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Relationship between seedling ages, total leaf number and leaf appearance rate of transplanted rice

B Amiri Larijani1,2, Z. Tahmasebi Sarvestani1 and Gh Nematzadeh3

1 Department of Agronomy, Agricultural Faculty, Tarbiat Modares University, Tehran, Iran
2
Haraz Extension and Technology Development Center, Agricultural Research, Education and Extension Organization, Ministry of Jihad-e-Agriculture, Iran.

3 Department of Plant Breeding, Natural Resources and Agricultural Sciences University, Sari, Iran
Corresponding author, e-mail: amiri_931@yahoo.com

Abstract

Seedling age at transplanting has a great impact on seedling establishment, plant growth and development. The correct age of seedlings used for transplanting is of primary importance for stand uniformity. The number of leaves on the main stem (LN) and leaf appearance rate (LAR) are important indicators for determination of physiological development stage and development rate of rice plant. To examine the effect of seedling age on leaf appearance rate and total leaf number of rice, a two year experiment has conducted in the Caspian Sea coastal area in Iran. The experiment was conducted as a split-plot, randomized complete block design with three replications. Main plot include three rice cultivars and subplot consist of three different seedling ages consist of young seedlings (3-3.5 leaf stage); medium seedlings (4-4.5 leaf stage) and mature seedlings (5.5-6.5 leaf stage). To raise seedling with different ages, cultivars were sown in 10-day intervals starting 21st April 2008 in a box nursery and transplanted at the same time, 25th May. Seedling ages influenced leaf appearance rate (LAR), decreasing the total number of leaves on the main culm at mature seedling. Average LN was 15.2, 16.0 and 15.7; Average LAR was 0.17, 0.20, 0.21 leaves/day and average phyllochron was 5.9, 5.0, 4.8 days/leaf for mature, medium and young seedlings respectively.

Key Words

Seedling age, leaf appearance rate (LAR), transplanted rice

Introduction

Transplanting has a considerable impact on the rice growth by generally delaying phenological development (Kotera et al. 2004). Seedling age at transplanting has a great impact on seedling establishment, plant growth and development. The correct age of seedlings used for transplanting is of primary importance for stand uniformity and seedling establishment, and Khakwani (2005) stated that "half of the success of rice cultivation depends upon the seedling". The establishment of transplanted rice seedlings and their subsequent growth depends on the growth of new root and leaf (Mishra and Salokhe 2008). The development stage of a plant defines its physiological age and is characterized by the formation of the various organs and their appearance (Bouman et al. 2001). Leaf appearance is one of the important aspects of development in rice. In cereal crops including rice, panicle emergence immediately follows the full appearance of the flag leaf. Thus, if final leaf number is known, flowering time can be predicted on the basis of leaf appearance (Yin and Kropff 1996). Leaf appearance rate (LAR) is the number of leaves that become visible on a stem per unit time. The integration of LAR on the main stem over time gives the number of emerged leaves (NL) on the plant main stem, which is an excellent measure of plant development (Streck et al. 2008; Streck et al. 2005). In small grains, NL is often represented by the Haun Score (HS), which is the number of fully expanded leaves plus a ratio of the length of the expanding leaf to the penultimate leaf (Haun 1973). The calculation of LAR is an important part of many crop simulation models, including those computing development and growth of rice. Vegetative development of plants, especially in small grains, is defined by main stem NL. The main stem NL in rice is related to the timing of several plant development stages such as tillering, panicle initiation, booting and anthesis (Streck et al. 2008). Temperature, photoperiod and genetic characteristics are major factors that drive LAR and final LN in rice (Yin and Kropff 1996; Streck et al., 2008; Sie et al. 1998). The number of leaves varies with the cultivar. Short-duration and photoperiod-insensitive varieties have fewer leaves than long-duration and photoperiod-sensitive varieties. According to Yoshida (1981) most early to medium duration rice cultivars produce 10 to 18 leaves on the main culm. Sie et al. (1998) observed variable number of leaves with different sowing dates, which was correlated with crop duration. The rate a plant produces leaves (LAR) is important for canopy development, since leaves are the source of carbohydrates for the plant (Khanal 2005). The objective of this study was to determine whether seedling age at transplanting time had an impact on main stem LAR and LN in rice plant.

Methods

Location and treatments of field experiment

A two-year field experiment was conducted at the Haraz Extension and Technology Development Center (HETDC) at Amol City, Mazandaran province, Iran (Lat. 36o 29/ N, Long. 52o 23/ E, Elev. 23.7 m) during the 2008-2009 growing seasons. This location was representative of the main irrigated paddy area at the Caspian Sea coast in north of Iran on a neutral (pH=7.61) loam soil with 20% clay, 44% silt and 36% sand. The climate of the site is semi-Mediterranean, characterized by a humid warm summer with low rainfall and a moderate winter with high precipitation. Monthly mean minimum temperature becomes more than 10oC from May, so that rice can grow without hazard. The nursery, however, is sometimes affected by cold weather below 10oC in April. Annual mean precipitation is about 800 mm. Since precipitation is only 177 mm during the dry summer from April to August, the irrigation requirement was 1152 mm.

The experiment was conducted as a split-plot design with 3 replications. Main plots were 3 rice cultivars: "Tarom", a local short duration indica type rice, a tall plant height and tasty; "Ghaem" short-duration improved indica rice; and "Fajr (IR7328)" a mid-duration high-yielding variety (indica) introduced from IRRI. Sub-plots were 3 seedling ages: young seedling (3 to 3.5 leaf stage), medium seedling (4 to 4.5 leaf stages) and mature seedling (5 to 6.5 leaf stages), from three sowing dates in the nursery at 10 day intervals, so that seedlings of different ages were transplanted at the same time on 25 May. For each date, pre-germinated seeds (1 day soaking and 1 day disinfection solution and 1 day incubation at growth chamber) were sown in standard plastic box (30*60*3cm) nursery with 150 g germinated seed per box on 21 April, 1 May and 11 May. Plots size for transplanting were 3 4 m2 with a hill spacing of 30 16 cm and 1 plant per hill in the puddled soil. Plots were irrigated after transplanting and a shallow water level maintained up to two weeks before harvest. Fertilizer as urea, potassium sulphate and triple superphosphate was applied, based on technical recommendation of the National Research Center taking account of the different nutrient requirements for each cultivar. Plots were clean weeded by hand and insect pest and disease controlled as necessary.

Data collection and analysis

Maximum and minimum water and air temperatures were monitored daily. Air temperatures were measured by a standard meteorological station located at about 100 m from the experimental field plots and water temperature was measured within nursery period and experimental plots after transplanting at three replications with a mercury maximum/minimum thermometer submerged in the floodwater slightly above ground level. The date at which successive leaf tips appeared was recorded and averaged from main culms of six hills per sub-plots based on weekly observation from transplanting to fully emergence of flag leaf. Leaf expansion is completed when the ligule becomes visible, which occurs after the first appearance of the next leaf. As one leaf appears, the one below is actively expanding, and the one below that is approaching full expansion. When the leaf was not fully developed, the following method of counting was adopted: for instance, when the 6th leaf on the main stem of a plant is not fully developed but it has reached 30% of its full length, the plant's age is counted as 5.3, and when it reaches 70% of its full length, the plant's age will be 5.7 (Haun 1973). At the time of counting, the full length of the 6th leaf is not known, and hence we must estimate it from the length of the 5th and 4th leaves. In order to count the number of leaves we placed a small mark with waterproof marker on the middle of every other leaf blade to assist the count. Leaf appearance rate were regressed against average water temperature or days after transplanting, using linear regression techniques by Excel software. Statistical analysis was performed with SAS software.

Results

Total number of leaves on main stem

The number of leaves on the main stem was recorded twice per week from transplanting to fully emergence of the flag leaf. When considering the two-year average of total leaf number of each cultivar, there was a significant difference between cultivar main plots, so that the varieties Fajr and Ghaem with 16.1 leaves had highest, and the short-duration variety Tarom with 14.8 leaves had lowest number of leaves on the main stem (Table 1). The interaction of cultivar seedling age wasn’t significant in 2008, but was significant (alpha=0.05) in 2009, apparently because transplanting time was delayed by 9 days in 2009.

Table 1. Analysis of variance and mean comparison between treatments for total number of leaves on the main stem.

source

Mean of Square

 

Tarom

Ghaem

Fajr

 

2008

2009

 

2008

2009

2008

2009

2008

2009

Mean

Block

0.114ns

0.541ns

S1

14.9cd

14.6c

15.3bcd

16.0b

15.8ab

14.8c

15.2

Cultivar

2.795**

8.834**

S2

15.6abc

14.3c

16.2a

16.8a

16.4a

16.4ab

16.0

Block*cultivar

0.206ns

0.0838ns

S3

14.6d

14.7c

15.6abc

16.5ab

16.3a

16.6ab

15.7

Seedling age

1.373**

1.781**

               

Cultivar* Seedling age

0.231ns

0.825*

Mean

15.0

14.5

15.7

16.4

16.2

15.9

 

Note1:**,*=significant difference at 1% and 5% level of Duncan test; ns=not significane
Note2: the numbers with at least one same letters denote no significant difference (Duncan test, alpha=0.05).

Effect of seedling ages on total number of leaves on the main stem

Number of leaves at transplanting time is indicating the seedling age, e.g. young (3 to 3.5 leaf stage), medium (4 to 4.5 leaf stage) and mature seedling (5 to 6.5 leaf stage). Based on results, seedling ages at transplanting had a significant effect on total leaf number of main stem in each year separately. On average medium seedlings with 16.0 leaves had the highest and mature seedling with 15.2 leaves had lowest leaf number on the main stem.

Leaf appearance rate (LAR) at different cultivars and seedling ages

Total number of leaves was linearly regressed against water temperature or days after transplanting. The slope of the linear regression was the LAR (leaves/ oC.day-. The cultivar main plots had different LAR from transplanting to panicle initiation, averaging 0.0108, 0.0113 and 0.0115 leaves/oC.day, respectively, for Fajr, Tarom and Ghaem. Seedling age has changed the leaf appearance rate of all cultivars. The average LAR after transplanting to full expansion of flag leaf was 0.0089, 0.0098 and 0.0101 leaves/oC.day for mature, medium and young seedlings respectively (Figure 1). The inverse of LAR, based on days after transplanting, results in number of days need for appearance of one leaf on the main stem (phyllochron). The effect of variety and seedling age on LAR and phyllochron are shown in Table 2.

Main stem leaf number

   

GDD

 

Figure 1. Leaf appearance rate (LAR) of main stem as a function of accumulated water temperature different seedling ages over time (after transplanting to flag leaf full expanding). G=Ghaem; T= Tarom; F= Fajr; S1, S2, S3 = mature, medium, young seedlings

Conclusion

Age of seedlings at transplanting often depends on weather conditions, availability of water, herbicide, labour and other inputs in farmers’ fields. Transplanting older seedlings induces a delay in the onset of linear dry matter accumulation and tiller or leaf emergence (Pasuquin et al. 2008). The extended stay of seedlings in the nursery delayed leaf emergence after transplanting by a duration similar to that between the two transplanting dates. Roots of the seedlings are injured by transplanting, and the seedlings lose the balance between water uptake and transpiration. This phenomenon is called transplanting shock (Kotera et al. 2004). Results showed LAR and LN are affected by seedling age, so that transplanting of mature seedlings (at a leaf stage > 5) reduced the total leaf number and leaf appearance rate of three rice cultivars. In generally, average LAR was 0.17, 0.20, 0.21 leaves/day and average phyllochron was 5.9, 5.0, 4.8 day/leaf for mature, medium and young seedlings respectively (Table 2). Based on these results, a short-duration variety such as Tarom with a low total leaf number on the main stem is more sensitive to seedling age at transplanting due to the likelihood of a greater reduction in LAR and LN with mature seedlings, result to reducing crop duration and grain yield. Therefore, short duration varieties with low total LN must be transplanted at young or medium seedling ages.

Table 2. The average of leaf appearance rate (leaves /day ) and phyllochron (day/leaf) of varieties at three seedling ages (average of two years)

Tarom

Ghaem

Fajr

mean

LAR

Phyl.

LAR

Phyl.

LAR

Phyl.

LAR

Phyl.

S1

0.17

6.0

0.18

5.7

0.17

6.1

0.17

5.9

S2

0.19

5.3

0.22

4.5

0.20

5.2

0.20

5.0

S3

0.21

4.9

0.22

4.6

0.21

4.8

0.21

4.8

mean

0.19

5.4

0.21

4.9

0.19

5.3

 

Note :LAR=Leaf Appearance Rate; Phyl.=Phyllochron: time intervals (day) for appearance of two successive leaves on the main stem

Acknowledgements

To the management of HETDC, in Amol City, Iran, Mr. Gh.R.Rayatpanah and staff of Agronomy group especially Mr.S.J.Hosseini for help and their assistance in conducting the field experiments.

References

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