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, 99 (5), 925-33

Development and Seed Number in Indeterminate Soybean as Affected by Timing and Duration of Exposure to Long Photoperiods After Flowering

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Development and Seed Number in Indeterminate Soybean as Affected by Timing and Duration of Exposure to Long Photoperiods After Flowering

Adriana G Kantolic et al. Ann Bot.

Abstract

Background and aims: Long photoperiods from flowering to maturity have been found to delay reproductive development in soybean (Glycine max) and to increase the number of seeds per unit land area. This study was aimed to evaluate whether sensitivity to photoperiod after flowering (a) is quantitatively related to the length of exposure to long days and (b) persists throughout the whole pod-setting period. It was also evaluated whether seed number was related to changes in the duration of post-flowering phenophases.

Methods: Two field experiments were conducted with an indeterminate cultivar of soybean of maturity group V. In expt 1, photoperiods 2 h longer than natural daylength were applied during different numbers of days from the beginning pod stage (R3) onwards, while in expt 2 these photoperiod extensions were imposed during 9 consecutive days starting at different times between R3 and R6 (full seed) stages.

Key results: There was a quantitative response of development to the number of cycles with a long photoperiod. The exposure to long photoperiods from R3 to R5 (beginning of seed growth) increased the duration of R3-R6 regardless of the timing of exposure. The stages of development comprised in the R3-R6 phase were delayed by current as well as by previous exposure to long days. A positive relationship was found between seed number and the duration of R3-R6, irrespective of the timing and length of exposure to the long photoperiod.

Conclusions: Sensitivity to photoperiod remained high during the reproductive period and was highly and positively coupled with the processes of generation of yield.

Figures

F<sc>ig</sc>. 1.
Fig. 1.
Schematic representation of treatments in expts 1 (top panel) and 2 (bottom panel). Each horizontal bar represents the reproductive growth cycle (R1–R8; Fehr and Caviness, 1977). Black sectors represent the period during which the photoperiod was extended by 2 h in relation to natural daylength. A time scale for the occurrence of reproductive stages is shown for the control (T0).
F<sc>ig</sc>. 2.
Fig. 2.
Duration of the R3–R6 phase in soybean plants grown under natural photoperiod (treatment T0, open circle) or subjected to exposures to long days after the R3 stage for increasing durations (treatments T1–T9, closed circles) including a treatment exposed to long days from R3 until maturity (T10, arrow on the ordinate). The s.e.m. is shown as a vertical line when larger than the symbols.
F<sc>ig</sc>. 3.
Fig. 3.
Duration of the phases R3–R4 (A), R4–R5 (B), R5–R6 (C) and R6 to maturity (D) in soybean plants grown under natural photoperiods (treatment T0, open circle) or subjected to exposures to long days after the R3 stage for increasing durations (treatments T1–T9, closed circles) including the treatment exposed to long days from R3 until maturity (T10, arrow head on the ordinate). The s.e.m. is shown as a vertical line when larger than symbols.
F<sc>ig</sc>. 4.
Fig. 4.
Relationship between seed number per unit land area and average duration of the R3–R6 phase, in soybean plants from expt 1 (circles) and expt 2 (triangles). Bilinear adjustment (r2 = 0·82, P < 0·01) excludes TD from expt 2 (open triangle). Inset is the relationship between yield and seed number per unit land area (r2 = 0·93, P < 0·001).
F<sc>ig</sc>. 5.
Fig. 5.
Relationship between the corrected duration of R3–R6 and the mean photoperiod of the phase for soybean plants from expt 1 (closed circles) and expt 2 (closed triangles). Data from early sowing dates from previous experiments of Kantolic and Slafer (2001) (open squares) and Kantolic and Slafer (2005) (open circles) are also included. Mean data from TD from expt 2 are shown as an open triangle.

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