Canopy stay-green and yield in non-stressed sunflower

Delayed leaf senescence during the grain filling phase, or stay green (SG), may be functional or cosmetic; the first being considered a valuable trait in breeding of many crop species. To establish whether canopy senescence patterns exhibited by two sunflower (Helianthus annuus L.) hybrids visually selected for slow post-anthesis canopy leaf area index (LAI) loss reflected functional, rather than cosmetic, SG, LAI dynamics and total biomass increase between anthesis and physiological maturity were followed in three separate experiments in which the putative SG hybrids were compared with standard (i.e., non stay green, NSG) hybrids exhibiting normal rates of canopy senescence under non- or minimal-water stress conditions. In two experiments, pairwise (i.e., one SG vs. one NSG) comparisons were made at two crop population densities. In the third experiment, grown at a single crop population density, four NSG hybrids were contrasted with the two SG hybrids. Canopy senescence dynamics were well described by fitted bilinear functions which discriminated between an initial, slow phase of leaf area loss and a second phase of rapid canopy senescence. No differences between hybrids in the rate of senescence during the first phase were found, but the putative SG hybrids exhibited a significantly slower rate of senescence during the second phase and a significantly higher LAI at physiological maturity (as % of LAI at anthesis). One NSG hybrid showed a greater rate of second-phase senescence than the remaining three hybrids in this category. Across experiments, the anthesis-physiological maturity increment in total oil-corrected biomass and radiation use efficiency (RUE) were significantly greater in the SG hybrids. Cluster analysis based on these four attributes, using data from Exp. 3, clearly discriminated between SG and NSG hybrids. Examination of specific leaf N (SLN) dynamics suggests that the lower RUE values observed in one of the NSG hybrids could be attributable to lower SLN values, but this was not the case for the remaining NSG hybrids. It is also possible that very rapid canopy LAI loss in another NSG hybrid may underlie its lower RUE. The failure of SG hybrids to translate greater post-anthesis biomass increment into grain yield across experiments was associated with their slightly shorter times to anthesis and their significantly lower biomass at anthesis. Grain number, the component of yield most strongly associated with yield, was associated with biomass at anthesis. A further contributing factor was that the duration of grain-filling tended to be shorter in SG hybrids. In the one experiment in which this variable was measured, resistance to stalk breakage was greater in the SG hybrids than the NSG one to which they were compared. We conclude that the SG observed in the hybrids with slower canopy senescence is functional and not cosmetic, and that for this to be translated into a yield advantage all hybrids need to reach anthesis at the same time and grain-filling duration also has to be the same across hybrids. Stay green is an interesting secondary trait to select for and should lead to higher and more stable yields in environments in which stem breakage (lodging) is a problem.