Genotypic variation in intrinsic transpiration efficiency correlates with sugarcane yield under rainfed and irrigated field conditions.

Intrinsic transpiration efficiency (i TE), the ratio of photosynthesis (A) to stomatal conductance (gs ), is considered a useful trait for improving productivity; however, higher i TE with high A is more desirable than that with low gs . This study dissects i TE of 20 sugarcane genotypes to understand its relationship with total dry matter (TDM) and cane yield (TCH) under irrigated and rainfed conditions. Water stress reduced mean A and gs by 56 and 61%, and mean TDM and TCH by 55 and 59%, respectively; however, genotype × irrigation treatment interaction was smaller than genotype variance. Mean i TE increased from 117.4 µmol mol-1 in the irrigated treatment to 130.6 µmol mol-1 in the rainfed treatment. In irrigated conditions, i TE had high heritability (H2 b  = 0.67) and significant genetic correlation with TDM (rg  = 0.58) and TCH (rg  = 0.72). Under water stress, at gs below 0.1 mol m-2  s-1 , non-stomatal limitation to A was evident and i TE had low heritability (H2 b  = 0.2). Whereas in the gs range of 0.1-0.4 mol m-2  s-1 , heritability of i TE (H2 b  = 0.63) and its genetic correlation with TDM (rg  = 0.78) and TCH (rg  = 0.75) were maximised. There was significant genotypic variation in photosynthetic capacity (Ac ), and the differences were related to TDM and i TE. Selecting genotypes with higher i TE and Ac could offer potential for improving productivity without the unfavourable effect of low gs .

Genetic variation in transpiration efficiency and relationships between whole plant and leaf gas exchange measurements in Saccharum spp. and related germplasm.

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.

Changes in photosynthesis and carbohydrate metabolism in sugarcane during the development of Yellow Canopy Syndrome.

Photosynthesis, stomatal conductance, electron transport, internal CO2 and sugar levels were determined in the leaves of Yellow Canopy Syndrome (YCS) symptomatic sugarcane (Saccharum spp.) plants. Two varieties from two different geographic regions in Australia, KQ228 and Q200 were used. Although visual yellowing was only evident in the lower leaves of the canopy (older than Leaf 5), photosynthesis and stomatal conductance were lower in both the yellowing leaves and those not yet showing any visible symptoms. On a canopy basis, photosynthesis was reduced by 14% and 36% in YCS symptomatic KQ228 and Q200 plants, respectively. Sucrose levels increased significantly in the leaves, reflecting some of the earliest changes induced in YCS symptomatic plants. The electron transport characteristics of dark-adapted leaves showed disruptions on both the electron acceptor and donor side of PSII. Some of these changes are characteristic of a degree of disruption to the protein structure associated with the electron transport chain. Based on the results, we propose that the first change in metabolism in the YCS symptomatic plants was an increase in sucrose in the leaves and that all the other changes are secondary effects modulated by the increased sugar levels.