Sink–Source Balance and Down-Regulation of Photosynthesis in Raphanus sativus: Effects of Grafting, N and CO2

Abstract

To clarify whether excessive accumulation of total non-structural carbohydrate (TNC) causes down-regulation of photosynthesis in Raphanus sativus, we manipulated sink-source balance to alter TNC levels in source leaves and examined its effects on photosynthetic characteristics, whole-plant biomass allocation and anatomical characteristics of leaves and petioles. Comet and Leafy varieties with large and small hypocotyls were reciprocally grafted to change hypocotyl sink strength. They were grown at high or low nitrogen (N) availability and at elevated or ambient CO2. Maximum photosynthetic rate, which was highly correlated with Rubisco and leaf N contents, was hardly correlated with TNC across the grafting combinations and growth conditions. Biomass allocation to petioles and hypocotyls and accumulation of TNC in each organ were significantly higher at low N. TNC and structural carbohydrates such as cellulose and hemicellulose were higher and the proportion of intercellular air space in source leaves was lower at low N and elevated CO2. We conclude that excess TNC does not cause severe down-regulation of photosynthesis, and cell walls and petioles are also major carbohydrate sinks responding to changes in sink-source and carbon-nitrogen balances, which contribute to alleviating further accumulation of TNC to avoid its negative effects in source leaves.

Effect of Seedling Nitrogen Condition on Subsequent Vegetative Growth Stages and Its Relationship to the Expression of Nitrogen Transporter Genes in Rice

“…In several crop species, the source–sink relationship is influenced by many factors such as nutrients, irrigation, light, defoliation [ 53 , 54 , 55 ]. Basically, roots are the source of inorganic N and are sinks for carbon (C), while leaves are often the major sink organs for N and sources of C [ 53 ].…”

“…Since direct sowing offers promise in place of transplanting in single rice cropping owing to its lower N requirement [49], this result will be helpful for rice production strategies. In several crop species, the source-sink relationship is influenced by many factors such as nutrients, irrigation, light, defoliation [53][54][55]. Basically, roots are the source of inorganic N and are sinks for carbon (C), while leaves are often the major sink organs for N and sources of C [53].…”

Elevated CO2 induces age-dependent restoration of growth and metabolism in gibberellin-deficient plants

“…Stem storage of excess photoassimilate during periods of low sink strength buffers against source-sink changes during the different stages of growth (Slafer 2003). Imbalance between source and sink can lead to downregulation of photosynthesis due to accumulation of non-structural carbohydrates (such as soluble sugars and starch) in leaves (Stitt and Krapp 1999; Ainsworth and Bush 2011; Sugiura et al 2017). Thus, the allocation of carbohydrates to the stem could be a way of delaying photosynthetic inhibition in gib-1 mutants.…”

Elevated CO2-induced changes in mesophyll conductance and anatomical traits in wild type and carbohydrate-metabolism mutants of Arabidopsis

“…The two or three frozen leaf discs were used to determine the contents of Rubisco and cell wall materials as described previously (Mizokami et al , 2015; Sugiura et al , 2017). The frozen discs were homogenized in Tris–HCl buffer (62.5 mM, pH 6.8) using a Multi-beads Shocker (Yasui Kikai) to extract Rubisco.…”