Photosynthetic capacity, irradiance and sequential senescence of sugar beet leaves

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Abstract
In field-grown sugar beet plants (Beta vulgaris L. cv. Dobrovická A), each of66 successive leaves produoed in the course of the vegetation period was different with respect to its photosynthetic capaoity (Pc), life span, duration of leaf area expansion, and longevity after its maximum leaf area (Amax) has developed. The proportionality between the seasonal changes in these characteristics was not the same if the sequential senescence of leaves was taken into account. With aging of individual leaves, Pc increased with the leaf area expansion having attained the peak value between 75% to 100% of Amax The rate of ontogenetic changes in Pc of each leaf was specified by the rate of its growth and development so that even at comparable ages the successive leaves constituted a series of different physiological units. The seasonal changes in quantum irradiance (PAR) were found to be responsible for differences in the growth characteristics between the successive leaves: Leaf expansion period was related with daily integrals of the incoming PAR (Io), while leaf longevity, after the Amax had been attained, was closely linked with PAR intercepted by the canopy (I). Pc expressed per the total leaf area of the plant was significantly correlated withI, while Pc calculated per unit leaf area of the plant was related toI o Leaf potential to adapt Pc correspondingly to changes in PAR was greatest during leaf blade expansion; after the leaf had ceased to expand, changes in Pc were independent of differences in leaf irradiance. The results stress, at least for field conditions, the inadmissibility of the extrapolation of attributes from one leaf to the other ones sequentially senescing on the plant.
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