Plant phenotypic plasticity—the Sede-Boker workshop (1998)
Thinning a population during synchronized development affects dry matter allocation — comparison of a corn species with a below-ground storer
Manfred Küppers1,2, Dieter Schmitt2, Markus Schreiber2 and Diethard Matthies3

1Institut für Botanik und Botanischer Garten, Universität Hohenheim, Germany D-70593 Stuttgart.
Institut für Botanik, Technische Universität Darmstadt, Germany.
3Institut fur Umweltwissenschaften der Universitat Zurich, Switzerland.

The effect of successive release from intraspecific competition by thinning was studied in the two different growth forms of herbs, the annual oat species Avena sativa L. (cultivar "Erbgraf"), and the annual Raphanus sativus L. var. sativus (radish, cultivar "Cherry Bell"). While Avena allocates carbohydrates into grain filling in the reproductive phase, Raphanus especially supports a hypocotyl as storage organ from the vegetative phase onwards. Therefore, thinning should have entirely different effects on the species-specific allocation patterns.

Avena was sown at a density of 400 plants m-2 in 9m x 9m field plots. In a randomized design the plots were thinned in parts of the plots at the vegetative and flowering stage to give densities of 100 plants m-2 and 25 plants m-2; thus, competition was released at different times and to different degrees. Thinning did neither vary the number of flowers per spikelet nor the number of panicles per shoot or the number of seeds per plant at both times. In contrast, total leaf area, above-ground dry matter, plant height, number of shoots per plant and total yield per plant was strongly affected. Early thinning increased the number of shoots per plant and above-ground dry matter in comparison to the unthinned control, but reduced total leaf area and the number of flowers per nodium, whereas later thinning increased the total yield per plant through an increased individual grain weight. However, thinning to 25 individuals m-2 did actually reduce growth and yield.

Raphanus seeds were germinated and 35 seedlings were planted into sand-filled containers, each, at a density of 455 plants m-2 and 4.8 cm between each individual. At day 22 and at day 31 after germination the populations in some of the 40 containers were thinned to 156 plants m-2, providing the remaining plants with double space in all directions. Untreated containers were harvested over the period of cultivation, some at the same time as thinning trook place. After 50 days all plants were harvested. Statistical tests revealed a striking significance of the influence of early thinning on hypocotyls as well as on leaves, whereas effetcs of late thinning were low. An almost linear weight gain over time was observed in the hypocotyl until final harvest, whereas other organs declined. This was furthermore supported by early release of competition, indicating that the hypocotyl is a high-priority organ for allocation.

Comparing both species indicates the significance of the time of thinning: While for Avena, thinning in the generative state is more favourable to achieve higher seed weight per plant, Raphanus profits of thinning in the early vegetative state by promoting the growth of leaves, and, therefore, improving yield of the hypocotyl.