No abstract

Mycorrhizal-induced growth depression of plants in high-P soil has been reported in many species. The carbon costs of factors contributing to this growth depression were analyzed in Volkamer lemon (Cifrus volkameriana Tan. & Pasq.) colonized by the mycorrhizal (M) fungus Clomus infraradices Schenck and Smith. M and nonmycorrhizal (NM) plants were each grown at two P-supply rates. Carbon budgets of M and NM plants were determined by measuring whole-plant carbon assimilation and respiration rates using gas-exchange techniques. Biomass, M colonization, tissue-P concentration, and total fatty acid concentration in the fibrous roots were determined. Construction costs of the fibrous roots were estimated from heat of combustion, N, and ash content. Rootgrowth respiration was derived from daily root growth and rootconstruction cost. M and NM plants grown in high-P soil were similar in P concentration, daily shoot carbon assimilation, and daily shoot dark respiration. At 52 d after transplanting (DAT), however, combined daily root plus soil respiration was 37% higher for M than for NM plants, resulting in a 20% higher daily specific carbon gain (mmol COz [mmol carbonl-' d-') in NM than M plants. Estimates of specific carbon gain from specific growth rates indicated about a 10% difference between M and NM plants. Absolute values of specific carbon gain estimated by whole-plant gas exchange and by growth analysis were in general agreement. At 52 DAT, M and NM plants at high P had nearly identical whole-plant growth rates, but M plants had 19% higher root dry weight with 10% higher daily rates of root growth. These allocation differences at high P accounted for about 51% of the differences in root/soil respiration between M and NM plants. Significantly higher fatty acid concentrations in M than NM fibrous roots were correlated with differences in construction costs of the fibrous roots. Of the 37% difference in daily total root/soil respiration observed between high-P M and NM plants at 52 DAT, estimated daily growth respiration accounted for only about 16%, two-thirds of which was associated with construction of lipid-rich roots, and the remaining one-third with greater M root growth rates. Thus, of the 37% more root/soil respiration associated with M colonization of high-P plants, 10% was directly attributable to building lipid-rich roots, Florida Agricultural Experiment Station Joumal Senes No. R-02500.

Genetic improvement in grain yield has been intensively studied in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.]. Such information is limited in rice (Oryza sativa L.). The objective of this study was to determine the trend in the yield of rice cultivars–lines developed since 1966. Twelve cultivars–lines were grown at the International Rice Research Institute (IRRI) farm and the Philippine Rice Research Institute farm during the dry season of 1996. Seven cultivars–lines were grown at IRRI farm in the dry season of 1998. Growth analyses were performed at key growth stages, and yield and yield components were determined at physiological maturity. Regression analysis of yield versus year of release indicated an annual gain in rice yield of 75 to 81 kg ha−1, equivalent to 1% per year. The highest yields obtained with the most recently released cultivars was 9 to 10 Mg ha−1, which is equivalent to reported yields of IR8 and other early IRRI cultivars obtained in the late 1960s and early 1970s at these same sites. Therefore, the 1% annual increase in yield may not represent genetic gain in yield potential. The increasing trend in yield of cultivars released before 1980 was mainly due to the improvement in harvest index (HI), while an increase in total biomass was associated with yield trends for cultivars–lines developed after 1980. Results suggest that further increases in rice yield potential will likely occur through increasing biomass production rather than increasing HI.

SummaryHigh night temperatures (HNTs) can reduce significantly the global rice (Oryza sativa) yield and quality. A systematic analysis of HNT response at the physiological and molecular levels was performed under field conditions.Contrasting rice accessions, N22 (highly tolerant) and Gharib (susceptible), were evaluated at 22°C (control) and 28°C (HNT). Nitrogen (N) and nonstructural carbohydrate (NSC) translocation from different plant tissues into grains at key developmental stages, and their contribution to yield, grain-filling dynamics and quality aspects, were evaluated. Proteomic profiling of flag leaf and spikelets at 100% flowering and 12 d after flowering was conducted, and their reprogramming patterns were explored.Grain yield reduction in susceptible Gharib was traced back to the significant reduction in N and NSC translocation after flowering, resulting in reduced maximum and mean grain-filling rate, grain weight and grain quality. A combined increase in heat shock proteins (HSPs), Ca signaling proteins and efficient protein modification and repair mechanisms (particularly at the early grain-filling stage) enhanced N22 tolerance for HNT.The increased rate of grain filling and efficient proteomic protection, fueled by better assimilate translocation, overcome HNT tolerance in rice. Temporal and spatial proteome programming alters dynamically between key developmental stages and guides future transgenic and molecular analysis targeted towards crop improvement.

The chlorophyll meter provides a simple, quick, and nondestructive method to estimate leaf N status of rice (Oryza sativa L.), but the linear relationship between leaf N concentration on a dry‐weight basis (Ndw) and the meter reading differs depending on developmental stage and genotype. The objective was to determine whether prediction of (Ndw) with the chlorophyll meter can be improved by a simple correction for specific leaf weight (SLW). Leaf N status was estimated by a chlorophyll meter (SPAD‐502) and measured directly by micro‐Kjel‐dahl procedure. Specific leaf weight was calculated as the ratio of dry weight to leaf area. In one field study ‘IR72’, measurements were taken at midtillering, panicle initiation, and flowering stages on the uppermost fully expanded leaves of both N‐deficient and N‐sufficient plants. There was a linear relationship between Ndw and SPAD values at each stage, but regression lines differed significantly between growth stages. Based on pooled data from all stages, the degree of linear fit was poor (r2 = 0.49). Adjusting SPAD values for SLW (SPAD/SLW) improved the prediction of Ndw (r2 = 0.93). For another set of measurements made on the flag leaves of five genotypes grown in the field and greenhouse, prediction of Ndw was also improved, from r2 = 0.51 based on SPAD values alone to r2 = 0.87 based on the SPAD/SLW ratio. These results demonstrate that SLW influences the prediction of Ndw by the chlorophyll meter, and that the adjustment of SPAD values for SLW greatly increases the accuracy of the prediction. However, when SPAD values are adjusted for SLW, the chlorophyll meter's estimate of Ndw is no longer as quick, simple, or nondestructive as the nonadjusted SPAD values.

Leaf hydraulic conductance (K ) and mesophyll conductance (g ) both represent major constraints to photosynthetic rate (A), and previous studies have suggested that K and g is correlated in leaves. However, there is scarce empirical information about their correlation. In this study, K , leaf hydraulic conductance inside xylem (K ), leaf hydraulic conductance outside xylem (K ), A, stomatal conductance (g ), g , and anatomical and structural leaf traits in 11 Oryza genotypes were investigated to elucidate the correlation of H O and CO diffusion inside leaves. All of the leaf functional and anatomical traits varied significantly among genotypes. K was not correlated with the maximum theoretical stomatal conductance calculated from stomatal dimensions (g ), and neither g nor g were correlated with K . Moreover, K was linearly correlated with g and both were closely related to mesophyll structural traits. These results suggest that K and g are related to leaf anatomical and structural features, which may explain the mechanism for correlation between g and K .

To understand the underlying mechanism for plasticity in root to shoot ratio (R/S) in response to drought stress, two rice cultivars, Zhenshan97 (drought susceptible) and IRAT109 (drought resistant), were grown hydroponically, and R/S, carbohydrate concentration and partitioning, and activities of enzymes for sucrose conversion in seedlings exposed to drought stress condition (DS) imposed by polyethylene glycol 6000 were investigated. The R/S significantly increased under DS in comparison with that under well-watered condition. The proportion of dry matter and soluble sugar of roots markedly increased under DS. The R/S was negatively correlated with proportion of soluble sugar in stems, and positively with the proportions of soluble sugar and starch in roots. Drought stress condition significantly increased leaf sucrose-phosphate synthase (EC 2.4.1.14) activity and root acid and neutral/ alkaline invertase (EC 3.2.1.26) activity. The R/S was positively correlated with leaf sucrose-phosphate synthase and root acid invertase activity, and negatively with leaf sucrose synthase activity in the cleavage direction. Our results indicate that the increase in R/S in response to DS is closely associated with the higher proportion of dry matter and soluble sugar in roots, and this occurs via an increase in leaf sucrose-phosphate synthase and root invertase activity, and thus more sucrose is available for transport from leaves to roots.

seedling-vigor is important for crop establishment. There have been reported quantitative trait locus (QTL) analyses on seedling-vigor related morphological traits. However, physiological understanding of these detected QTLs is rather limited. In this study, we employed a recombinant inbred population to detect QTLs for seedling-vigor traits and physiological traits related to seedling-vigor. Germination rate and seedling growth were measured to quantify seedling-vigor. Total amylase activity, alpha-amylase activity, reducing sugar content, root activity and seed weight were determined. Correlations were observed between the seedling-vigor and physiological traits. QTL analysis reveals that the intervals of RG393-C1087-RZ403 on chromosome 3, C246-RM26-C1447 and R830-R3166-RG360-C734b on chromosome 5, and the interval of Waxy on chromosome 6 are the four main chromosomal regions controlling seedling-vigor. Several QTLs for amylase activities, reducing sugar content and root activity were localized in the similar regions as the QTLs for seedling-vigor. The results suggest that these traits were under the control of pleiotropic and/or closely linked QTLs. The implications of the results in the understanding of the physiological basis of seedling-vigor were discussed.