Effect of exogenous application of cytokinin on yielding ability of developing grains at different locations within same spike or spikelet in wheat.
The position of grain within a spike of wheat to some extend determines its final grain weight which can range from 20 to 60 mg. The grains from spikelets in the middle region of the spike and from the basal region within each spikelet are more towards the upper level of this range . Endogenous hormones in the grains play crucial roles in regulating their filling pattern especially, during the early phases of their development. It was evident from the preceding section on hormones that bolder grains, endowed with a higher dry matter precipitation capacity, also contained relatively higher levels of growth promoters, i.e., auxins, gibberellins and cytokinins [2,15,16]. Presence of relatively lower levels of the above growth promoting substances in smaller grains plausibly may be the cause for variable metabolic events which ultimately led to a reduction in their yielding ability. Davies  confirmed that auxin in combination with cytokinins stimulated cell division and their higher levels in the sinks could create a renewed vibrancy and impetus for growth of grains. Furthermore, higher cytokinins contents in the grains may promote the division of endosperm cells at the early grain filling stages thus building a powerful sink  thereby enhancing assimilate migration and its accumulation in the developing grains . Therefore, it might be possible to improve grain filling by increasing in vivo quantitative upgradation in plant growth promoter levels in grains, especially at the early filling stages either through breeding or by crop management. To substantiate the contention that it was the hormonal regulation of metabolism which determined the potential of a grains to grow, support is also drawn from the work of  who mooted that the exogenous applications of plant growth regulating substances was accompanied by a transformation of sink capacity along with a change in the grain metabolism in buckwheat. Evaluation the effect of exogenous application of cytokinins on dry matter accumulation and growth rate of individual grains at different grain type and position will be important to identifying the exact role of plant growth regulators on differences in dry matter accumulation of grains within a spike, which could be the key in developing wheat with higher grain yield potential. It will also help to improving management practices to result in additional wheat yield. Hence, the objective of this study was to evaluate the dry matter accumulation and growth rate of individual grains as affected by exogenous application of kinetin at different grain type and position within a spike of PBW-343 wheat.
Materials and methods
Single plants of the wheat (Triticum aestivum L. var. PBW-343) were grown in plastic containers with a diameter of 4.5 cm and depth of 20 cm. The pots were filled with a pasteurized soil which classified as a clay loam with 28.1% Sand, 25.7% Clay and 46.2% Silt, an electrical conductivity ([EC.sub.e]) of 1.2 dS [m.sup.-1], a pH of 7.1 (saturated paste), and organic C of 0.62%. The plants were grown in a screen covered hall under otherwise natural conditions. The pots were watered as described by Houshmandfar et al.,  and fertilized once a week with half strength Peter's solution (NPK = 10:10:10) . An aqueous solution of 100 ppm kinetin was sprayed at 3 ([+ or -] 0.2) ml [plant.sup.-1] at panicle initiation. Ten labelled spikes were sampled nine times, four-day intervals started from seventh day after anthesis (DAA) up to 35th DAA, and at maturity. Spikes were divided into three grain positions included proximal (spikelet number 1 to 5), middle (spikelet number 6 to 15), and distal (spikelet number 16 to 20) regions, and further into two grain types included basal (bold) (grain No. 1 and 2) and apical (small) (grain No. 3 upward). The samples were dried in an oven at 70 [degrees]C for 72 h, and then weighed for dry matter accumulation. Relative growth rate (RGR)  and absolute growth rate (AGR) , were calculated using the following equations:
AGR (mg [day.sup.-1]) = [W.sub.2] - [W.sub.1]/[T.sub.2] - [T.sub.1]
RGR (mg [mg.sup.-1] [day.sup.-1]) = [log.sub.e][W.sub.2] - [log.sub.e][W.sub.1]/[t.sub.2] - [t.sub.1]
Where, [W.sub.1] = Total dry matter of grain at time [t.sub.1], [W.sub.2] = Total dry matter of grain at time [t.sub.2], [t.sub.1] = Time of first observation, and [t.sub.2] = Time of second observation. The data were analysed statistically using analysis of variance and critical differences (CD) at 5 percent level were computed.
Application of kinetin had an insignificant effect on changing the dry matter accumulation potential in both the types of grains during the initial stages of grain growth (Table 1). Though bolder grains showed an increase to the tune of 11.8, 10.8 and 13.5 percents at proximal, middle and distal segments respectively at 19th DAA, the changes were statistically insignificant. However, the corresponding increase in smaller grains at 19th DAA was highly significant and it was to the tune of 30.0, 21.9 and 43.4 percents in proximal, middle and distal segments respectively. The data hint that smaller grains were comparatively more receptive to an exogenous application of kinetin irrespective of their positions in the spike. The response of smaller grains was maximum in distal segment of spike followed by the proximal and middle segments respectively. Further, as the grains progressed in age the smaller grains recorded a significant increase in their dry weight, in all the three segments of spike with a booster close of kinetin and at maturity the recorded increment was to the tune of 18.9, 14.8 and 32.1 percents in proximal, middle and distal segments respectively. Application of kinetin reduced the disparity in dry weight between the bold and small grains as it was to the tune of 9.8, 10.0 and 10.2 percents (lesser in smaller grains) in proximal, middle and distal segments of spike respectively. The corresponding values in control series were 17.8, 15.7 and 26.0 percents at the similar segments of spike respectively at the time of maturity. The whole exercise unequivocally shows that smaller grains tended to gather more mass under the influence of kinetin while bolder grains tended to be non-receptive to this exogenous application. Application of kinetin also created a cascade effect which led to enhancement of growth rate (Table 2). It led to higher initial growth rate of smaller grains as compared to bolder grains. There was a deduction in growth rate in later stages of grain growth in both the types of grains. The maximum enhancement in growth rate was at 15th DAA which was to the tune of 41.2 and 18.3 percents in proximal, 29.2 and 19.1 percents in middle and 63.6 and 22.1 percents in distal segments of spike in small and bold grains respectively.
A scrutiny of data (Table 3) revealed that kinetin application improved relative growth rate of bold and small grains in initial stages of grain growth. The improvement of relative growth rate was relatively more in smaller grains as compared to bolder grains.
Cytokinins play important roles in regulating plant growth and development such as intensity and direction of assimilate flow, cell elongation, and cell division rate . In cereals, high levels of cytokinins are generally found in the endosperm of developing seeds, which may be required for active cell division during the early phase of grain setting . We have investigated the effect of exogenous application of cytokinin (kinetin) on dry matter accumulation and growth rate of individual grains at different grain type and position within a spike of PBW-343 wheat. The exogenous applications of the cytokinin, wherever applicable, improved the precipitation potential of smaller grains with more vigour as compared to the bolder grains e.g., the increase in weight of smaller grains at maturity ranged between 14.8 to 32.1 percents in three different segments of spike. However, the augmentation in case of bolder grains was in the range of 7.5 to 8.8 percents. Furthermore, the results point in no uncertain words that the bolder grains were distinct from smaller grains by some diagnostic features like higher growth rates in relation to smaller grains. Whenever these characteristics were measured, under the influence of exogenous application of the cytokinin in smaller grains, the same seemed to drift in the direction as in operation in the bolder grains with a simultaneous increase in dry matter precipitation. The partial responsiveness of bolder grains to exogenously applied cytokinin, possibly hints to the fact that the bold grains were possibly operating at a saturation level with regard to this endogenous promoter. Higher cytokinins contents in the grain at the grain filling stage may promote the division of endosperm cells , thus constitute a powerful sink  and enhance assimilate transport and its accumulation in the developing grains ,. Michael and Seiler-Kelbitsch, reported that transient grain cytokinin content is correlated with final grain yield of barley. The findings of Alizadeh et al.,  showed an increase in grain yield of wheat cultivars by the application of exogenous cytokinin. Yang et al.,  suggested that differences in sink strength due to cytokinins and indole-3-acetic acid (IAA) are responsible for variations in grain filling between superior and inferior spikelets of rice. In conclusion, the results suggest that a higher grain weight as indicative of capacity of grain to grow, was associated with higher endogenous levels of cytokinins and an exogenous application of cytokinin drifted the metabolism of smaller grains in the direction what was in operation in bolder grains.
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(1) Alireza Houshmandfar and Davood Eradatmand Asli
Department of Agronomy and Plant Breeding, Islamic Azad University, Saveh Branch, Saveh, Iran.
Alireza Houshmandfar and Davood Eradatmand Asli: Effect of exogenous application of cytokinin on yielding ability of developing grains at different locations within same spike or spikelet in wheat.
Alireza Houshmandfar, Department of Agronomy and Plant Breeding, Islamic Azad University, Saveh Branch, Saveh, Iran.
Table 1: Grain dry matter accumulation (mg [grain.sup.-1]) in individual grains of wheat (Triticum aestivum L. var. PBW-343) as affected by exogenous application of kinetin (KN), isolated from different regions of the same spike at different intervals of time after anthesis (mean of ten replications). Days after Proximal anthesis(DAA) Basal Apical 7th 6.52(+5.2) 4.39(+9.8) [-29.8] 11th 11.45(+6.0) 7.29(+10.4) [-36.3] 15th 21.75(+11.5) 17.57 * (+26.4) [-19.2] 19th 33.09(+11.8) 29.24 * (+30.0) [-11.6] 23rd 43.07(+10.4) 38.02 * (+24.6) [-11.7] 27th 50.21(+8.9) 44.12 * (+21.2) [-12.1] 31st 52.53(+8.1) 46.69 * (+20.3) [-11.1] 35th 54.00(+8.0) 48.62 * (+19.4) [-10.0] Maturity 54.84(+8.4) 49.48 * (+18.9) [-9.8] Days after Middle anthesis(DAA) Basal Apical 7th 6.83(+5.1) 4.55(+8.3) [-33.4] 11th 12.19(+5.1) 8.22(+9.6) [-32.6] 15th 22.90(+11.2) 18.28(+19.5) [-20.2] 19th 34.35(+10.8) 29.63 * (+21.9) [-13.7] 23rd 44.54(+9.7) 38.71 * (+18.7) [-13.1] 27th 51.69(+8.4) 45.20 * (+16.8) [-12.5] 31st 53.90(+7.8) 47.59 * (+16.1) [-11.7] 35th 55.05(+7.7) 49.32 * (+15.5) [-10.4] Maturity 55.47(+7.5) 49.93 * (+14.8) [-10.0] Days after Distal anthesis(DAA) Basal Apical 7th 6.40(+6.7) 4.18 (+12.9) [-34.7] 11th 10.92(+6.0) 6.94(+15.7) [-36.4] 15th 21.33(+13.4) 17.74 * (+43.1) [-16.8] 19th 32.57(+13.5) 29.12 * (+43.4) [-10.6] 23rd 41.81(+10.2) 37.53 * (+36.0) [-10.2] 27th 49.51(+10.1) 44.19 * (+34.7) [-10.7] 31st 51.80(+9.0) 46.18 * (+33.5) [-10.8] 35th 53.37(+8.9) 47.94 * (+32.8) [-10.2] Maturity 54.10(+8.8) 48.60 * (+32.1) [-10.2] Values within parenthesis indicate percentage increase (+) in dry weight of grains over control and values within the square brackets denote the relative disparities in small grains over bold grains growing in the same spikelets; CD at 5% level: Age: 5.62, Position/ Type: 4.42, Age x Position/Type: 8.58; *: Significant at 5% level over control. Table 2: Absolute growth rate (mg [day.sup.-1]) in individual grains of wheat (Triticum aestivum L. var. PBW-343) as affected by exogenous application of kinetin (KN), isolated from different regions of the same spike at different intervals of time after anthesis (mean of ten replications). Days after Proximal anthesis(DAA) Basal Apical 11th 1.23(+ 7.0) 0.72(+ 10.8) [- 41.5] 15th 2.58(+ 18.3) 2.57(+ 41.2) [- 0.4] 19th 2.84(+ 12.7) 2.92(+ 35.8) [+ 2.8] 23rd 2.50(+ 6.4) 2.20(+ 10.0) [- 12.0] 27th 1.78(+ 1.2) 1.52(+ 2.7) [- 14.6] 31st 0.58(- 6.4) 0.64(+ 6.7) [+ 10.3] 35th 0.37(+ 5.7) 0.48(+ 3.1) [+ 29.7] Maturity 0.21(+ 40.0) 0.22(+ 48) [+ 4.8] Days after Middle anthesis(DAA) Basal Apical 11th 1.34(+ 4.8) 0.92(+ 12.2) [- 31.3] 15th 2.68(+ 19.1) 2.520(+ 29.2) [- 6.0] 19th 2.86(+ 10.0) 2.84(+ 26.2) [- 0.7] 23rd 2.55(+ 6.2) 2.27(+ 9.1) [- 11.0] 27th 1.79(+ 0.6) 1.62(+ 6.6) [- 9.5] 31st 0.55(- 5.2) 0.60(+ 3.4) [+ 9.1] 35th 0.29(+ 3.6) 0.43(+ 2.4) [+ 48.3] Maturity 0.10(- 16.7) 0.15(- 25.0) [+ 50.0] Days after Distal anthesis(DAA) Basal Apical 11th 1.13(+ 4.6) 0.69(+ 23.2) [- 38.9] 15th 2.60(+ 22.1) 2.70(+ 63.6) [+ 3.8] 19th 2.81(+ 13.3) 2.84(+ 43.4) [+ 1.1] 23rd 2.31(- 0.4) 2.10(+ 15.4) [- 9.1] 27th 1.92(+ 9.7) 1.66(+ 27.7) [- 13.5] 31st 0.57(- 8.1) 0.50(+ 11.1) [- 12.3] 35th 0.39(+ 2.6) 0.44(+ 15.8) [+ 12.8] Maturity 0.18(- 1.1) 0.16(- 11.1) [- 11.2] Values within parenthesis indicate percentage increase (+) or decrease (-) in dry weight of grains over control and values within the square brackets denote the relative disparities in small grains over bold grains growing in the same spikelets; CD at 5% level: Age: 0.29, Position/Type: 0.18, Age x Position/Type: 0.37. Table 3: Relative growth rate (mg [mg.sup.-1] [day.sup.-1]) in individual grains of wheat (Triticum aestivum L. var. PBW-343) as affected by exogenous application of kinetin (KN), isolated from different regions of the same spike at different intervals of time after anthesis (mean of ten replications). Days after Proximal anthesis(DAA) Basal Apical 7th-11th 0.140(+ 0.7) 0.127(+ 0.8) 11th-15th 0.160(+ 8.1) 0.220(+ 18.3) 15th-19th 0.105(+ 1.0) 0.127(+ 5.8) 19th-23rd 0.066(- 4.3) 0.067(- 13.2) 23rd-27th 0.038(- 7.3) 0.037(- 15.9) 27th-31st 0.011(- 21.4) 0.014(- 12.5) 31st-35th 0.007(- 5.2) 0.010(- 16.7) 35th-Maturity 0.004(- 19.5) 0.004(- 20.0) Days after Middle anthesis(DAA) Basal Apical 7th-11th 0.145(+ 4.5) 0.148(+ 2.1) 11th-15th 0.158(+ 9.7) 0.200(+ 12.3) 15th-19th 0.101(- 1.0) 0.121(+ 4.3) 19th-23rd 0.065(- 3.0) 0.067(- 8.2) 23rd-27th 0.037(- 7.5) 0.039(- 9.3) 27th-31st 0.010(- 16.7) 0.013(- 7.1) 31st-35th 0.005(- 9.2) 0.009(- 5.2) 35th-Maturity 0.002(- 5.1) 0.003(- 2.3) Days after Distal anthesis(DAA) Basal Apical 7th-11th 0.126(- 6.7) 0.126(+ 5.0) 11th-15th 0.167(+ 11.3) 0.231(+ 26.9) 15th-19th 0.106(+ 2.3) 0.128(+ 4.1) 19th-23rd 0.062(- 11.4) 0.063(- 18.2) 23rd-27th 0.042(- 15.0) 0.041(- 4.6) 27th-31st 0.011(- 21.4) 0.011(- 15.4) 31st-35th 0.007(- 14.1) 0.009(- 18.2) 35th-Maturity 0.003(- 12.2) 0.003(- 25.0) Values within parenthesis indicate percentage increase (+) or decrease (-) in relative growth rate of grains over control; CD at 5% level: Age: 0.120, Position/Type: 0.051, Age x Position/Type: 0.186.
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|Title Annotation:||Original Article|
|Author:||Houshmandfar, Alireza; Asli, Davood Eradatmand|
|Publication:||Advances in Environmental Biology|
|Date:||Apr 1, 2011|
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