Xylem sap analysis reveals new facts of salt tolerance in rice genotypes

Abstract

Salinity damage in rice and other salt-sensitive species is due to excessive transport of NaCl through the root system to the leaves and consequently low salt transport to the shoot can be a major trait determining salt resistance. Since the rapid uptake of sodium ions is such a crucial part of the response of rice to salinity, physiological experiments were carried out to compare bypass flow in two genotypes of rice (IR4630 and IR15324) differing in salt tolerance, because it has been suggested that an apoplastic pathway, bypass flow, is a major contributory pathway for Na + entrance into rice plants. Experiments on the youngest fully expanded photosynthetic leaf (the third from the base), using PTS as a tracer for apoplastic movement and Philaenus spumarius (a xylem-feeding insect) as a means to sample the xylem sap, did not demonstrate any apparent difference in bypass flow between the two lines. The similarity of Na + concentration in the xylem sap of both genotypes paralleled the results of PTS (a fluorescent dye used as an apoplastic tracer for the transpiration stream) measurements. Despite the similarity of Na + concentration in the xylem sap of the third leaves, the Na + concentration in the bulk of these leaves of IR15324 plants (the sensitive line) was about twice that of IR4630 (the tolerant line). Measurements of transpiration over 8 d of salinisation showed the similarity of rates in both lines providing evidence that the greater accumulation of NaCl in IR15324 than in IR4630 plants was unlikely to be due to a difference in the delivery of salt to the leaves by an apoplastic route. Results of the current work suggest that the difference in salt tolerance might be a consequence of damage to leaves 1 and 2 of IR15324 that allowed Na + to leak into the phloem -and consequently move to leaf 3. Key words: bypass flow, Na + transport, Oryza sativa, Philaenus spumarius, salt stress, xylem concentration A análise da seiva do xilema revela novos fatos sobre a tolerância a sais em genótipos de arroz: Danos por salinidade em arroz e em outras espécies sensíveis ao estresse salino são devidos ao transporte excessivo de NaCl, desde o sistema radicular até as folhas; assim, baixo transporte de sais para os ramos pode ser uma característica importante na determinação da resistência salina. Uma vez que a absorção rápida dos íons sódio é uma parte crucial da resposta do arroz à salinidade, conduziram-se experimentos para comparar-se o fluxo (apoplástico) pelo desvio (bypass flow) de dois genótipos de arroz (IR4630 e IR15324) diferindo na tolerância salina, visto ser esse fluxo uma das principais rotas pelas quais o Na + entra nas raízes de arroz. Experimentos com as folhas mais jovens completamente expandidas (terceira a partir do ápice), usando-se o PTS (um corante fluorescente como um traçador apoplástico para a corrente transpiratória) e o inseto Philaenus spumarius para amostragens da seiva do xilema, não demonstraram qualquer diferença aparente no chamado fluxo pelo desvio entre os dois genótipos. A similarida...

Overexpression of a C3HC4‐type E3‐ubiquitin ligase contributes to salinity tolerance by modulating Na+ homeostasis in rice

“…This difference was not observed under normal conditions but was lower in OsRFPHC‐4 ‐overexpressing plants than in WT and osrfphc‐4 plants under 100 mM NaCl (Figure 5C,F). The Na + transport capacity from the roots to the shoots in plants is an important factor in determining salt tolerance (Roshandel, 2007). Therefore, we performed xylem sap analysis to identify the transport of Na + and K + in the three genotypes of plants using ICP analysis under 0 or 100 mM NaCl (Figure 5G‐I).…”

“…Xylem sap analysis may be useful for evaluating salt tolerance by measuring the uptake of Na + ions by the plant xylem. For example, the salt‐tolerant variety showed approximately two times lower Na + content in the xylem than the salt‐sensitivity variety in rice (Roshandel, 2007). In our study, xylem sap analysis in OsRFPHC‐4 ‐overexpressing plants showed low absorption of Na + and high K + content compared to WT plants under saline conditions; however, osrfphc‐4 showed high Na + and low K + content compared to WT plants.…”