victor.sadras@sa.gov.au

From a developmental perspective, connectance is defined as the level of linkage between organs or traits (Biosystems 56, 1-11). Development is not a continuous process but alternates stabilisation during discrete stages (i.e., phenophases) and dismantling of relational networks between organs in the intervening, shorter critical periods (Theory in Biosciences 123, 17).

In this context, the critical period is a transient phase of isolation of the system that enables its evolution towards equilibrium; the transition from dissipative to isolated system is the source of newly emerging dissipative structures, i.e., new phenophases, in which environmental or developmental disturbances are adaptively integrated. We calculated the connectance of 17 traits in a historic collection of 13 wheat cultivars grown in four field environments.

The frequency distribution of connectance, which varied from 0.09 to 4.2, conformed to a power law with slope = -1.67. Several traits, including time to anthesis and seed weight, clustered below 0.3. Traits related to growth during the pre-anthesis period including shoot biomass at anthesis, nitrogen uptake at anthesis, evapotranspiration from sowing to anthesis, heads per m2 and grains per head, clustered in the range from 0.3 to 0.5.

Shoot biomass at maturity had the largest connectance (2.1) and related traits including grain yield, nitrogen uptake at maturity, biomass per unit evapotranspiration at maturity, all had connectance above 1.

Genotype-dependent median connectance varied 2.4-fold, from 0.28 in Condor, the first semi-dwarf cultivar in the sampled series, to 0.67 in Gladius. Connectance of yield increased at 0.028 per year over the period 1958 to 2007.