Leaf gas exchange and tuber yield in Jerusalem artichoke (Helianthus tuberosus) cultivars

Abstract

Abstract Ten cultivars of Jerusalem artichoke (Helianthus tuberosus L.) were tested in pot and field experiments for maximum leaf photosynthesis, response of photosynthesis to photon flux density and leaf ageing, chlorophyll concentration, nutrient effects, and the relationship between growth parameters, tuber yield and photosynthesis. The high biomass productivity of Jerusalem artichoke is at least partly due to its above-average photosynthetic capacity. Photosynthetic rates of the youngest fully developed leaves amount to 29–40 μM CO2m−2s−1, thus equalling or slightly exceeding the rates if sunflower selections. Photosynthetic rates of cultivars are positively correlated with chlorophyll concentrations in the leaves (0.3–0.6 g m−2). Light saturation of photosynthesis in young leaves is not complete at 1700 μE m−2 s−1, but above 1000 μE m−2 s−1 the increase is only slight. Water-use efficiency is stable above 300 μE m−2 s−1. In ageing plants, photosynthetic capacity of the leaves is highly dependent on leaf position. During the stage of most intense tuber growth, the gas exchange rates of leaves below the top 50 cm of the plant rapidly drop below 50% of those at the top. This apparent decrease of whole-plant photosynthesis in late summer and autumn might negatively influence tuber growth, and could be caused by nitrogen deprivation of old leaves, as has been observed in sunflower. Slow but steady N release in the soil might be essential for maintaining high canopy photosynthesis in Jerusalem artichoke. Nutrient studies have shown high dependence of photosynthesis on adequate nitrogen supply, whereas deficiency of potassium and phosphorus influence tuber yield but not leaf photosynthesis. Varietal tuber dry-matter yields were most closely correlated to stem dry-matter in early summer and leaf dry-matter in autumn. In spite of significant differences in the mean photosynthetic rates of the cultivars, there was no positive correlation to autumnal tuber yields. It is proposed that, in existing cultivars, tuber formation ability is frequently not maximized. Although the potential apparently exists for improving Jerusalem artichoke by exploitation of superior photosynthetic capacity at the leaf, plant and canopy level, screening for photosynthetic characteristics will prove successful only when the sink capacity of the tubers is also improved.