Sugarcane response to row spacing-induced competition for light

Information about crop response to row spacing (RS) in sugarcane is often contradictory and there is a lack of understanding of competition for light. This study aimed to gain a better understanding of how competition for light affects leaf and tiller development, radiation capture and conversion to biomass, and its partitioning between leaves and stalks. A first ratoon crop of cultivar NCo376 was started on 29 August 2003 (when the plant crop was cut back) at Mount Edgecombe, South Africa (29°42′18.4″S, 31°02′48.5″E, 105 m). A wagon wheel design was used with RS ranging from 0.36 m to 2.66 m and the crop received adequate water and nutrients. Radiation interception was measured within cane rows (FIINTRA) and across cane rows (FIINTER). FIINTRA was not affected by RS and maximum FIINTRA (defined as the first value above 0.9) was reached simultaneously for all RS. The progression of FIINTER differed substantially between the different RS. Maximum green leaf number was attained when FIINTRA reached 90% and coincided with the occurrence of peak tiller density. Thereafter there was a gradual decline in green leaf number caused by intra-row competition for light. As soon as FIINTER exceeded a value of 90%, there was a drastic reduction in green leaf number due to a sharp acceleration in leaf senescence rate induced by inter-row competition for light. Inter-row competition had an effect on aboveground biomass accumulation of RS of less than 1.37 m from before the first sampling at 730 °Cd (base temperature of 16 °C). For RS of 1.73 m the competition effect commenced sometime between the first and second sampling (at 948 °Cd). There appeared to be no inter-row competition effect for RS of 2.15 m and more. Radiation use efficiency seemed unaffected by RS (average value of 1.50 g MJ−1), although the increase in biomass yield (22% per metre reduction in RS) could not be fully explained by a concurrent increase in intercepted radiation (source size—18% per metre reduction in RS). Sink size (stalk density) responded more to a decrease in RS than source size (intercepted radiation) and the latter became insufficient to fill the sink to capacity when row spacing was less than 1.37 m. Conversely, sink size appeared to be the limiting factor for wider RS as evidenced by a similar mass per stalk for different RS. The stalk partition fraction of biomass increments was unaffected by row spacing, although the start of stalk growth was delayed with decreasing RS in terms of the threshold biomass required for stalk partitioning to commence. This resulted in lower final stalk fractions for narrower RS. The information obtained in this study enabled important new insights into underlying mechanisms of row spacing-induced competition effects on sugarcane growth and development. It should be useful for improving crop models’ ability to predict crop response to row spacing more accurately and in a more functional manner. The improved understanding could highlight avenues for effective crop improvement (e.g. ideal canopy development sub-traits) and crop management (e.g. weed and water management).