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Salinity is a worldwide problem for rice (Oryza sativa L.) cultivation, and a number of breeding programs targeting increased salt tolerance are ongoing. A new trait-based mathematical model for salt stress on rice was recently proposed, characterized by a high level of detail in the description of physiological mechanisms dealing with crop response to salinity. In this study, dedicated growth chamber experiments were performed where three rice cultivars with different degrees of tolerance were grown under different salinity levels. The aim was to improve the understanding of physiological mechanisms like Na⁺ uptake and sequestration in structural tissues, and to validate the model using new datasets where temporal dynamics in plant response to salt stress were analyzed. Model evaluation demonstrated strong agreement between measured and simulated dry weights of plant organs (e.g., R² = 0.88–0.97 for aboveground biomass), [Na⁺] in plant tissues (R² = 0.73–0.88), and green leaf area index (R² = 0.71–0.99). These results demonstrate the reliability of the model and support its adoption within studies aimed at analyzing or predicting the response of different cultivars to temporal dynamics of Na⁺ concentration in soil and water.

Delaying leaf senescence is an effective strategy for increasing cereal production, particularly under water- limited conditions. A set of 72 nonsenescent (stay green) genotypes of sorghum [Sorghum bicolor (L.) Moench] was evaluated for pattern of postflowering leaf senescence in replicated field experiments during the 1998-1999 and 1999-2000 post-rainy seasons at the International Crops Research Institute for Semi-Arid Tropics to identify superior sources of stay green. Individual leaves of three plants per plot were scored visually for leaf senescence at weekly intervals from flowering until harvest maturity. Leaf senescence patterns were determined by fitting logistic or linear functions to weekly percent green leaf area (% GLA), from which % GLA at 15, 30, and 45 d after flowering were estimated. On the basis of estimated % GLA at 15, 30, and 45 d after flowering in the two years, genotypes clustered into five groups. The clusters retained 74% of the total variation in % GLA, despite differences in the pattern of development of stress and subsequent leaf senescence pattern between the two years. Cluster I genotypes, which combined nonsenescence with drought escape, through early flowering, were superior to Cluster 2 genotypes, which were nonsenescent but late flowering, in the more severely stressed year when drought escape was important, but not in the milder stress year when escape was less of a factor. The experiment identified several (e.g., IS 22380, QL 27, QL 10, E36 X R16 8/1) tropically adapted lines with stay-green expression equivalent to those of the best temperate lines B 35 and KS 19.

Visual ratings that can be taken quickly in the field provide information for evaluation and selection that could not otherwise be obtained due to time or cost constraints of quantitative techniques. We evaluated the accuracy of two subjective visual rating systems that assess green leaf retention in sorghum [Sorghum bicolor (L.) Moench] under drought stress in the field: Green leaf retention (GLR) estimates the percentage of total laminar leaf area that is green; number of green leaves (NGL) estimates the number of green leaves

An evaluation was made of leaf diffusive resistance and chlorophyll concentrations of dark green (normal) and pale green (chlorophyll deficient) phenotypes of a soybean [Glycine max (L.) Merrill] mutant when grown 10- and 30-klux light intensities. When grown at the 10-klux intensity the two leaf types had similar mesophyll and total CO₂ diffusive resistances and similar net photosynthetic rates per unit leaf area at normal ambient CO₂ concentrations (e.g., 300 µl/liter). Chloropyhll concentration of the pale green leaf was one-fourth that of the dark green leaf. On a chlorophyll basis, rate of CO₂ fixation was four times faster in the pale green type than in the dark green type. The similarity in net photosynthetic rates per unit leaf area of the two phenotypes when grown at the 10-klux light intensity was the result of (1) similar CO₂ diffusive resistances and (2) the ability of the photosynthetic apparatus of the pale green leaf to fix CO₂ as rapidly as that of the dark green leaf, inspite of a much lower amount of chlorophyll. When grown at the 30-klux light intensity, chlorophyll concentration of the pale green phenotype progressively declined, to one-tenth the amount at the 10-klux intensity. Chlorophyll concentration of the dark green phenotype was not significantly affected by light intensity. Photosynthesis of the pale green leaf grown at the 30-klux intensity became CO₂ saturated at concentrations well below 300 µl/liter. Therefore, light intensity during leaf growth strongly influenced the relative CO₂ assimilation rates of the dark green and pale green phenotypes.