Many rainfed wheat production systems are reliant on stored soil water for some or all of their water inputs. Selection and breeding for root traits could result in a yield benefit; however, breeding for root traits has traditionally been avoided due to the difficulty of phenotyping mature root systems, limited understanding of root system development and function, and the strong influence of environmental conditions on the phenotype of the mature root system. This paper outlines an international field selection program for beneficial root traits at maturity using soil coring in India and Australia. In the rainfed areas of India, wheat is sown at the end of the monsoon into hot soils with a quickly receding soil water profile; in season water inputs are minimal. We hypothesised that wheat selected and bred for high yield under these conditions would have deep, vigorous root systems, allowing them to access and utilise the stored soil water at depth around anthesis and grain-filling when surface layers were dry. The Indian trials resulted in 49 lines being sent to Australia for phenotyping. These lines were ranked against 41 high yielding Australian lines. Variation was observed for deep root traits e.g. in eastern Australia in 2012, maximum depth ranged from 118.8 to 146.3 cm. There was significant variation for root traits between sites and years, however, several Indian genotypes were identified that consistently ranked highly across sites and years for deep rooting traits.
Selection for rapid leaf area growth has the potential to increase wheat biomass, and both water-use efficiency and weed competitiveness early in the season. Several morphological components contribute to increased seedling leaf area, including rapid seedling emergence and production of longer, wider leaves. Early emergence of a large coleoptile tiller has also been demonstrated to increase plant leaf area and biomass in wheat and other grass seedlings. Yet little is known of the extent and nature of genotypic variation for coleoptile tiller growth in wheat. A random set of 35 wheat, barley, and triticale genotypes was evaluated in glasshouse and outdoor studies for seedling characteristics, including coleoptile tiller growth and total plant leaf area. Coleoptile tillers were produced more reliably for seedlings grown outdoors and when supplied with additional soil nitrogen. Genotypic differences in coleoptile tiller frequency and leaf area were large, ranging from 0 to 78% and from 0.0 to 1.4 cm2, respectively at very early growth stages. Australian commercial wheats tended to produce fewer coleoptile tillers of smaller size than overseas germplasm where the coleoptile tiller accounted for up to 12% of total seedling leaf area. This compared favourably with mainstem tiller leaf area, which ranged from 0 to 3.5 cm2 and accounted for up to 16% of plant leaf area. Broad-sense heritabilities were high for coleoptile tiller presence and size in favourable conditions (c. 75%) but low (c. 40%) for seedlings evaluated across nitrogen content-varying soils. Generation means analysis was used to investigate genetic control for coleoptile tiller growth across multiple populations. Significant (P < 0.05) differences were observed among generations for coleoptile tiller frequency and growth (numbers of leaves, leaf area, and biomass). These differences reflected strong additive genetic control with little evidence for any gene action × year interaction. Increases in coleoptile tiller frequency and mass were correlated with larger embryo size and wider seedling leaves to increase seedling leaf area (rg = 0.89). Comparisons between reciprocal F1 and F2 generation means indicated strong maternal effects for coleoptile tiller growth in some but not all crosses. Screening in favourable environments will increase heritability and aid in selection for progenies producing large coleoptile tillers. Evidence for additive genetic control should permit early generation selection but not without some progeny-testing for coleoptile tiller growth together with other early vigour components associated with increased plant leaf area.
• Background and Aims The gibberellin-insensitive Rht-B1b and Rht-D1b dwarfing genes are known to reduce the size of cells in culms, leaves and coleoptiles of wheat. Resulting leaf area development of gibberellin-insensitive wheats is poor compared to standard height (Rht-B1a and Rht-D1a) genotypes. Alternative dwarfing genes to Rht-B1b and Rht-D1b are available that reduce plant height, such as the gibberellin-responsive Rht8 gene. This study aims to investigate if Rht8 has a similar dwarfing effect on the size of leaf cells to reduce leaf area. • Methods The effect of Rht8 on cell size and leaf area was assessed in four types of epidermal cells (interstomatal, long, sister and bulliform) measured on leaf 2 of standard height (rht8) and semi-dwarf (Rht8) doubled-haploid lines (DHLs). The DHLs were derived from a cross between very vigorous, standard height (rht8) (‘Vigour18’) and less vigorous, semi-dwarf (Rht8) (‘Chuan-Mai 18’) parents. • Key Results Large differences were observed in seedling vigour between the parents, where ‘Vigour18’ had a much greater plant leaf area than ‘Chuan-Mai 18’. Accordingly, ‘Vigour18’ had on average longer, wider and more epidermal cells and cell files than ‘Chuan-Mai 18’. Although there was correspondingly large genotypic variation among DHLs for these traits, the contrast between semi-dwarf Rht8 and tall rht8 DHLs revealed no difference in the size of leaf 2 or average cell characteristics. Hence, these traits were independent of plant height and therefore Rht8 in the DHLs. Correlations for leaf and average cell size across DHLs revealed a strong and positive relationship between leaf width and cell files, while the relationships between leaf and cell width, and leaf and cell length were not statistically different. The relative contribution of the four cell types (long, sister, interstomatal and bulliform) to leaf size in the parents, comparative controls and DHLs is discussed. • Conclusions Despite a large range in early vigour among the DHLs, none of the DHLs attained the leaf area or epidermal cell size and numbers of the vigorous rht8 parent. Nonetheless, the potential exists to increase the early vigour of semi-dwarf wheats by using GA-sensitive dwarfing genes such as Rht8.
Gibberellin (GA)-insensitive dwarfing genes Rht-B1b and Rht-D1b that are responsible for the 'Green Revolution' have been remarkably successful in wheat improvement globally. However, these alleles result in shorter coleoptiles and reduced vigour, and hence poor establishment and growth in some environments. Rht18, on the other hand, is a GA-sensitive, dominant gene with potential to overcome some of the early growth limitations associated with Rht-B1b and Rht-D1b. We assessed progeny from both a biparental and a backcross population that contained tall, single dwarf, and double dwarf lines, to determine whether Rht18 differs from Rht-D1b and hence verify its value in wheat improvement. Progeny with Rht18 had an almost identical height to lines with Rht-D1b, and both were ~26% shorter than the tall lines, with the double dwarf 13% shorter again. However, coleoptile length of Rht18 was 42% longer than that of Rht-D1b. We detected no differences in time to terminal spikelet and anthesis, and few differences in stem or spike growth. Both dwarfing genes diverted more dry matter to the spike than tall lines from prior to heading. No differences were detected between Rht18 and Rht-D1b that could prevent the adoption of Rht18 in wheat breeding to overcome some of the limitations associated with the 'Green Revolution' genes.
The present study was designed to analyse the effect of the length of exposure to a long photoperiod imposed c . 3 weeks after sowing in spring wheat ( cv . UQ189) and barley ( cv . Arapiles) to (i) establish whether the response to the number of cycles of exposure is quantitative or qualitative, (ii) determine the existence of a commitment to particular stages well before the stage has been observable, and (iii) study the interrelationships between the effects on final leaf number and phyllochron when the stimulus is provided several days after seedling emergence. Both wheat and barley seemed to respond quantitatively to the number of long-day cycles they were exposed to. However, wheat showed a requirement of approximately 4 long-day cycles to be able to produce a significant response in time to heading. The barley cultivar used in the study was responsive to the minimum length of exposure. The response to extended photoperiod cycles during the stem elongation phase was due to the ‘memory’ photoperiod effects being related, in the case of wheat, to the fact that the pre-terminal spikelet appearance phase saturated its photoperiod response well before that stage was reached. Therefore, the commitment to the terminal spikelet appearance in wheat may be reached well before this stage could be recognized. As the response in duration to heading exceeded that of the final leaf number, and the stem elongation phase responded to memory effects of photoperiod, the phyllochron of both cereals was responsive to the treatments accelerating the average phyllochron when exposed to longer periods of long days. The response in average phyllochron was due to a switch from bi-linear to linear models of leaf number v . time when the conditions were increasingly inductive, with the phyllochron of the initial (6–8) leaves being similar for all treatments (within each species), and from then on increased.
Selection for altered stomatal conductance has potential to improve wheat grain yields in dry and well- watered environments. Yet the slow speed with which conductance is typically measured has limited studies reporting genetic parameters for leaf conductance. A viscous air-flow porometer that measures resistance to mass flow through a leaf was used to provide rapid estimates of leaf conductance. These estimates were obtained prior to anthesis on irrigated plants representing different generations of crosses between the low conductance parent, Quarrion, and 3 high conductance varieties, Hartog, Genaro 81, and Matong. Sampling for leaf conductance was done between 08 00 and 12 00 hours under cloud-free conditions. Significant (P < 0.01) genetic differences were observed between generation means for conductance measured in different crosses and on different days. Gene action was complex with both additive and non-additive (dominance and additive-based epistasis) genetic effects important for expression of leaf conductance. There was a greater reduction in leaf conductance for Quarrion and backcross-Quarrion progeny with sampling later into the day. In turn, genetic variances for leaf conductance increased with later sampling. Family-mean heritabilities varied in size (0.06–0.70), depending on cross and time of sampling. It is suggested that breeders selecting for altered leaf conductance maximise genetic gain by delaying screening of populations until later in the day, and repeat measurements across a minimum of 2 days. Large populations of inbred families should be evaluated in order to minimise confounding through dominance and increase the probability of recovering families containing desirable non-allelic gene combinations.
A recessive gene (tin) that inhibits tillering in wheat (Triticum aestivum L.), and that may be important in the redirection of assimilate from unproductive to productive tillers, has been reported. However, this gene has also been associated with a fatal condition known as 'stunting'. The severity of this phenomenon has been shown to increase when plants are grown under long photoperiods and at low temperatures. The objectives of this study were to observe how the expression of the tin gene varied in different genetic backgrounds, in addition to obtaining a better understanding of environmental factors that may affect both tillering and stunting in lines with the tin gene. Plants were grown outdoors in Canberra, Australia, at various times throughout the year, as well as under controlled conditions where photoperiod, temperature and light intensity were varied. The inhibition of tillers resulting from the presence of the tin gene was most extreme in summer, autumn and spring (up to 90% reduction in tillering). However, when sown in late autumn and winter, tillering was reduced by between 30-50% for the tin lines compared with their near-isogenic parents. Reduced tillering in the tin lines was due to an earlier cessation of tillering rather than a reduced rate. Stunting was frequently observed in some lines more than others when plants were grown under long days and when temperatures were low. The daily minimum temperature, rather than the average daily temperature, was associated with stunting. The duration of the dark period also influenced stunting, with a longer dark period reducing the incidence of stunting from almost 100% to 0%. In all experiments where irradiance was increased, stunting also increased. In addition, elevated CO2 also increased growth characteristics associated with stunting. It is concluded that stunting is associated with a high assimilate supply to the main stem shoot apex before the time of floral initiation. This is caused by an inhibition of tillering and a high photothermal quotient. Leaf length was found to be a good indicator of stunting severity, with stunted plants producing shorter leaves than those plants which failed to stunt. Measurements of leaf length indicated that stunting is induced when the second leaf is expanding.
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