Low sink demand limits photosynthesis under Pi deficiency

Abstract

The role of the demand for carbon assimilates (the 'sink') in regulating photosynthetic carbon assimilation (Pn: the 'source') in response to phosphate (P(i)) deficiency was examined in tobacco (Nicotiana tabacum L.). P(i) supply was maintained or withdrawn from plants, and in both treatments the source/sink ratio was decreased in some plants by darkening all but two source leaves (partially darkened plants). The remaining plants were kept fully illuminated. P(i)-sufficient plants showed little variation in rate of Pn, amounts of P(i) or phosphorylated intermediates. Withdrawal of P(i) decreased Pn by 75% under the growing conditions and at both low and high internal CO2 concentration. Concomitantly, P(i), phosphorylated intermediates and ATP contents decreased and starch increased. RuBP and activity of phosphoribulokinase closely matched the changes in Pn, but Rubisco activity remained high. Partial darkening P(i)-deficient plants delayed the loss of photosynthetic activity; Rubisco and phosphoribulokinase activities and amounts of sucrose and metabolites, particularly RuBP and G6P, were higher than in fully illuminated Pi-deficient plants. Rates of sucrose export from leaves were more than 2-fold greater than in fully illuminated P(i)-deficient plants. Greater sucrose synthesis, facilitated by increased G6P content, an activator of SPS, would recycle P(i) from the cytosol back to the chloroplast, maintaining ATP, RuBP and hence Pn. It is concluded that low sink strength imposes the primary limitation on photosynthesis in P(i)-deficient plants which restricts sucrose export and sucrose synthesis imposing an end-product synthesis limitation of photosynthesis.

Phosphorus Stress in Common Bean: Root Transcript and Metabolic Responses

“…Phosphate content and photosynthesis are related in several ways, and alteration of photosynthesis as a result of P starvation has been shown for several plant species, including common bean (Rychter and Randall, 1994;Mikulska et al, 1998). It has been shown that tobacco plants grown under P deficiency have reduced photosynthate demand in sink organs, resulting in carbohydrate accumulation and decrease in net photosynthesis (Pieters et al, 2001). Our data support the proposition of Morcuende et al (2007) that repression of photosynthesis may be a secondary response linked to lower demand of photosynthate and higher sugar levels during P limitation.…”

Physiological differences in Rhododendron calophytum seedlings regenerated in mineral soil or on fallen dead wood of different decaying stages

“…We found differences in foliage NSC composition among seedlings regenerated on different substrates, with seedlings on DW m having a higher starch concentration than those in S, and seedlings in S having a higher concentration than those on DW s , while the soluble sugars showed a contrary pattern. Many previous studies have reported that low P would reduce the translocation of calvin cycle intermediates out of the chloroplasts into the cytoplasm, where they are metabolized into starch and accumulated within the chloroplast, thus resulting in an increase in the starch concentration in the leaves (Pieters et al 2001;AlHamdani and Sirna 2008;Hammond and White 2008). The accumulation of starch within chloroplasts was found to be accompanied by the damage and disorientation of grana and thylakoids, as confirmed in our ultrastructural investigation (Fig.…”

“…Furthermore, we found that seedlings on DW s had a higher CE value than did seedlings in S, and seedlings in S had a higher value than those on DW m . Considering the similar photosynthetic pigment concentrations among the seedlings, we confirmed the view that phosphorus deficiency apparently does not affect photosynthesis through the chlorophyll content, but mainly by affecting the carboxylation capacity (Pieters et al 2001;Lin et al 2009). Moreover, low P might diminish the transportation of calvin cycle intermediates via the P translocator (Wissuwa et al 2005), leading to starch accumulation and the destruction of chloroplast, which, in return, would decrease the photosynthetic capacity.…”

“…Whether the differences in NSC levels are passively controlled through a simple source-to-sink flow or are actively regulated for coping with the ambient environment is another consideration when interpreting NSC levels (Bansal and Germino 2008). The observed decrease of net photosynthetic capacity in seedlings on DW m was accompanied by an increase in starch accumulation (Table 5), as previously reported in tobacco (Pieters et al 2001). This indicated that the foliage NSC levels in R. calophytum were not passively controlled through a simple source-to-sink flow, but, in contrast, the accumulation of starch within the chloroplasts was accompanied by damage and disorientation of grana and thylakoids, which, in return, led to a decrease in photosynthetic capacity.…”

Hastes florais de helicônia sob deficiência de macronutrientes

“…Pieters et al (2001) observaram que a omissão de P resultou em decréscimo na exportação de carbono das folhas (fonte), que foi quatro vezes inferior à exportação de carbono das folhas de plantas com suprimento adequado de P. O acúmulo do produto final (carboidratos) foi um fator limitante da fotossíntese em plantas sob deficiência de P. Existem evidências de que as plantas usam os carboidratos como recurso energético para o período reprodutivo e que a alocação de carboidratos pode variar consideravelmente entre espécies (Yee & Tissue, 2005).…”