Use of a controlled-nutrient experiment to test heterosis hypotheses.

A controlled-nutrient (CN) experiment was conducted to test three heterosis hypotheses with reference to tomato yield, and its components, for a set of two inbred lines and their hybrid that had previously exhibited considerable heterosis under field conditions. The CN treatments consisted of periodic applications of differential doses of nutrient solution to plants reared individually in containers filled with vermiculite. Ripe fruit were harvested, counted and weighed over a period of 340 days. The data permitted the partitioning of yield into a closed system of five component variables. Heterosis was not exhibited by yield, nor yield components, at any of the four nutrient levels. Hence the total heterosis phenomenon was classified as nutrient-dependent: heterosis occurring under field conditions, but not under the nutritional restrictions of the CN experiment. Three heterosis hypotheses were examined for their ability to explain all of the nutrient-dependent aspects of the heterosis phenomenon. Hypothesis 1: Heterosis is a consequence of a more efficient hybrid metabolic system in that it can produce more product with equal input. Hypothesis 2: Heterosis is a consequence of the somatic multiplication of additive component traits. Hypothesis 3: Heterosis is a consequence of a faster hybrid growth rate. Although none of the hypotheses are rejected by the field data, the first two are rejected by the CN experimental results. The third hypothesis fits all aspects of the nutrient-dependent heterosis phenomenon remarkably well. It is speculated that the indeterminate pattern of plant development responsible for yield and its components is due to two major gene systems: genes that determine morphogenetic, and genes that determine growth rate manifestations of growth. Under this hypothesis, the CN technique permits separation of the responses due to these two gene systems.