Plasticity of seed weight in winter and spring rapeseed is higher in a narrow but different window after flowering

Abstract

Breeding heavier seeds while maintaining seed number has been proposed as an effective strategy to improve oil crop production. Recent studies assessing rapeseed source-sink (S-Sratio) reduction after flowering suggest a window in the early seed-filling period that can compensate, either partially or totally, for a decrease in the number of seeds. However, little evidence has been reported in relation to the key window for seed plasticity in rapeseed. Furthermore, such studies have only assessed spring rapeseed genotypes and there is a lack of information on winter rapeseed. This study aims to elucidate the occurrence of a narrow window when seed plasticity is greatest, which is hypothesized to occur during the early stages of seed filling after the beginning of flowering in both winter and spring rapeseed genotypes. To asses this hypothesis, two winter and two spring rapeseed hybrids were evaluated under three S-Sratio treatments in two experiments performed in field conditions in Valdivia, Chile. The winter hybrids Trust and Mercedes were assessed under different nitrogen levels, and the spring hybrids Lumen and Solar were assessed using two sowing dates. S-Sratio treatments in both rapeseed genotypes consisted of a control without S-Sratio manipulation, a reduced S-Sratio from the beginning of flowering [BBCH 61] to 15 days after flowering (DAF) and a reduced S-Sratio from 15 to 30 DAF. The S-Sratio was reduced by shading the crop with black nets to intercept 75 % of the incoming solar radiation. Shading treatments decreased seed yield relative to the control between 13 and 42 % for the winter genotypes and from 23 to 44 % for the spring genotypes. The negative effect of shading on seed number under both S-S reduction timings explains the seed yield reduction. Interestingly, different windows of time for seed weight plasticity were found in response to the lower S-Sratio for winter and spring genotypes (i.e., 15–30 and 0–15 DAF, respectively). In the winter-type genotypes, seed weight increased between 28 and 33 % under the 15–30 DAF shading treatment, while seed weight in spring-type genotypes increased from 15 to 39 % during the 0–15 DAF treatment. Seed weight change was almost negligible outside of the sensitivity window as this trait increased only 2% at 0–15 DAF in winter-type genotypes and 7% at 15–30 DAF in spring-type genotypes, across treatments. In parallel, the S-S reduction decreased seed number by 23.6 and 40 %, at the 0–15 and 15–30 DAF treatment, respectively. The study showed statistical differences for seed oil and protein concentration across treatments, but both quality traits remained highly stable to the S-S reduction and showed similar behavior in winter and spring rapeseed types. Seed oil concentration changed little while seed protein concentration was more sensitive to seed weight response. The findings of the present study suggest that seeds per silique and seed weight are determined by the plant assimilate S-Sratio; therefore, seed number and seed weight determinations overlap as has been demonstrated in other crops like wheat, barley, sorghum and sunflower.