Controlled environment determination of nitrogen uptake and its efficient utilisation by selected South African sugarcane varieties.
Increasing nitrogen use efficiency (NUE) of sugarcane (Saccharum officinarum L.) has a potential of reducing farmers input costs associated with nitrogen (N) fertilizers. Although there is evidence for some genetic variability in NUE of sugarcane in South Africa, plant growth and physiological mechanisms underlying this variability are currently unknown. The study investigated the genetic variation in sugarcane for NUE (N-uptake efficiency; NUpE x N-utilisation efficiency; NUtE) as this could provide a basis for breeding varieties with reduced N demand. The study consisted of two separate successive pot trials at the South African Sugarcane Research Institute (SASRI) under outdoor conditions. A randomised block design preliminary trial (trial 1) was conducted in September 2013 to screen NUtE (biomass production/unit tissue N) of fifteen sugarcane varieties at three destructive (biomass) harvests that were conducted at four month intervals. Plants established from single nodal stem cuttings were, in six replicates, planted into pots that were immersed in metal troughs (5 pots/trough), which contained liquid nutrients (Schumann et al., 1998) with low (14.40 g N/pot) and high (28.80 g N/pot) N supply. In the subsequent trial (trial 2) conducted in November 2014, eight varieties were subjected to four N treatments, a no N (0 g N/pot), low (1.94 g N/pot), medium (5.81 g N/pot) and high (11.61 g N/pot) N, herein referred to as NN, LN, MN and HN, respectively. The trial was arranged in a randomised complete block design (RCBD) design with five replications. Non-destructive measurements (stalk height, stalk population, leaf relative chlorophyll (soil plant analysis development (SPAD) and leaf N, P and K concentration) were conducted at specific time intervals. Destructive measurements (whole plant sampling) were performed at 180 days after transplanting (DAT) to determine, green leaf counts and area (GLA), shoot biomass production, biomass partitioning, root length and NUE. Nitrogen concentration (% [g N/100 g DM]) in the tissue components was determined using the LECO TruSpec N analyser at the Fertilizer Advisory Service at SASRI. The data for trial 1 were not included in the thesis. The results of trial 2 showed that N supply significantly affected stalk height and counts, leaf counts, GLA, leaf SPAD and root length traits hence that varieties also differed significantly with respect to the physiological measurements. Stalk height was significantly enhanced by NN and LN supply whereas stalk counts were similar among the LN, MN and HN treatments. Variety N41 had taller stalks than N12 and N37 whilst the two latter varieties had higher stalk counts than the former. There were significant N level x variety interactions with respect to green leaf counts and GLA but not for SPAD and root length. The LN treatment increased the number of leaves more than the other treatments. Variety N37 and N12 had the highest number of leaves as compared with N32. The GLA and leaf SPAD increased linearly with increasing N supply. Amongst the test varieties, N12 had significantly greater GLA and NCo376, N48 together with N41 had higher leaf SPAD values as compared with other varieties. There was also a significant N level x variety interaction with respect to root fresh biomass and shoot, root and whole-plant dry biomass. Significant increases in shoot and whole-plant dry biomass occurred and plateaued with LN supply. Although N41 ranked the third in terms of root dry biomass, the variety ranked the highest in terms of shoot and whole-plant dry biomass. The NN and LN treated plants allocated greater proportions of biomass to the stalk component, whilst the MN and HN treated plants allocated greater biomass to green leaves. Significant N level x variety interaction was observed for shoot N concentration and N content, NUpE, NUtE and the overall NUE. Shoot N concentration and content of test plants increased linearly with increasing N supply. Contrarily, NUpE, NUtE and NUE decreased with increasing N supply. Overall genetic variability in NUE was greater under LN supply and can be explained mainly by differences in NUtE rather than NUpE. Among the varieties, N41 had the highest NUE when compared with the N37 and NCo376 which ranked the lowest. It is concluded that N supply has a significant effect on sugarcane growth, dry biomass yield and allocation, N allocation and NUE.