefficients, Rij, into direct and indirect effects. The following simultaneous equations were solved to determine the path coefficients, pij (the subscripts i and j indicate the seven measurements).

R57 = p57+r56p67
R56=p56+r54p46
R15=p15+r12p25+r13p35+r14p45
R46 = p46+r45p56
R67=p67+r65p57
R25=p25+r23p35+r24p45+r21p15
R35 = p35+r32p25+r34p45+r31p15
R45=p45+r42p25+r43p35+r41p15

In the equation R57=p57+r56p67, for example, the p57 is the direct effect of characteristic 5 on 7 (the path coefficient) while r56p67 is the indirect effect of characteristic 5 on 7 via 6. It was assumed that the causal relationship between measurements was based on ontogeny of wheat plant (Fig. 1). Photosynthesis was measured on the flag leaf at the end of different salinity periods with an IRGA (HCM-1000 Model, Walz Co.).

Results and discussion

The main effect on the level of salinity was on floret abortion, and subsequently grains/spike and grain weight (Table 1). Different levels of salinity (100, 200,and 300 mol/m3) caused 31.0%,32.0% and 50.0% reduction in the number of grains/spike in comparison with control treatment, respectively, but differences between these levels were not significant (Table1). These results indicated that the number of grains/spike is very susceptible to salinity, because its reduction was considerable, even in the lowest level of salinity. Grain weight was also reduced at each level of salinity and ranged from a 24% reduction at 100 mol/m3 to a 48% reduction at 300 mol/m3. Path coefficients indicated that grain yield per spike was a function of grain number/spike primarily (p57=0.571**) and mean grain weight in the second instance (p67=0.485**)(Fig1). Maas and Grieve (1990) also showed that the decrease of grain number is the main cause of grain yield reduction under salinity stress.

Different durations of exposure to salt had a similar main effect on grain yield as the severity of salinity. Exposing the plants to salinity up to stem elongation had no effect on spikelet number, and a small effect on grain set. Increasing the duration of salinity reduced grains/spike by increasing floret abortion. It was concluded that exposing the plants to salinity during the primary developmental stages has an important role in determining final grain yield. Similar results have reported by Francois et al (1992).

The effect of the grain number reduction on yield was not reduced by a compensatory effect on mean grain weight (Table1). The direct effect of the durations of the vegetative and reproductive developmental stages were not significant, but the duration of the grain filling period had a significant effect on mean grain weight (R46=0.794). Salinity also reduced the production of carbohydrate during important stages of growth. There was a reduction in the number of leaves, the leaf area, and leaf area duration in salt stressed individuals (data not shown).

Table 1. The main effects of the level of salinity and the duration of the salinity treatment on the yield and yield components of wheat cv.Falat. Means followed by the same letter are not significantly different (P=0.05)

 

Spikelet abortion
(%)

Spikelet/ Spike

Floret abortion (%)

Grains/
spike

Single Grain weight (g)

Grain yield/spike
(g)

Salinity level

Control

33.0a

15.2a

32.9a

30.4a

0.0308a

0.94a

100

43.9a

13.2b

44.1b

20.9b

0.0235b

0.58b

200

43.0a

13.7b

43.0b

20.7b

0.098bc

0.42b

300

47.9b

14.0b

47.8b

14.9b

0.016c

0.26b

Duration of salinity treatment

2-leaf to end

45.3a

12.6b

45.5a

16.8b

0.0153b

0.25b

2-leaf to pollination

42.7a

13.8a

42.9a

23.3ab

0.0244b

0.58ab

2-leaf to first node

37.8a

15.7a

37.8a

25.3a

0.0277a

0.71a

Control

33.0a

15.2a

32.9a

30.4a

0.0308a

0.94a

Fig1. ontogenic relationships between wheat yield components (Numbers are path coefficients and**and*indicate significant relationships at 0.1% and 1% level, respectively). Numbers in circles are components numbers in equations.

Kingsbury et al. (1984), indicated that carbohydrate shortage is the main cause of leaf area decreasing, which is consistent with results of Francois et al (1994). Maas and Grieve (1989), also, have reported leaf number reduction under salinity stress The rate of photosynthesis was significantly reduced by increased level of salinity by about 29, 60, 78% at 100, 200 and 300 mol/m3 respectively in comparison with control treatment in this experiment (Fig2). The reduction in photosynthesis was associated with increased spikelet and fertile floret abortion (Table1). Path coefficients indicated that grains/spike is a function of fertile floret number (grains/spikelet) firstly (p35=0.887**) and spikelet/spike in secondly (p25=0.307*) both of which are related to carbohydrate supply during the growth of the spike.

The results indicated that reductions in grain number per spike occurred in response to source limitation, which was caused by many factors that reduced total photosynthetic production, such as leaf number, area and area duration and reduction and photosynthesis rate. This was the most important reason for final grain yield reduction in salt stressed individuals. These results are supported by Mass and Grieve (1990). It seems that the salinity-induced source limitation will reduce yield primarily by a severe reduction in grain number and secondly by a reduction in grain weight. These results were demonstrated by path coefficients (Fig. 1). Therefore, it seems that if salinity stress can be avoided in first developmental stages of wheat (from first node up to pollination), damage to reproductive sinks will be decreased. Therefore if approaches such as the use of high and low quality irrigation water either in combination or as separate applications, it is better that the low quality water be applied during the least susceptible stages ,so that there will not be any considerable reduction of yield. At the sensitive development stages, high quality water or a mixture of high and low quality water should be used. These recommendations are based on pot trials and these concepts need to be tested field level experiments. Francois et al. (1994) have reported similar results in a field experiment, which suggests the concept is a feasible one.

Fig2. Photosynthesis rates of the flag leaf of wheat grown at different levels of salinity. Values with different letters are significantly different from one another.

References

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