Understanding growth and development of lucerne crops (Medicago sativa L.) with contrasting levels of perennial reserves

Abstract

This research examined seasonal patterns of growth and development of lucerne crops grown with different levels of crown and taproot reserves in a cool temperate environment. The approach required derivation of explanatory mathematical relationships between crop physiological processes and the main environmental variables of temperature, photoperiod and incoming radiation.

To create crops with contrasting levels of perennial reserves (i.e. carbon and nitrogen stored in crowns and taproots), four defoliation treatments were applied to an established 'Kaituna' lucerne crop at Lincoln University, Canterbury, New Zealand in the 2002/03 and 2003/04 growth seasons. Treatments consisted of a factorial combination of a (i) 28 day (short, S) or (ii) 42 days (long, L) regrowth cycle during (i) spring/mid-summer and/or (ii) mid-summer/autumn. The treatments had acronyms of LL, LS, SL or SS, in which each letter refers to the frequency of defoliation in the first and second period of the season, respectively.

Regardless of defoliation treatment, perennial dry matter (DMper) differed seasonally. In LL crops, DMper increased from <3 t/ha in mid-summer to >5 t/ha in autumn. Frequent defoliations (SS) reduced both the DMper by 20-30% and the concentration and amount of soluble sugars, starch and nitrogen in taproots. As a result, the annual shoot yield ranged from 23 t/ha (LL) to 14 t/ha (SS) in 2003/04. Most of this difference was explained by changes in the weight of each individual shoot. Despite shoot yield differences, plant population declined at similar exponential rates in all crops from 140 plants/m² in August 2002 to 60 plants/m² in October 2004. Final shoot populations were conservative at ~780 shoots/m² in all crops.

Physiologically, yield differences were mostly explained (R²=0.84) by the accumulated intercepted photosynthetic active radiation (ΣPARi). The ΣPARi was limited in frequently defoliated crops because these crops had lower leaf area index (LAI) than LL crops but similar canopy architecture, with an extinction coefficient for diffuse light (kd) of 0.81. Differences in LAI were a direct effect of harvesting before full canopy cover (i.e. critical LAI of 3.6) in short regrowth cycles. In addition, leaf area expansion rates (LAER) in spring were reduced from 0.016 (LL) to 0.011 m²/m²/°Cd (SS) as taproot DM declined from 3.0 to 1.5 t/ha, respectively. The reduction in LAER was a result of smaller leaves after the 5th main stem-node and a reduction in the leaf area of branches. At the same time the phyllochron was conservative at 34°Cd/leaf (Tb of 5°C) when photoperiod was greater than 12.5 h but increased to 40°Cd (LL and SL) and 60°Cd (LS and SS) at lower photoperiods. All crops had a similar maximum rate of branching of 6.8 leaves/main-stem node and leaf senescence rates at 0.48 leaves/main-stem node.

Shoot radiation use efficiency (RUEshoot) was 1.6 g DM/MJ PARi for pooled annual yield. However the seasonal RUEshoot was inconsistent being 1.8 g DM/MJ PARi in spring/summer and only 1.0-1.2 g DM/MJ PARi in autumn for LL crops. Frequent defoliations decreased RUEshoot by 20% during summer. The average RUE for total dry matter (RUEtotal) was 2.2 g DM/MJ PARi but this was affected by defoliation treatments and temperature. Measurements of RUE from 23 regrowth cycles were then used to validate a previously proposed temperature framework for RUE. That framework was used to estimate the RUEtotal and fractional partitioning of DM to perennial organs (Pper). In SS crops, RUEtotal was 50% of the values of LL crops in four of eight analysed regrowth cycles. This was consistent with the measured reduction of 20% in the saturated photosynthetic rate (Pmax) of SS crops in the summer/autumn of 2002/03 season. Photosynthetic rates at 1000 µmol photon/m²/s (Pn1000) were also ~20% lower during the first 150°Cd of regrowth in SS crops in comparison with LL crops. Differences in Pn1000 were mostly explained by the specific leaf nitrogen and chlorophyll content of these leaves. The estimated Pper increased from <0.05 in winter/early-spring to a maximum of 0.50 in mid-summer and declined slightly to 0.40 in late-autumn for LL crops. Photoperiod (Pp) and the ratio between soil and air temperature (Tsoil/Tair) were tested as predictors of Pper. A hysterisis model based on increasing and decreasing Pp was necessary to relate Pper to Pp but a single relationship (R²=0.52) was found between Pper and Tsoil/Tair.

The relationships derived from the field experiment were then integrated in a simple computer simulation model. The model accurately predicted seasonal primary leaf appearance, LAI and shoot yield of LL crops. Accurate simulation of perennial DM required a seasonal root maintenance respiration (Rm) rate of ~0.03 DM/day in spring/summer and <0.01 DM/day in autumn/winter.

In this thesis, the response of lucerne physiological processes to environmental signals and to the level of perennial reserves was quantified and integrated into mathematical relationships. These relationships provided a sound framework to better understand and predict seasonal yield and development of lucerne crops grown in cool temperate climates.