Assimilate partitioning in leaves of the raffinose-storing herb Lamium album L.: photosynthesis and carbon partitioning throughout the photoperiod

Abstract

-(Assimilate partitioning in leaves of the raffinose-storing herb Lamium album L.: photosynthesis and carbon partitioning throughout the photoperiod). Lamium album accumulates starch, sucrose and raffinose-family oligosaccharides (RFO) as the major products of photosynthesis. These products were measured in leaves throughout a sixteen-hour photoperiod and under various irradiance conditions. There was continuous accumulation of sucrose and starch. The rate of gas exchange was higher at 500 µEm 2 s -1 and 900 µEm 2 s -1 than at 300 µEm 2 s -1 . The rate of photosynthesis did not decline over the sixteen-hour photoperiod, which suggested that there was no short-term feed back inhibition due to sucrose accumulation in this plant. When the products of photosynthesis were compared at the end of the photoperiod, only sucrose increased in abundance at high irradiance. The RFO pool in leaves was shown to contain raffinose, stachyose and verbascose; galactinol was also present. 14 CO 2 feeding demonstrated that roots and flowers were the major sinks. The middle leaves were major source leaves whilst young leaves acted as both sources and sinks.Key words -Lamium album, partitioning, raffinose family oligosaccharides, starch, sucrose RESUMO -(Partição de assimilados em folhas de Lamium album L., uma planta herbácea que acumula rafinose: fotossíntese e partição de carbono ao longo do fotoperíodo). Lamium album acumula amido, sacarose e oligossacarídeos da família rafinósica (OFR) como produtos principais da fotossíntese. Estes produtos foram medidos em folhas durante um fotoperíodo de 16 horas e sob várias condições de irradiância. Houve um acúmulo contínuo de sacarose e amido. As taxas de trocas gasosas foram maiores a 500 e 900 µEm -2 s -1 do que a 300 µEm -2 s -1 . A taxa de fotossíntese não declinou durante o fotoperíodo de 16 horas, o que sugere que não existe um mecanismo inibitório de retroalimentação de curto prazo devido ao acúmulo de sacarose nesta planta. Quando os produtos da fotossíntese foram comparados ao final do fotoperíodo, apenas a sacarose aumentou em abundância em alta irradiação. Os OFR foram analisados e são compostos por rafinose, estaquiose e verbascose; galactinol também esteve presente. O fornecimento de 14 CO 2 demonstrou que raízes e flores foram os principais drenos. As folhas do meio foram as principais fontes enquanto folhas jovens atuaram como fonte e dreno ao mesmo tempo.Palavras-chave -amido, Lamium album, oligossacarídeos da família rafinósica, partição, sacarose 1.

Assimilate partitioning in leaves of the raffinose-storing herb Lamium album L.: the effects of altering source-sink balance

“…L. album was used because raffinose family oligosaccharides (RFOs) are major products of photosynthesis along with sucrose and starch, and this species therefore provides a contrast between high starch leaves and those of low-starch fructanaccumulators. Measurement of carbohydrate content showed that sucrose and starch were more abundant than RFOs in leaves of L. album grown under moderate irradiance with high sink activity (Kochhar et al 2003). We suggested that only sucrose was synthesized at the beginning of the light period, with RFO and/or starch synthesis becoming significant later in the photoperiod.…”

“…Plant cultivation and physiological treatments -Plants of Lamium album L. (white dead nettle) were grown hydroponically from cuttings in controlled environment conditions as described previously (Kochhar et al 2003). Growth temperature was 20 ºC, photoperiod 12 h, irradiance 500 -550 µEm -2 s -1 .…”

“…Duplicate plants were harvested after 1h (immediately after feeding), 6 h (after 6 h illumination), 12 h (after 12 h illumination), 18 h (12 h light, 6 h of the subsequent dark period) and 24 h (12 h light, 12 h of the subsequent dark period). Extraction, fractionation and measurement of radioactivity was carried out as described previously (Kochhar et al 2003). Soluble material was fractionated by ion exchange chromatography into an acidic fraction (organic acids plus acid-stable phosphorylated intermediates); a basic fraction (mainly amino acids) and a neutral fraction (carbohydrates).…”

“…Soluble material was fractionated by ion exchange chromatography into an acidic fraction (organic acids plus acid-stable phosphorylated intermediates); a basic fraction (mainly amino acids) and a neutral fraction (carbohydrates). The neutral fraction was separated further by TLC, sugars were identified by co-chromatography with markers and radioactivity quantified by autoradiography/densitometry (Kochhar et al 2003). Insoluble material was counted directly following solubilisation and aliquots were also digested with amyloglucosidase/ á-amylase and the glucose released from starch separated by TLC (Kochhar et al 2003).…”

“…Studies on fructanaccumulating monocots (which store very little starch in the leaves) indicate that photosynthetic downregulation is minimal, perhaps due to the vacuolar sequestration of carbohydrate (Farrar et al 2000). In a previous paper, we described the partitioning of carbon in leaves of Lamium album L. (Kochhar et al 2003). L. album was used because raffinose family oligosaccharides (RFOs) are major products of photosynthesis along with sucrose and starch, and this species therefore provides a contrast between high starch leaves and those of low-starch fructanaccumulators.…”