Growth and development of lucerne with different fall dormancy ratings

Abstract

The main aim of this research was to understand differences in the growth and development of three lucerne genotypes with different fall dormancy (FD) ratings; FD2 (dormant), FD5 (semi-dormant) and FD10 (winter-active). To do this, one field experiment was undertaken over two years; a seedling establishment phase followed by multiple regrowth cycles (October 2014 to January 2017). By the end of the seedling phase, the FD10 genotype had produced 20% more shoot and 16% more root biomass than the other two genotypes. Lucerne physiology was examined to see if the yield advantage of FD10 was maintained during subsequent regrowth cycles. After the seedling phase, a second treatment of defoliation frequency regime (DF) at 28 (DF28), 42 (DF42) and 84 (DF84) days was used to create different levels of root reserves, to examine whether treatments affected the yield and quality potential of crops. Annual shoot yields ranged from 4.4 t DM/ha in DF28 crops to 17.5 t DM/ha in DF84 crops. Most of this difference was due to changes in the rates of shoot growth in response to temperature and photoperiod (Pp). When crops were growing into an increasing Pp, growth rate was 3.5 kg DM/ha/°Cd for DF28 crops compared with 7.5 kg DM/ha/°Cd for DF42 and 8.8 kg DM/ha/°Cd for DF84 crops. The leaf stem ratio (LSR) declined by 0.82 for each one ton increase in shoot DM. The CP and ME accumulation in whole shoots or in leaf, soft stem and hard stem followed an allometric relationship. As DM increased, CP and ME increased in a similar pattern for all treatments. By third year 2016/17, crops defoliated at 42 and 84 day intervals produced 1.3 t CP/ha and 55 GJ ME/ha greater than a 28 day regrowth crop. Thus, quality was unaffected by FD ratings and explained allometrically by the leaf and stem ratio, associated with shoot DM. Physiologically, the differences in shoot yield among DF crops were explained mainly by differences in the amounts of radiation intercepted. More frequent defoliation caused lower radiation interception because short regrowth cycles reduced leaf area index (LAI). There was no difference in canopy architecture with an extinction coefficient of 0.83 for all treatments. The lower LAI in DF28 canopies was explained by a lower leaf area expansion rate (LAER). This was 50 and 62% slower during the main spring-summer growth periods than for DF42 and DF84 crops, respectively. The lower LAER in DF28 crops was caused by slower development of individual leaf area and a longer phyllochron. However, other LAI components including branching, shoot population, leaf senescence and reproductive development (flowering) were relatively consistent for all crops. The shortest defoliation interval reduced the amounts and levels of root reserves (DMroot) by 40 and 60% in relation to the DF42 and DF84 regimes. The smaller root reserves in DF28 crops were caused by a decline in the fractional DM partitioning in roots (Proot). Therefore post defoliation, a lack of root reserves reduced RUEshoot and consequently reduced biomass accumulation for DF28 crops. There was no interaction between the effects of the FD and DF treatments. Irrespective of the DF regime, the FD10 genotype produced 23% higher shoot yield in autumn. During this period, stem expansion rate of FD10 was 0.99 mm/°Cd which was faster than FD5 (0.70) and FD2 (0.53). Autumn mean leaf area was 226, 369, and 489 mm2 for FD2, FD5 and FD10, respectively. Therefore the higher autumn shoot yield of the FD10 genotype came from a faster stem elongation rate and larger leaves. However, individual leaf area was similar for all genotypes during the main spring-summer growth. In the 28 day defoliation treatment, FD10 had lower shoot growth rates during the spring-summer period 2016/17 and grew ∼3 kg DM/ha/0Cd lower than the FD2 and FD5 genotypes. The difference in Proot among genotypes was possibly due to different in base photoperiod response. The FD2 genotype showed the most response to Pp direction. This suggests this genotype had a lower base photoperiod response. Therefore this more dormant genotype recharged root reserves at all times of year. In contrast, the winter-active (FD10) genotype had a higher base photoperiod and therefore had less time to recharge root reserves than the FD2 genotype, FD5 was intermediate. This explains the faster decline in root reserves for FD10 growing in colder months in the DF28 regime. Consequently, the root reserves of FD10 declined over time to 1.5 t DM/ha by the end of the experiment in January 2017. It is likely that this progressive reduction in root reserves is the cause of reported decreases in persistence of FD10 genotypes over time. Ongoing monitoring of this experiment will be used to test this hypothesis. Thus this research showed growth differed among FD ratings. Phenological development (phyllochron, branching, canopy structure and leaf senescence) and reproductive development were conservative among FD ratings. In contrast, vegetative growth (leaf area expansion and stem elongation) was most closely correlated with fall dormancy ratings during atumn period.