Water relationships of castor bean (Ricinus communis L.) seeds related to final seed dry weight and physiological maturity

Castor bean (Ricinus communis L.) seeds develop inside capsules from sequential racemes. The indeterminate growth habit causes seeds to have different developmental stages among different orders of racemes. Determining the moment of physiological maturity (PM) for the whole plant and at the crop level presents a challenge. Also, it is unclear if a relationship exists between the final seed size of different orders of racemes and the seed-filling rate, seed-filling duration, or maximum seed water content (MWCmg, absolute amount of water per seed). The aim of the present work was to study the dynamics of seed dry weight (SDW) and seed water content (absolute WCmg) with the objectives of (i) associating final SDW to its seed-filling rate, seed-filling duration and maximum WCmg; (ii) identifying the timing of PM on the basis of seed water concentration (WC%); and (iii) predicting PM at the crop level using simulation models based on plant WC% and SDW relationships. Two genotypes (FAUBA 1 and AL GUARANI 2002), contrasting in seed size and branching degree, were studied under field conditions in Buenos Aires in 2006–2007. The evolution of SDW and WCmg was followed in racemes of first (R1), second (R2) and third (R3) order, from the beginning of flowering to harvest maturity. Bi-linear functions were fitted to SDW/WC% relationships, and the model was validated. Across both genotypes and all orders of racemes, final SDW (FSDW) was associated with seed-filling duration (expressed in both days and thermal time) to a greater extent than with seed-filling rate. Also, a clear positive association between FSDW and MWCmg was observed for a broad range of seed sizes (250–600 mg seed−1). Physiological maturity was attained when seeds exhibited a WC% of 21.8 ± 2.4% (n = 232; R2 = 0.94; P < 0.001). The model was successfully validated with independent data (R2 = 0.96). Simulations were performed to explore the effects of variations in the timing of flowering between racemes at the plant and crop level on fruit WC% at maximum yield. At the crop level, 22% WC% corresponded to 90–92% of the maximum attainable seed yields when harvest was delayed until the last growing seeds reached PM. We conclude that a relationship exists between FSDW and MWCmg, and ca. 22% of seed WC% is a good predictor of PM in castor bean at seed and crop level.

► Seed water relations were studied in two genotypes of castor bean.
► Three orders of raceme were studied.
► Final seed dry weight was associated with filling duration and maximum water content.
► Physiological maturity (PM) was attained with a seed water concentration of 22%.
► This indicator of PM was consistent at crop level when upscaling was done.