THE EFFECT OF DWARFING GENES ON MORPHOLOGICAL AND PHYSIOLOGICAL-CHARACTERISTICS OF STEM AND EAR OF WINTER-WHEAT CULTIVAR MIRONOVSKA

The available knowledge about the effects of Rht genes on morphologica l and physiological characteristics of stem as well as on the formatio n of yield elements does not yet enable a general explanation. Allan ( 1983), King et al. (1983), McClung et al. (1986), Bush, Evans (1988) a scribed varying results to the dependence of the Rht gene effects on e xternal conditions, developmental stage and genetic background. The ob jective of this paper was to assess the effect of various dwarfing gen es (Rht) on the development of the main stem and ear in near-isogenic lines of winter wheat cultivated in the field trials at Praha-Ruzyne. Near-isogenic lines carrying dwarfing genes Rht 1, Rht 2, Rht 3, and R ht 1 + Rht 2 were produced in the winter wheat background of the long- stem variety Mironovska (gene sources: Siete Carros, Avalon, W 3A, Yec ora). The lines as well as the control rht lines of the variety Mirono vska were assessed in field trials conducted in 1992 and 1993. Ear mor phogenesis was assessed according to specific morphologic criteria of a decimal scale described by Natrova, Jokes (1993) for the evaluation of the development of wheat ear. Furthermore, the effects of Rht genes have been assessed on stem and ear growth, leaf area size, dry matter formation and allocation. In all the Rht lines, the stem height has b een reduced, however, the degree of reduction differed in the individu al lines. The Rht 1 and Rht 2 genes induced a reduction to the range o f 76 cm to 82 cm which corresponds to the typical height of short stem types. The Rht 1 + Rht 2 and Rht 3 genes reduced the stem height up t o 46 cm to 58 cm producing thus typical dwarf lines. All the Rht genes induced reduction of the leaf size in lower insertions. Furthermore, a reduction of ear length was found in dwarf lines in 1993 when less f avorable climatic conditions prevailed (higher temperatures, lower pre cipitation). The effects of Rht genes on stem dry mass were comparable to those on stem height. However, considerable changes of dry matter allocation were found inducing lower allotment of stem dry mass and hi gher ratio of dry mass of leaves and ear. The changes in dry matter al location were consistent with an absolute increase in ear dry mass of most lines at anthesis. Different values were found only in dwarf line s in 1993, when the total stem dry mass has been reduced to a maximum. This high reduction brought about no changes or only minor reduction of ear dry mass, although the ratio of ear dry mass to total stem dry mass was increased. Rht genes also modified ear morphology. Although t he total number of initiated florets per ear varied among Rht lines, t he number of florets reaching the highest development stage at termina l spikelet (TS), i.e. florets with fully differentiated anthers, was s ignificantly increased in the Rht lines. Because during the ear growth , i.e. during the period from TS initiation to anthesis, only florets with fully developed anthers further developed, it may be supposed, th at the higher number of these florets increased the sink strength brin ging about a higher ability of the ear to compete for assimilate. With regard to the theory about the dependence of the number of florets su rvived at anthesis on kernel dry mass (Brooking, Kirby, 1981), our int erpretation of this association is slightly different. Our results pro ved that the higher number of fertile florets of the Rht lines was det ermined at the time of ear formation already because of the effect of Rht genes on the number of florets with fully differentiated anthers. Such a modification of ear development together with the influence of Rht genes on lowering the stem ability to compete for assimilate induc ed an increase in the dry matter allocation into the ear. In this way, the Rht lines possess both a higher number of fertile florets and hig her ear dry mass at anthesis. However, the relationships between highe r number of fertile florets and higher ear dry mass at anthesis was no t found in all the Rht lines. Although the number of fertile florets o f the line with Rht 3 genes was higher than that of the control variet y in 1993 its ear dry mass remained unchanged. But even in this case, the number of fertile florets was very close to those reaching the hig hest development stage at the time of TS formation. At anthesis, the R ht lines had higher number of fertile florets per ear than the control variety. The only exception was the line with Rht 1 + Rht 2 genes wit h no changes of the floret number in 1993. Most of the lines with high er number of fertile florets had also a higher proportion of grain set . A difference was found only in 1993 with the dwarf line carrying the Rht 3 gene. Because of the higher temperature at anthesis the reducti on of fertile florets was higher that were otherwise able to develop k ernels, reducing thus the number of kernels per ear to that of the con trol variety. Higher number of kernels per ear of lines with Rht 1 and Rht 2 genes brought about a reduction in kernel dry mass only in 1992 when the climatic conditions were less favorable for kernel growth. O n the contrary, the kernel dry mass was reduced in dwarf lines in the two growing seasons. The Rht 2 gene positively affected the kernel dry mass per ear in the two years. It may be concluded that the higher dr y mass of kernels per ear in the Rht 2 line and higher number of kerne ls per ear in the Rht lines was brought about by modifications of stem growth and ear morphogenesis.