Is the remobilization of S and N reserves for seed filling of winter oilseed rape modulated by sulphate restrictions occurring at different growth stages?

Abstract

How the remobilization of S and N reserves can meet the needs of seeds of oilseed rape subject to limitation of S fertilization remains largely unclear. Thus, this survey aims to determine the incidence of sulphate restriction [low S (LS)] applied at bolting [growth stage (GS) 32], visible bud (GS 53), and start of pod filling (GS 70) on source-sink relationships for S and N, and on the dynamics of endogenous/exogenous S and N contributing to seed yield and quality. Sulphate restrictions applied at GS 32, GS 53, and GS 70 were annotated LS(32), LS(53), and LS(70). Long-term (34)SO(4)(2-) and (15)NO(3)(-) labelling was used to explore S and N partitioning at the whole-plant level. In LS(53), the sulphur remobilization efficiency (SRE) to seeds increased, but not enough to maintain seed quality. In LS(32), an early S remobilization from leaves provided S for root, stem, and pod growth, but the subsequent demand for seed development was not met adequately and the N utilization efficiency (NUtE) was reduced when compared with high S (HS). The highest SRE (65 ± 1.2% of the remobilized S) associated with an efficient foliar S mobilization (with minimal residual S concentrations of 0.1-0.2% dry matter) was observed under LS(70) treatment, which did not affect yield components.

Fig. 1.

Schematic diagram of the experimental design. Mineral S restriction [low S (LS)] was applied at GS 32 (bolting stage) for LS₃₂, GS 53 (visible bud stage) for LS₅₃, or GS 70 (start of pod filling) for LS₇₀, until the end of the growth cycle (GS 99). During different periods (from GS 16 to GS 32, GS 16 to GS 53, or GS 16 to GS 70), the plants were supplied with ³⁴SO₄²⁻ (1 atom% excess) and ¹⁵NO₃⁻ (2 atom% excess) in order to obtain plants with homogeneous ³⁴S and ¹⁵N labelling before applying treatments. Plants were harvested at GS 32, GS 53, GS 70, GS 81, and GS 99.

Fig. 2.

³⁴S partitioning (expressed as the percentage of total ³⁴S labelling) in the different tissues of the plants from GS 32 to GS 81 in HS₃₂ and LS₃₂ (A), HS₅₃ and LS₅₃ (B), and HS₇₀ and LS₇₀ conditions (C). Vertical bars indicate ±SE of the mean (n=4). The foliar S remobilization efficiency (SRE_leaf, corresponding to the foliar loss of ³⁴S between two growth stages, as a percentage of total ³⁴S labelling) is indicated for each chase period, and asterisks indicate that mean values in LS conditions are significantly different from control (P <0.05).

Fig. 3.

¹⁵N partitioning (expressed as the percentage of total ¹⁵N labelling) in the different tissues of the plants from GS 32 to GS 81 in HS₃₂ and LS₃₂ (A), HS₅₃ and LS₅₃ (B), and HS₇₀ and LS₇₀ conditions (C). Vertical bars indicate ±SE of the mean (n=4). The foliar N remobilization efficiency (NRE_leaf, corresponding to the foliar loss of ¹⁵N between two growth stages, as a percentage of total ¹⁵N labelling) is indicated for each chase period, and asterisks indicate that mean values are significantly different from control (P <0.05).

Fig. 4.

Flows as a percentage of remobilized S (determined on the basis of ³⁴S enrichment; see Materials and methods for details) and S taken up (estimated from the unlabelled S), and the S amount present at GS 70 and GS 81 in seeds (in mg in seeds), pod walls, floral stems, stems, leaves, and roots of oilseed rape for control plants (HS₇₀; A) and S-deficient plants (LS₇₀; B) during the reproductive stage. Values are given as the mean ±SE (n=16 for the flows, n=4 for the S amounts in tissues). In S-deficient plants (B), the uptake of sulphate was nil during the duration of the experiment. The thickness of the arrows represents the relative importance of each flow to or from a tissue related to the S taken up or S remobilized, and asterisks indicate that mean values are significantly different from control (P <0.05).

Fig. 5.

Flows as a percentage of remobilized N (determined on the basis of ¹⁵N enrichment; see Materials and methods for details) and N taken up (estimated from the unlabelled N), and the N amount present at GS 70 and GS 81 in seeds (in mg in seeds), pod walls, floral stems, stems, leaves, and roots of oilseed rape for control plants (HS₇₀; A) and S-deficient plants (LS₇₀; B) during the reproductive stage. Values are given as the mean ±SE (n = 16 for the flows, n=4 for the S amounts in tissues). The thickness of the arrows represents the relative importance of each flow to or from a tissue related to the N taken up or N remobilized, and asterisks indicate that mean values are significantly different from control (P <0.05).

Fig. 6.

Residual S (A) and N (B) concentrations of dead leaves in oilseed rape grown under a high level of sulphate (HS) or under a low level of sulphate (LS₃₂, LS₅₃, and LS₇₀). Horizontal bars indicate ±SEM (n=8) when larger than the symbol. Asterisks indicate that mean values are significantly different from control (P <0.05). The intersection points between S and N concentrations were illustrated for each treatment by dotted lines corresponding to the transition between N/S <1 and N/S>1.