Transpiration efficiency, W, the ratio of plant carbon produced to water transpired and carbon isotope discrimination of leaf dry matter, ∆ d , were measured together on 30 lines of the C 4 species, Sorghum bicolor, in the glasshouse and on eight lines grown in the field. In the glasshouse, the mean W observed was 4.9 mmol C mol -1 H 2 O and the range was 0.8 mmol C mol -1 H 2 O. The mean ∆ d was 3.0‰ and the observed range was 0.4‰. In the field, the mean W was lower at 2.8 mmol C mol -1 H 2 O and the mean ∆ d was 4.6‰. Significant positive correlations between W and ∆ d were observed for plants grown in the glasshouse and in the field. The observed correlations were consistent with theory, opposite to those for C 3 species, and showed that variation in ∆ d was an integrated measure of long-term variation in the ratio of intercellular to ambient CO 2 partial pressure, p i /p a . Detailed gas exchange measurements of carbon isotope discrimination during CO 2 uptake, ∆ A , and p i /p a were made on leaves of eight S. bicolor lines. The observed relationship between ∆ A and p i /p a was linear with a negative slope of 3.7‰ in ∆ A for a unit change in p i /p a . The slope of this linear relationship between ∆ A and p i /p a in C 4 species is dependent on the leakiness of the CO 2 concentrating mechanism of the C 4 pathway. We estimated the leakiness (defined as the fraction of CO 2 released in the bundle sheath by C 4 acid decarboxylations, which is lost by leakage) to be 0.2. We conclude that, although variation in ∆ d observed in the 30 lines of S. bicolor is smaller than that commonly observed in C 3 species, it also reflects variation in transpiration efficiency, W. Among the eight lines examined in detail and in the environments used, there was considerable genotype x environment interaction.

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their expression, and may demonstrate variation that is subsequently modified by adaptive traits. Adaptive traits In the rainfed lowlands, rice (Oryza sativa L.) develops roots under will be defined as those, such as root penetration index anaerobic soil conditions with ponded water, prior to exposure to aerobic soil conditions and water stress. Constitutive root system or osmotic adjustment (Zhang et al., 2001), which are development in anaerobic soil conditions has been reported to have expressed in response to water deficit or soil physical/ a positive effect on subsequent expression of adaptive root traits chemical barriers. Less research attention has been and water extraction during water stress. We examined effects of given to constitutive traits than to adaptive traits. phenotyping environment on identification of quantitative trait lociA deep and thick root system has been thought advan-(QTLs) for constitutive root morphology traits using 220 doubledtageous for improved drought tolerance in the rainfed haploid lines (DHLs) from the cross of 'CT9993-5-10-1-M' (CT9993; lowland ecosystem, based on extrapolation from experijaponica, upland adapted) ϫ 'IR62266-42-6-2' (IR62266; indica, lowence with upland rice (O'Toole, 1982; and Fukai and land adapted) in four greenhouse experiments. Broad sense heritabil-Cooper, 1995). Under anaerobic well-watered condiity (h 2 ) was 75, 60, and 64% on average for shoot biomass, deep root tions, root system development had a positive effect on morphology, and root thickness traits, respectively. Quantitative trait loci analysis identified 18 genomic regions associated with deep root subsequent plant growth during progressive water stress morphology traits, but only three were identified consistently across (Azhiri-Sigari et al., 2000; Kamoshita et al., 2000; and experiments. Three out of a total of eight QTLs for root thickness Hoque and Kobata, 1998). Azhiri-Sigari et al. (2000) traits were found in more than one experiment. The maximum genetic and Kamoshita et al. (2000) demonstrated genotypic effects caused by a single QTL were increments of 0.05 g of deep variation in constitutive root traits, and subsequent reroot mass below a 30-cm soil depth, 0.9% of deep root ratio, 1.6 cm sponses of adaptive root traits, especially in deeper soil of rooting depth, and 0.09 cm of root thickness, with phenotypiclayers. Greater root elongation to depth resulted in imvariation explained by a single QTL ranging from 6.8 to 51.8%. The proved water extraction. Improved seedling vigor was results demonstrate the importance of phenotyping environment and also valuable to growth afterward (Mitchell et al., 1998).suggest prospects for selection of QTLs for deep root morphology,

Developmental plasticity in lateral roots may be one of the key traits for the growth of rice plants under soil moisture fluctuations. We aimed to examine responses in seminal root system development to changing soil moisture for diverse rice cultivars. Special attention was paid to the two different types of lateral roots ; the generally long, thick L type capable of branching into higher orders, and the non-branching S type. Plants were grown in half-split polyvinyl chloride tubes fixed with transparent acrylic plate for root observation under glasshouse conditions. When plants were grown first under drought conditions, then rewatered, the seminal root system development in terms of dry weight and total length was promoted as compared with plants grown under continuously well-watered conditions in IRAT 109 and Dular, drought tolerant cultivars. Promoted production of L type lateral roots mainly contributed to the development of the longer seminal root system. Plants exposed to soil submergence before they were grown under drought conditions did not show such promoted responses in these two cultivars. However, in KDML 105, a drought tolerant cultivar, the production of especially L type laterals was substantially promoted under drought and rewatered conditions. Honenwase was characterized by the shallow root system and great reduction in root system length when soil moisture becomes limited. These facts show that genotypic variations exist in the plastic response of rice seminal root system and that the L type lateral root plays a key role in manifestation of this plasticity.

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