Radiation use efficiency and shoot:root dry matter partitioning in seedling growths and regrowth crops of lucerne (Medicago sativa L.) after spring and autumn sowings

Storage of carbohydrates in roots is important for lucerne survival and regrowth rate, begins during initial growth. This study examined whether variation in radiation use efficiency (RUE) and shoot:root partitioning (Pshoot) occurring for various types of lucerne canopy, including seedling crops and regrowths in various seasons, was significant or not. Seedling and regrowth crops were monitored for four sowing dates in fully irrigated experiments on chalky soils in north-eastern France for three years. Pshoot was calculated as the ratio between increments in aerial dry matter (ADM) and total dry matter (TDM) for successive crop measurements. Leaf Area Index (LAI) was regressed against cumulative thermal time (CTh) using a 5 °C base temperature, which provided the best fit (R2 = 0.98). The increase in LAI was exponential up to 0.7 m2 of green leaves per m2 of soil (350 CTh) and then became linear for seedling crops, but was linear for regrowths. During spring and summer, the slope of the linear phase of LAI increase was almost constant for all the seedling growths (0.011–0.014 CTh−1) except the last summer sowing, but was significantly variable (from 0.007 to 0.025 CTh−1) for regrowths. The RUE based on ADM and TDM (RUEa and RUEt, respectively) reached a linear phase for seedling growths when ADM exceeded 1.0 t DM ha−1. RUEa was equal to 1.72 g DM MJ−1 irrespective of sowing date, while RUEt was higher at 2.28 and 2.84 DM MJ−1 for canopies from spring and summer sowings, respectively. The RUE of seedling crops always exceeded those of regrowth crops present at the same period. Hence seedling crops of lucerne were slower to establish their LAI than regrowths, but used cumulative intercepted radiation as efficiently as the latter. The seedling Pshoot varied slightly according to crop age and season unlike the regrowth Pshoot which decreased significantly vs. CTh5 (−8.9 × 10−4 CTh−1; R2 = 0.66); while the ratio was also influenced by the daylength of the period. The Pshoot of seedling and regrowth canopies evolved very differently with respect to CTh, the former remaining quite stable and the latter declining. This suggests a change in plant behaviour between seedling and regrowth crops, probably triggered by crop defoliation.

► Seddling and regrowth crops of Lucerne were compared for four sowing dates in fully irrigated experiments in France.
► Seddling crops were slower to establish their LAI according cumulative thermal time (CTh).
► Seddling used intercepted radiation more efficiently than regrowth crops for both aerial and total dry matter accumulation.
► The shoot:root dry matter partitioning of seedling crops was almost stable but regrowth crop one linearly declined vs CTh.
► A shift occurred in crop ontogeny between seedling and regrowth crops.