Effects of seed size and carbon dioxide enrichment on expansion and initiation rates of wild radish (Raphanus raphanistrum) leaves.

Abstract

Rising atmospheric carbon dioxide concentrations could lead to selection for certain genotypes which are best able to respond to the changing conditions. For some plant species, early growth rates in seedlings is a good indicator of fitness. By attaining a large size early in life plants can reduce their susceptibility to water stress and shading by competitors. Two components of early growth rate that would minimize the possibility of shading by competitors are expansion and initiation rates of leaves. Past studies have shown that early growth rates can be increased when plants are grown from relatively large seeds or in an elevated CO2 environment. However, this study of early growth rates of the third, fourth, and fifth leaves of Raphanus raphanistrum, wild radish, detected no correlation between seed size and leaf expansion or initiation rates. 183 plants were grown in open-top field chambers, eighty-three at ambient CO2 (360 ppm) and one hundred at an elevated carbon dioxide concentration of approximately 700 ppm. The plants grown under elevated CO2 conditions showed a decrease in early growth. Fourth and fifth leaves grew significantly slower in elevated CO2 (P=0.0001 and 0.0002 respectively), and the initiation rate between the third and fourth leaves was also significantly slower (P=0.020). The two most likely reasons for this decrease in growth are that the leaves grew thicker instead of longer, and that higher proportions of assimilated carbon were invested into below ground growth in order to take up greater amounts of nutrients.