Comparison of nitrogen utilization of diploid and tetraploid perennial ryegrass genotypes using a hydroponic system

Abstract

In the present study, genetic variation in growth rate of perennial ryegrass (Lolium perenne L.) was evaluated on a nutrient flow system (hydroponics) with computer controlled supply of nitrogen. The control algorithms were based on a simulation model designed to achieve a nitrogen supply proportional to growth. The setpoints of the growth rates, governed by limiting nitrogen supplies, were 20%, 50% and 100% of the optimal growth rate. Twelve tetraploid and 12 diploid cultivars were tested in a split plot design with 14 replicates. The leaf area indices at four successive harvests predicted by the model corresponded with the measurements within 15% deviation margins. The tetraploid cultivars yielded more at the first harvest but this trend was reversed at the fourth harvest, owing to a lower tillering rate in the tetraploids. Growth analyses at the different N\supply rates led to the following conclusions:

• A low N-supply leads to a low tillering rate. The decline is most manifest at organic N-contents below 2%. At comparable organic N-contents, the number of tillers of the tetraploids was smaller than that of the diploids. At the fourth harvest, the average tiller density of the tetraploid cultivars was reduced to 60% of the diploid tiller density.

• The leaf weight ratio (LWR) increased linearly with the organic N-content of the leaves. The effect was equal for diploid and tetraploid cultivars. The LWR increased from 0.40 at 1.5% N to 0.80 at 3.5% N.

• Specific leaf area (SLA) decreased with a decreasing N-content. The SLA of tetraploid cultivars was 10%-15% lower than that of diploid cultivars.

• Photosynthesis was linearly related to the organic N-content. The photosynthesis of the diploid cultivar Wendy did not differ from that of the tetraploid cultivar Condesa. However, a test of 6 tetraploid and 6 diploid perennial ryegrass cultivars under optimal N-supply revealed that the light-saturated net photosynthesis of tetraploid cultivars on a leaf area basis was 30% higher than that of diploid cultivars.