Analysis of specific leaf area and photosynthesis of two inbred lines of Plantago major differing in relative growth rate

Two inbred lines of Plantago major L., differing in relative growth rate (RGR), were studied in order to elucidate physiological factors responsible for this difference. The slow growing inbred line of P. major ssp. major L. (line W9) had a lower specific leaf area (SLA) compared with that of the fast growing P. major ssp. pleiosperma Pilger (line A4). Photosynthetic activity per unit leaf area was higher than that of A4, but expressed per unit leaf dry weight W9 exhibited a lower activity than A4. Photosynthetic capacity (per unit leaf dry weight or nitrogen) was identical for both lines. The amount of chlorophyll a per unit dry weight or leaf area was higher for line W9. Differences in the chemical composition of the leaf dry weight of the two lines did not explain the lower SLA or lower photosynthetic activity per unit leaf dry weight of the slow growing line. The lower SLA of W9 was mainly caused by the lower water content per unit dry weight. A higher amount of cell wall material per unit leaf dry weight was found. We conclude that the lower SLA of W9 resulted in a greater shading within the leaf, a lower light absorption and lower photosynthetic rate per unit leaf dry weight. Consequently, this resulted in a lower RGR. The significance of these characters for growth under trampling and soil compaction is discussed.