Daniel J. Miralles

Floret development in near isogenic wheat lines differing in plant height

Abstract The effects of Rht1 and Rht2 alleles on the dynamics of floret development in isogenic lines (dwarf, DD; semi-dwarf, SD and standard height, SH) of spring wheat were investigated. Studies were conducted on wheat grown in the field in each of 4 years and where water and nutrients were non-limiting. The number of grains per spike was significantly greater in the lines with Rht alleles than in the SH lines. Grain number for each line was such that DD>SD>SH. Grains per spike varied with the number of grains per spikelet rather than number of spikelets per spike. Grains per spikelet in turn varied with the number of fertile florets at anthesis. Florets were considered fertile when male and female reproductive organs had developed green anthers and bifidum stigma, respectively. The dwarfing genes had no effect on the percentage of fertile florets setting grain. Increased number of fertile florets per spikelet due to the presence of Rht1 and Rht2 alleles in the genome was a consequence of the higher number of relatively distal primordia, to progress to the stage of fertile floret at anthesis in the DD and SD than in the SH lines. This ability to allow that a greater proportion of distal florets maintain a normal rate of development was related to the fact that Rht alleles produced a more favourable assimilate partitioning to the spike during the pre-anthesis period associated with the reduction in stem growth imposed by Rht alleles. This allowed a higher proportion of the later-initiated floret primordia to produce fertile florets at anthesis.

PAPER PRESENTED AT INTERNATIONAL WORKSHOP ON INCREASING WHEAT YIELD POTENTIAL, CIMMYT, OBREGON, MEXICO, 20–24 MARCH 2006 Sink limitations to yield in wheat: how could it be reduced?

Ponencia presentada al International Workshop on Increasing Wheat Yield Potential, CIMMYT, Obregon, Mexic, del 20 al 24 de marc de 2006.

Duration of the stem elongation period influences the number of fertile florets in wheat and barley.

Yield increases in temperate cereals have been associated with increases in grain number. An experiment was conducted to investigate whether altering the duration of the stem elongation period in wheat and barley increases floret fertility and thereby grain number. A photoperiod-responsive spring wheat (UQ189) and spring barley (Arapiles) were grown in a naturally lit phytotron. Plants were grown at constant (9, 13 and 19 h) and reciprocally interchanged photoperiod [at terminal spikelet (TS, wheat) or triple mound (TM, barley)] to alter the duration of the stem elongation period. An increased duration of the late reproductive phase from TS to heading in wheat resulted in more fertile florets per spike. A similar relationship was observed in barley but only for a limited range of duration of the stem elongation period. Shorter photoperiods reduced the rate of floret development and extended the time to reach the fertile floret stage. The fact that the duration of the late reproductive phase during which the spike and stem competing for assimilates was associated with the number of fertile florets per spike suggests that extending the stem elongation period in cereals could be a way to reduce assimilate competition and thereby increase the number of fertile florets and grain yield.

Individual grain weight responses to genetic reduction in culm length in wheat as affected by source-sink manipulations

Abstract This paper describes the effects of Rht alleles in isogenic lines of Maringa spring wheat (Triticum aestivum L.) on number of grains per spike, average grain weight and weight of individual grains from different positions within the spike. Plants were grown under five environmental conditions in the field (in the 1991, 1992 and 1993 growing seasons, with two sowing dates in the first two years), aiming to determine causes of lower average grain weight commonly observed in semi-dwarf (SD) compared with standard-height (SH) cultivars. The number of grains per spike was significantly greater in SD and dwarf (DD) lines than in the SH line due to differences among lines in number of grains per spikelet. Therefore, the relative contribution of proximal grains was affected negatively by Rht alleles while the contribution of distal grains was affected positively. Average grain weight was reduced with increases in the dose of Rht alleles, and this trait was negatively correlated to number of grains per spike. To understand the possible causes of this negative relationship, individual grains from specific positions within the spike were analysed. Basal grains in central spikelets were heavier than those in near apical and near basal spikelets. Within central spikelets, the ranking of individual grain weights were grains 2 > 1 > 3 > 4, numbered from the most proximal to the most distal positions. Proximal grains (1 and 2) were heaviest in SH, lightest in DD, and intermediate in the SD line. In general, the relative differences between the lines were smaller than that found for the average of all grains of the spike, in particular when grains of different positions within the central spikelets were considered. For all these grains, differences in weight due to Rht alleles were due to differences in rate of grain filling, with the effective duration of grain filling being almost unchanged. An increased source-sink relationship did not significantly modify the weight of any of the analysed grains in SD and SH lines (and just slightly increased rate of grain filling in the DD line). Therefore, the smaller grains in SD and DD lines could hardly have been due to an increased competition for assimilates. Alternatively, SD and DD lines had a greater proportion of grains from distal positions than the SH line, and these grains were always smaller than proximal grains. The reduction in average grain weight produced by Rht alleles therefore was due to a combination of effects on the potential size of each grain and on the contribution of grains from distal positions within the spike, with no effects attributed to an increased competition for assimilates.

Autophagy regulated by day length determines the number of fertile florets in wheat

SUMMARY The wheat spikelet meristem differentiates into up to 12 floret primordia, but many of them fail to reach the fertile floret stage at anthesis. We combined microarray, biochemical and anatomical studies to investigate floret development in wheat plants grown in the field under short or long days (short days extended with low-fluence light) after all the spikelets had already differentiated. Long days accelerated spike and floret development and greening, and the expression of genes involved in photosynthesis, photoprotection and carbohydrate metabolism. These changes started while the spike was in the light-depleted environment created by the surrounding leaf sheaths. Cell division ceased in the tissues of distal florets, which interrupted their normal developmental progression and initiated autophagy, thus decreasing the number of fertile florets at anthesis. A massive decrease in the expression of genes involved in cell proliferation, a decrease in soluble carbohydrate levels, and an increase in the expression of genes involved in programmed cell death accompanied anatomical signs of cell death, and these effects were stronger under long days. We propose a model in which developmentally generated sugar starvation triggers floret autophagy, and long days intensify these processes due to the increased carbohydrate consumption caused by the accelerated plant development.

Wheat floret survival as related to pre-anthesis spike growth

Further improvements to wheat yield potential will be essential to meet future food demand. As yield is related to the number of fertile florets and grains, an understanding of the basis of their generation is instrumental to raising yield. Based on (i) a strong positive association between the number of fertile florets or grains and spike dry weight at anthesis; and (ii) the finding that floret death occurs when spikes grow at maximum rate, it was always assumed that floret survival depends on the growth of the spike. However, this assumption was recently questioned, suggesting that assimilates diverted to the spike do not determine the number of florets and grains and that the onset of floret death may instead be a developmental process that is not associated with spike growth. In this study, the relationships between the fate of floret primordia and spike growth from six independent experiments that included different growing conditions (greenhouse/field experiments, growing seasons, photoperiod/shading treatments during the floret primordia phase) and diverse cultivar types (winter/spring, semi-dwarf/standard-height, photoperiod sensitive/insensitive) were re-analysed together. Onset of floret death was associated with the beginning of spike growth at the maximum rate in c. 80% of the cases analysed; and the rate of floret death (the main determinant of floret survival) showed a negative quantitative relationship with spike weight at anthesis. As floret death and survival were shown to be linked to pre-anthesis spike growth, the strategy of focusing on traits associated with pre-anthesis spike growth when breeding to increase wheat yield potential further is valuable.

Photoperiod Sensitivity during Stem Elongation as an Avenue to Raise Potential Yield in Wheat

Worldwide wheat yields have been only slightly, and non-significantly, increasing during the 90’s, suggesting that they may be levelling off. Considering that there is consensus that large new growing areas will be not introduced and management improvements will be increasingly harder to obtain, genetic improvement would play a more important role to keep rising wheat yields in the future than in the past. In this scenario, the use of physiological bases as a complementary tool to identifying alternative ways for breeding seems to be crucial for breaking the apparent barriers in wheat yields. In this presentation, we attempted to envisage, from published and recent unpublished evidences from our lab, using studies carried out under both controlled and field conditions, some opportunities to manipulate the rate of crop development during the late reproductive phase. This phase has been recognised as critical in terms of yield generation, and the idea of manipulating its response to photoperiod as a tool for increasing yield potential in wheat is reviewed.

Radiation interception and radiation use efficiency of near-isogenic wheat lines with different height

Three near isogenic lines of spring wheat grown to determine whether Rht dwarfing alleles alter radiation interception. A field study (involving two sowing dates in two growing seasons) with lines containing different allelic dosage of Rht1 and Rht2 (i.e. dwarf, DD; semi-dwarf, SD and tall SH), was conducted without water and nutritional deficiencies. Dwarfing genes did not modify the timing of occurrence of phenological events. Above-ground biomass at anthesis was reduced by 22% in the DD line in relation of the rest of the lines. However, at maturity accumulated biomass of the DD and SH lines were not significant different. Dwarfing genes increased the light attenuation coefficient (k, with values of 0.48, 0.62 and 0.78 for the SH, SD and the DD line respectively). A similar trend was followed to the leaf thickness (estimated by the specific leaf weight, SLW). Despite the differences observed among the lines, both in k and SLW values, they did not differ significantly in the proportion of incoming radiation intercepted by the canopy, nor in the cumulative intercepted radiation during the pre and post-anthesis periods. Radiation use efficiency (RUE) differed significantly among the lines. While RUE during pre-anthesis was the lowest in the DD line, from anthesis to maturity the lines with Rht alleles showed higher RUE values than the SH line. The low pre-anthesis RUE in the DD lines could be associated with (i) poor canopy architecture due to reductions in leaf sheath and internode lengths and/or (ii) reduced canopy CO2 exchange rate. Post-anthesis RUE was lower than that recorded pre-anthesis in all lines. But the magnitude of the reduction was inversely related to the doses of the Rht alleles. Post-anthesis RUE appeared to be closely and positively associated with the number of grains set per unit biomass at anthesis. This relationship suggests a regulatory effect of the sink size on the efficiency of the crop to convert radiation into biomass during this period.

Pre-anthesis development and number of fertile florets in wheat as affected by photoperiod sensitivity genes Ppd-D1 and Ppd-B1

Lengthening the late reproductive phase (LRP) of stem elongation in wheat (Triticum aestivumL.), by changing its photoperiod sensitivity independently of the preceding phases, would improve the yield potential through increasing spike weight and the number of fertile florets at anthesis. This paper presents results of a two-year field experiment designed to determine the impact of Ppd-D1and Ppd-B1on (i) the duration of three pre-anthesis developmental phases, and (ii) spike weight and the number of fertile florets at anthesis under two photoperiods during the LRP (natural and an extension of six hours over that). Near isogenic lines of Mercia and single chromosome recombinant lines of Cappelle Desprez were used. Under natural photoperiod, Ppd-D1hastened time to anthesis ca. 500∘C d in both backgrounds by reducing each of the three pre-anthesis phases. Ppd-B1hastened the time to anthesis under natural photoperiod by 178∘C d, mainly by reducing the early reproductive phase. The response to photoperiod of the LRP under extended daylength depended on the Ppdlocus present: Ppd-D1was insensitive while Ppd-B1and the recessive controls were sensitive. For all lines, photoperiod treatments and years, the number of fertile florets was associated with spike dry weight at anthesis (R2≅ 80%, p< 0.01) which, in turn, was positively related to the intercepted radiation accumulated during the LRP (R2 45%, p< 0.05). Changing the duration of the LRP through extended photoperiod or through Ppd-D1produced similar results in both backgrounds and years. Thus, altering the duration of the LRP by manipulating photoperiod sensitivity may be an alternative to changing the fertile floret number in wheat. Nevertheless, as no particular allele was responsible for the photoperiod sensitivity only during the LRP, new alleles should be studied to identify the control of photoperiod sensitivity of individual phases to fine-tune the pre-anthesis wheat development.

Vernalization and photoperiod responses in wheat pre-flowering reproductive phases

Abstract It has been established that photoperiod influences the rate of wheat development well beyond the end of the vegetative phase. Conversely, vernalization effects are still assumed to be mainly during the vegetative phase. The aim of the study was to assess the effects of different combinations of vernalization and photoperiods on the developmental characteristics of wheat pre-flowering reproductive phases under field conditions. For this purpose, three high yielding cultivars (ProINTA Puntal, ProINTA Super and Klein Pegaso) were subjected to two vernalization treatments (unvernalized (V0) and vernalized during 56 days in a cool chamber (V56)) and four photoperiod regimes. The photoperiod treatments consisted of the natural photoperiod of the season (NP+0) and daylength extensions over that of 2 (NP+2), 4 (NP+4), and 6 (NP+6) hours. Photoperiod influenced the duration of early (collar—first node detectable) and late (first node detectable—heading) pre-flowering reproductive phases, reducing their durations as photoperiod increased. When cultivars with strong vernalization response (ProINTA Puntal and ProINTA Super) were not vernalized, duration of the early and late pre-flowering reproductive phases increased and the rate of spikelet initiation decreased. Thus, when vernalization requirements were not satisfied, the spikelet initiation rate was even slower than the leaf initiation rate (both expressed in a thermal time base). Results showed significant interactions between photoperiod and vernalization (vegetative phase), or clear, though non-significant, trends to interactive effects (late pre-flowering reproductive phase). Thus, the length of vegetative and late pre-flowering reproductive phases changed in response to photoperiod depending on the level of satisfaction of the vernalization requirements. The relationship between the cumulative number of leaves on the main stem and thermal time fitted a bi-linear model whenever the final leaf number (LNf) was large, e.g. when photoperiod was not extended or vernalization requirements not satisfied. Thus, responses during the pre-flowering reproductive phases were accompanied by an increased phyllochron for leaves emerging after the first 7–8 leaves. This study demonstrated with field grown plants that the length of the late pre-flowering reproductive phase in wheat is sensitive to photoperiod and that the response to this environmental factor could be changed by the level of satisfaction of the vernalization requirements.