Abstract
Fifty-one genotypes of sugarcane (Saccharum spp.) or closely related germplasm were evaluated in a pot experiment to examine genetic variation in transpiration efficiency. Significant variation in whole plant transpiration efficiency was observed, with the difference between lowest and highest genotypes being about 40% of the mean. Leaf gas exchange measurements were made across a wide range of conditions. There was significant genetic variation in intrinsic transpiration efficiency at a leaf level as measured by leaf internal CO2 (Ci) levels. Significant genetic variation in Ci was also observed within subsets of data representing narrow ranges of stomatal conductance. Ci had a low broad sense heritability (Hb = 0.11) on the basis of single measurements made at particular dates, because of high error variation and genotype × date interaction, but broad sense heritability for mean Ci across all dates was high (Hb = 0.81) because of the large number of measurements taken at different dates. Ci levels among genotypes at mid-range levels of conductance had a strong genetic correlation (−0.92 ± 0.30) with whole plant transpiration efficiency but genetic correlations between Ci and whole plant transpiration efficiency were weaker or not significant at higher and lower levels of conductance. Reduced Ci levels at any given level of conductance may result in improved yields in water-limited environments without trade-offs in rates of water use and growth. Targeted selection and improvement of lowered Ci per unit conductance via breeding may provide longer-term benefits for water-limited environments but the challenge will be to identify a low-cost screening methodology.
Highlights
In sugarcane (Saccharum spp.), as with other crop species, the ability of a crop to produce high biomass and yield per unit of available water is potentially important in affecting profitability and yield, in both irrigated and rainfed production systems (Robertson and Muchow, 1994; Inman-Bamber and McGlinchey, 2003; Inman-Bamber and Smith, 2005; Basnayake et al, 2012)
We propose a general hypothesis that attaining higher yields by improving transpiration efficiency (TE) in crop improvement programmes requires an understanding of the extent of genetic variation in the key mechanisms causing the observed variation, and that selection in breeding programmes is best applied to subcomponents of TE, rather than selection for TE per se
Genetic variation was high for both components of TE—total biomass and water use—and highest for biomass of roots, as indicated by the genetic CVs (Table 2)
Summary
In sugarcane (Saccharum spp.), as with other crop species, the ability of a crop to produce high biomass and yield per unit of available water is potentially important in affecting profitability and yield, in both irrigated and rainfed production systems (Robertson and Muchow, 1994; Inman-Bamber and McGlinchey, 2003; Inman-Bamber and Smith, 2005; Basnayake et al, 2012). The concept of water use efficiency is depicted in the use of various formats of the simple framework: Yield = water transpired x transpiration efficiency (TE) x harvest index (Eqn1) This framework or slight modifications of it have been widely used among researchers to conceptualize and interpret causes of yield variation in water-limited environments Passioura, 1977; Condon and Richards, 1993; Richards et al, 2002) This has proven to be useful in interpreting results investigating variation in crop growth. According to this framework, growth in water-limited environments may be increased if there is more transpiration (i.e. more water accessed and acquired by roots and transpired) or if there is more biomass produced per unit water transpired
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