Most semidwarf wheat (Triticum aestivum) cultivars currently in commercial use in northern New South Wales and Queensland have reduced coleoptile length when compared with stand^d height wheat cultivars. Some semidwarf wheat cultivars have demonstrated poor seedling establishment in the field, particularly after deep sowing or under conservation tillage practices. In rain-fed semi-arid environments, deep sowing to provide sufficient soil moisture for the germinating seed or conservation tillage practices to preserve the soil moisture received before sowing, is often necessary. Long coleoptile is a desirable plant attribute for satisfactory seedling establishment for semidwarf wheat in such planting situations.
This study aimed to identify optimal physical conditions for maximum expression of coleoptile length, to investigate the genetic variability for the character coleoptile length and its inheritance in semidwarf wheat and to identify the maximum sowing depth possible without significant yield loss for a number of current semidwarf wheat cultivars.
To identify the optimal physical conditions for maximum expression of coleoptile length, a range of temperatures (5 30°C), growth periods (7-19 day) and relative humidities of the growth cabinet (58 98%) in which the tests were conducted, were investigated. Eight semidwarf wheat cultivars, having either the Rht1 or Rht2 semidwarf allele, and known to have either short or long coleoptiles, were chosen for testing. Grains were germinated in 'cigars' placed in growth cabinets at different preset temperatures and relative humidity levels and coleoptile lengths were measured after pre-determined growth periods for each 'cigar' Four laboratory trials were conducted in 1996 at The University of Queensland Gatton Campus.
Coleoptile lengths were maximized at 14±1°C after 17 - 19 days with the relative humidity of the growth cabinet around 80%. This regime can be used for routine screening of wheat cultivars for coleoptile length and has been used in the subsequent trials in this study.
In the study of genetic variability of coleoptile length among semidwarf wheats, a total of 152 wheat cultivars, grown at thirteen sites in the wheat growing region of southern Queensland in the 1996, 1997, 1998 and 1999 growing seasons were included. Heads of 53 cultivars were collected from Inglestone, Jimbour, Lundavra and Wyaga sites in 1996; 58 cultivars from Condamine, Haystack and Macalister sites in 1997 and 55 cultivars from Jimbour, Macalister and Pirrinuan sites m 1998. For the 1999 growing season, the mean coleoptile length for each cultivar from Billa Billa, Jimbour and Moonie was measured using only one 'cigar' for each experimental unit and the data have been supplied by Dr Phillip Banks, Leslie Research Centre, Toowoomba.
The cultivars represented three maturity groups, viz. quick, intermediate and slow maturity. Coleoptile lengths were measured using the optimal physical conditions identified in this study. The effect of the grain-filling environment on the expression of coleoptile length was investigated by genotype-environment analysis, using the joint linear regression approach. Only 13 semidwarf wheat cultivars were common in all four seasons. In the 1997 growing season, there was pre-harvest rain at the Macalister and Haystack sites and those sites were not included in any subsequent analyses. Mean coleoptile lengths for the 13 common cultivars over eleven sites were regressed on their respective mean grain yield to identify a more reliable grain-filling environment for sampling for subsequent coleoptile length measurement.
Coleoptile lengths of the 152 wheat cultivars varied from 39.5 to 129.5 mm. The grain-filling environment influenced the expression of coleoptile length in each season, regardless of maturity group. Coleoptile length measured from the grains collected from a single site in a single season may not therefore be representative of the cultivar. Grains from a high yielding site are apparently more reliable for the estimation of potential coleoptile length. The observed moderate to high broad sense heritability and high observed genotypic variance, particularly for the quick growing cultivars, indicated that selecting for long coleoptile length in semidwarf wheat is feasible.
In the inheritance study, six semidwarf wheat cultivars, Warigal (Coleoptile length: Long, Rht allele: Rht1), QT 5633 (Long, Rht1), 11 IBWSN 45 (Short, Rht1), Spear (Long, Rht2), Dagger (Long, Rht2) and Hartog (Short, Rht2) were chosen. Four crosses, Warigal x 11 IBWSN 45 (Long x Short; Rht1 common), Warigal x QT 5633 (Long X Long; Rht1 common). Dagger x Hartog (Long x Short; Rht2 common) and Spear x Dagger (Long x Long; Rht2 common) were made in the 1997 growing season. The F1s and parents were grown in the 1998 growing season and F2 and BCIFI seeds were produced from the same Fl plant for each cross. The cultivars, QT 5633, 11 IBWSN 45, Dagger and Hartog were backcrossed to the respective Fl plants. Coleoptile lengths were measured using the optimal physical conditions identified in this study and Mendelian models for two non-allelic genes segregation were tested. Amongst these crosses, coleoptile length appeared to be controlled by two non-allelic genes, one dominant gene for "longness" in one locus, epistatic to the dominant gene for longness in another locus. In the case of the Rht1 semidwarfing allele, the observed F2 and BCIFI ratios for the Long x Long and Long x Short combination crosses supported an alternative gene segregation model: one dominant gene for longness in one locus, epistatic to a dominant gene for shortness. Modifiers in BCIFI populations and transgressive segregation over long coleoptiles in F2 populations were also observed.
To investigate the relationship between coleoptile length and seedling establishment in the field, twelve field trials were conducted under different planting situations at four sites in wheat growing regions of Queensland. Ten semidwarf wheat cultivars, varying in coleoptile length, and three sowuig depths, viz. 4-5, 7-8 and 9-11 cm were used. Seedling numbers per square meter (m2) and grain yield (t/ha) data were analysed.
The sowing depth effect was significant for seedling number in all trials and for grain yield in three of the 12 trials. Seedling number in all trials and grain yield in three of 12 frials was significantly reduced at the deepest sowing (9-11 cm). Sowing depths less than 8 cm did not influence grain yield. Coleoptile length had a marked influence on seedling establishment, particularly at deeper sowing depths.
Hence, long coleoptile is an important character for satisfactory wheat crop establishment and subsequent gram yield, particularly for rain-fed semi-arid environments if deep sowing is necessary to provide sufficient soil moisture to the germinating seeds. Breeding of semidwarf wheat cultivars (having either Rht1 or Rht2 semidwarfing allele) with long coleoptile is possible. Coleoptile length in wheat breeding populations can be improved by selecting from the present variability. Selection can commence in the F2 or F3 generations since a large proportion of the genetic control of this character appears to be simple. In routine wheat breeding programs, coleoptile length can be measured efficiently using the optimal physical conditions identified in this investigation.