Photosynthetic limitations imposed by self-shading in field-grown sugarcane varieties

- Canopy photosynthesis was evaluated in field-grown sugarcane plants.
- There is large genotypic variation in photosynthetic capacity.
- Photosynthetic plasticity due to self-shading is genotype-dependent.
- Light availability determines the CO2 uptake by sugarcane.
- Photosynthetic capacity is not reached under natural conditions.

As a result of self-shading, a large fraction of a plant's leaf area is exposed to low light levels, causing photosynthesis to occur under limiting light conditions. The aim of this study was to evaluate photosynthesis in the canopy of two sugarcane varieties grown under field conditions in view of the effects of self-shading on photosynthetic metabolism. Photosynthesis response curves to increases in CO2 partial pressure (A/Ci) and light intensity (A/Q) were obtained for leaves from the upper (leaf +2) and lower (leaf +5) canopy layers, and the leaf nitrogen and chlorophyll content was also evaluated. The IACSP93-2060 genotype, which shows higher self-shading, did not exhibit differences in photosynthetic capacity between canopy layers, presenting similarities in C4 maximum RubisCO capacity (Vmax) and A/Q-derived parameters when the canopy layers were compared. This pattern was most likely the result of light acclimation, with IACSP93-2060 showing increased photosynthetic nitrogen use efficiency in the lower canopy layer. Despite higher light availability due to a lower level of self-shading, the IACSP95-3028 genotype showed a decrease in photosynthetic capacity in the lower layer. This response was related to a decrease in Vmax, indicating a biochemical limitation of photosynthesis due to self-shading. Our data on stomatal conductance did not show any differences in the stomatal limitation of photosynthesis in both canopy layers of both varieties. In general, self-shading reduced the chlorophyll a and b and leaf N concentrations without significant changes in the PEPCase activity (given by the initial slope of the A/Ci curve) in the lower canopy layer. In addition to presenting a higher photosynthetic capacity, IACSP95-3028 also received more light in the lower canopy layer, supporting its higher biomass production compared with IACSP93-2060.