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Carbon Dioxide Exchange Rates, Transpiration, and Leaf Characters in Genetically Equivalent Ploidy Levels of Alfalfa¹

The influence of polyploidization on CO₂ exchange, transpiration, leaf diffusion resistance, stomatal size, stomatal density, and leaf area was examined in diploid (2x = 16), tetraploid (4x = 32), and octoploid (Sx = 64) alfalfa (Medicago sativa L.). Carbon dioxide exchange rates (CER) of intact leaves were measured with an open system by differential CO₂ analysis. Leaf resistance to water vapor diffusion was determined by differential water vapor analysis in a flow-through leaf chamber and leaf temperature measurements. 2x plants derived by haploidy from cultivated 4x alfalfa were colchicine doubled to form 2x – 4x genetically equivalent paired sets. A different group of cultivated 4x alfalfa was chromosomally doubled to the 8x level to form 4x – 8x paired sets. The CER of 4x plants grown in a growth chamber were similar to the corresponding 2x plants, and 8x plants were similar to corresponding 4x plants even though the stomatal densities were higher for the lower ploidy level in each case. Leaf areas, leaf weights, and specific leaf weights generally were unchanged with polyploidization. In an experiment comparing parents and hybrids at respective 2x and 4x levels, hybrids at the 4x level had greater leaf areas, transpiration per leaf, and CER per leaf than their corresponding 2x hybrids, but CER and transpiration per unit leaf area were unchanged with polyploidization. In both hybrids and parents, 2x plants had greater stomatal densities and shorter guard cell lengths than corresponding 4x plants. When F_t progenies (both 2x and 4x) were compared to parents, hybrids generally were higher in transpiration per leaf, CER per leaf, and leaf area.

Key Words: Medicago sativa L. • Polyploid • Specific leaf weight • Net photosynthesis • Leaf diffusion resistance • Stomatal density • Stomatal size

² Former graduate research assistant and professors of agronomy, respectively, Univ. of Wisconsin; Madison, WI 53706.

Received for publication May 26, 1977.

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