| Abstract | Genotypic variability in photosynthesis per unit of leaf area may be useful in increasing gross productivity of crop plants, but only if it can be demonstrated to be measurable and related to growth in field stands and if it is not physiologically linked to compensating differences in leaf area production. To examine these issues, six genotypes of pea (Pisum sativum L.), selected for differences in photosynthetic CO₂ exchange per unit of leaf area (CER), were planted in replicate field plots at densities of 25 and 100 seeds m⁻². Mean CER, leaf area index (LAI), and relative growth rate (RGR) were determined for each genotype ✕ density combination over six weeks of growth in two years. At the higher planting density, LAI increased by 72%, but CER and RGR decreased by 22 and 24%, respectively. Genotypes differed in all three traits, but the variability in LAI was related to a twofold difference between two developmentally distinct groups that did not differ in either CER or RGR. Although CER of all genotypes decreased at high density, the magnitude of the CER response to increased LAI differed. In both years, mean RGRs of plants in the twelve genotype ✕ density treatments were significantly correlated (P < 0.01) with mean CERs, and the genotype CER and RGR means from all data were also correlated (P < 0.05). That these correlated differences were not due to differences in photosynthetic area was shown by the superiority in CER (38%) and RGR (34%) of genotype PI 269810 over genotype PI 269770, even though the two genotypes did not differ in mean LAI. |
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