Photoperiod-sensitivity genes shape floret development in wheat

Abstract

Lengthening the pre-anthesis period of stem elongation (or late-reproductive phase, LRP) through altering photoperiod sensitivity has been suggested as a potential means to increase the number of fertile florets at anthesis (NFF) in wheat. However, little is known about the effects that the Ppd-1 genes modulating plant response to photoperiod may have on reproductive development. Here, five genotypes with either sensitive (b) or insensitive (a) alleles were grown in chambers under contrasting photoperiods (12 h or 16 h) to assess their effects. The genotypes consisted of the control cultivar Paragon (three Ppd-1b) and four near-isogenic lines of Paragon with Ppd-1a alleles introgressed from: Chinese Spring (Ppd-B1a), GS-100 (Ppd-A1a), Sonora 64 (Ppd-D1a), and Triple Insensitive (three Ppd-1a). Under a 12-h photoperiod, NFF in the genotypes followed the order three Ppd-1b > Ppd-B1a > Ppd-A1a > Ppd-D1a > three Ppd-1a. Under a 16-h photoperiod the differences were milder, but three Ppd-1b still had a greater NFF than the rest. As Ppd-1a alleles shortened the LRP, spikes were lighter and the NFF decreased. The results demonstrated for the first time that Ppd-1a decreases the maximum number of florets initiated through shortening the floret initiation phase, and this partially explained the variations in NFF. The most important impact of Ppd-1a alleles, however, was related to a reduction in survival of floret primordia, which resulted in the lower NFF. These findings reinforce the idea that an increased duration of the LRP, achieved through photoperiod sensitivity, would be useful for increasing wheat yield potential.

Keywords: Ppd-1 genes; Floret development; floret initiation; floret survival; photoperiod sensitivity; wheat.

Fig. 1.

Box-and-whiskers plot for number of fertile florets per spike produced by each genotype under either a long (16-h) or short (12-h) photoperiod. The boxes consist of the 25th to 75th percentiles, with the mean indicated by ‘+’ and the median indicated by the vertical line. The whiskers extend from the minimum to the maximum.

Fig. 2.

Fertility profiles of most and least fertile spikes (‘+ fertile’ and ‘- fertile’, respectively) for the Paragon and Triple Insensitive genotypes under long (16-h) and short (12-h) photoperiods. The graphs show the number of fertile florets at anthesis for a given spikelet position for one half of the spike, with zero on the y-axis representing the central spikelet of the spike.

Fig. 3.

Relationship between the number of fertile florets per spike and the spike dry weight at anthesis for the different genotypes. Open and closed symbols refer to long (16-h) and short (12-h) photoperiods, respectively. The horizontal bar indicates the minimum significant difference for Tukey’s test (α=0.05). (This figure is available in colour at JXB online.)

Fig. 4.

Number of living florets in central spikelets for the different genotypes grown under a short (12-h) photoperiod in relation to thermal time from terminal spikelet. Data are means (±SE) of four replicates. The last data point for each genotype represents the final number of fertile florets in the spikelet. (This figure is available in colour at JXB online.)

Fig. 5.

Number of fertile florets in central spikelets for the different genotypes as related to (a) floret initiation, and (b) floret survival. Data are means (±SE) of two replicates. (This figure is available in colour at JXB online.)

Fig. 6.

Floret development stages in central spikelets of the Triple Insensitive near-isogenic line (NIL, left) and photoperiod-sensitive Paragon (right) grown under a short (12-h) photoperiod. The vertical scale indicates the thermal time (°Cd) from terminal spikelet to anthesis when each of the images was taken (note the differences in the duration). At each time-point, the florets are arranged in a ‘virtual spikelet’, with the centre of the figure (the thermal-time axis) representing the position of the rachis. The first floret (F1, the most proximal to the rachis) is placed closest to the axis and the fifth floret (F5, when observable) is placed furthest from the axis. Scale bars are shown for each group of florets at a given time-point. (This figure is available in colour at JXB online.)

Fig. 7.

Development of florets (Waddington score) with thermal time for the different genotypes during the late-reproductive phase for the third (F3), fourth (F4), and fifth (F5) florets counted from the rachis of the central spikelets of each genotype grown under a short photoperiod (12 h). Individual florets are plotted from the moment at which they became distinguishable through to anthesis. The dotted line indicates the stage of development at which we considered a floret to be fertile at anthesis. For each curve, the last data point represents the most advanced stage of development that particular floret primordium reached. Data are means (±SE) of four replicates. (This figure is available in colour at JXB online.)

Fig. 8.

Maximum floret development (Waddington score) achieved by the first (F1) through to the fifth (F5) floret for each genotype grown under a short photoperiod (12 h) as related to (a) LRP duration, and (b) the floret position relative to the rachis. Data are means (±SE) of two replicates.