Seed Mass and the Evolution of Early-Seedling Etiolation

The influence of seed mass on the evolution of seedling-foraging strategies for light acquisition under deep shade was assessed in a comparative study of etiolation behavior. This was done across 50 Australian species varying in seed reserve mass by eight orders of magnitude during the first week after germination. Proportional increase in hypocotyl length in shade compared to light was similar across the range of seed reserve mass. Etiolation did not lead to increase in hypocotyl length per wet mass, in other words, etiolated hypocotyls were not thinner. However, hypocotyl length per dry mass did increase, more so in smaller-seeded species. Thus, part of the hypocotyl elongation was because of increased water content, which would increase vulnerability to loss of turgor. There was also reallocation of dry matter from root to hypocotyl, again more so in smaller-seeded species, which would decrease anchorage strength and increase vulnerability to soil drying. Results were very similar when considered as correlated evolutionary divergences, compared to the cross-species patterns. The higher-risk etiolation behavior of smaller-seeded species can be understood through their having little to lose. Because they hold less reserve resource uncommitted and attempt a faster initial growth rate, their chances of sustained longevity in shade below the compensation point are very low.