Sunflower ecophysiology: some unresolved issues

Major unresolved issues in sunflower ecophysiology constrain efforts to imp rove crop modelling, management, genetic analysis and breeding. Three issue s are used here to illustrate this point. Firstly much of the work on the d uration of the emergence to flowering phase has considered the phase as a w hole. It is argued that a more detailed analysis based on sub-phases is req uired, particularly in view of possible intraspecific variability in the du rations of the basic vegetative and juvenile phases and evidence that photo period responses before, during and after floral initiation may differ betw een genotypes and even be of opposite sign for the same genotype. Secondly, contras ring responses of grain oil proportion to manipulation of plant po pulation density and incident radiation appear to be linked to variations i n kernel oil proportion rather than to kernel: hull ratio, and responses of grain oil proportion to changes in sowing date seem to have a similar orig in. More effort should be focused on understanding the controls of oil mass per kernel. it is speculated that there may be a genotype-dependent limit to this variable. A third unresolved issue relates to the nature and streng th of the linkage between post-anthesis stay-green and leaf photosynthetic functionality These variables are poorly related during pre-anthesis senesc ence of leaves in the lower portion of closed canopies, and for sunflower t his linkage appears much weaker than in other crop species. Current interes t in post-anthesis stay-green as a possibly useful crop attribute requires clarification of this uncertainty.