Loss of blade photosynthetic area and of chloroplastsʹ photochemical capacity account for reduced CO2 assimilation rates in zinc-deficient sugar beet leaves

The effects of zinc deficiency on sugar beet chloroplastsʹ ultrastructure and photochemical ability, as well as on whole-leaf CO2 assimilation rates, were evaluated. It is shown that increasing zinc shortage firstly induces progressive disorganisation of the chloroplastsʹ internal membrane system, followed by degradation of both lamellae and stromal components. This zinc deficiency-triggered premature senescence process leads to cell death and, ultimately, to chlorotic/necrotic blade lesions, thus reducing the photosynthetically-active leaf area; chloroplasts in the remaining leaf area show slight impairment of their capacity to reduce methyl viologen using water as electron donor. Whole-leaf CO2 assimilation rates expressed on an area basis are observed to decrease with zinc deficiency, but do not do so significantly when expressed per unit weight of chlorophyll. It is proposed that the partial loss of chloroplastsʹ photochemical capacity, aggravated by loss of photosynthetically-competent blade area in severely-stressed leaves, accounts for depressed CO2 assimilation rates measured in the zinc-deficient sugar beet leaves.