Search

Genetic gain is characteristically slow when selecting directly for increased grain yield under water-limited conditions. Genetic increases in grain yield may be achieved through increases in aerial biomass following selection for greater transpiration efficiency (TE as aerial biomass/water transpired). Strong negative correlations between TE and carbon isotope discrimination (Δ) in wheat (Triticum aestivum L.) suggest that selection of progeny with low Δ may increase TE and aerial biomass under water-limited conditions. This study investigated how early generation, divergent selection for Δ affected aerial biomass and grain yield among 30 low- and 30 high-Δ, `Hartog'-like, B[C.sub.2][F.sub.4:6] progeny and the recurrent, high-Δ parent Hartog. Lines were evaluated in nine environments varying for seasonal rainfall (235-437 mm) and hence grain yield (1.3-6.2 Mg/ha). Selection for low Δ in early generation progeny was associated with significantly (P < 0.01) smaller Δ, higher grain yield (+5.8%), aerial biomass (+2.7%), harvest index (+3.3%), and kernel size (+4.8%) in tested lines. Kernel number was the same for low- and high-Δ selected groups. Grain yield advantage of the low Δ group increased with reductions in environment mean yield (r = -0.89, P < 0.01) and total seasonal rainfall (r = -0.85, P < 0.01) indicating the benefit of low Δ, and therefore high TE for genetic improvement of grain yield in lower rainfall environments. Narrow-sense heritability on a single-plot basis was much greater for Δ ([h.sup.2] = 0.63 ± 0.10) than for either aerial biomass (0.06 ± 0.05) or grain yield (0.14 ± 0.04). Strong genetic correlations between Δ and both aerial biomass ([r.sub.g] = -0.61 ± 0.14) and grain yield (-0.58 ± 0.12) suggest Δ could be used for indirect selection of these traits in early generations. Selection of low Δ (high TE) families for the advanced stages of multiple-environment testing should increase the probability of recovering higher-yielding wheat families for water-limited environments., WHEAT IS COMMONLY GROWN in regions where grain yields are limited by low seasonal rainfall (Stephens and Lyons, 1998; van Ginkel et al., 1998). Growers in these regions rely on [...]

Greater yield per unit rainfall is one of the most important challenges in dryland agriculture. Improving intrinsic water-use efficiency ([W.sub.T]), the ratio of C[O.sub.2] assimilation rate to transpiration rate at the stomata, may be one means of achieving this goal. Carbon isotope discrimination (Δ[sup.13]C) is recognized as a reliable surrogate for [W.sub.T] and there have now been numerous studies which have examined the relationship between crop yield and [W.sub.T] (measured as Δ[sup.13]C). These studies have shown the relationship between yield and [W.sub.T] to be highly variable. The impact on crop yield of genotypic variation in [W.sub.T] will depend on three factors: (i) the impact of variation in [W.sub.T] on crop growth rate, (ii) the impact of variation in [W.sub.T] on the rate of crop water use, and (iii) how growth and water use interact over the crop's duration to produce grain yield. The relative importance of these three factors will differ depending on the crop species being grown and the nature of the cropping environment. Here we consider these interactions using (i) the results of field trials with bread wheat (Triticum aestivum L.), durum wheat (T. turgidum L.), and barley (Hordeum vulgare L.) that have examined the association between yield and Δ[sup.13]C and (ii) computer simulations with the SIMTAG wheat crop growth model. We present details of progress in breeding to improve [W.sub.T] and yield of wheat for Australian environments where crop growth is strongly dependent on subsoil moisture stored from out-of-season rains and assess other opportunities to improve crop yield using WT., YIELDS WOULD BE GREATER in many cropping regions if more water was available for crop growth. Water is essential to plant growth because it provides the medium within which most [...]

Seedling characteristics between germination and the two leaf stage are principally responsible for variation in early leaf area development, or vigor, among temperate cereals. There is compelling evidence that this greater vigor confers substantial benefits in crops grown in dryland Mediterranean-type environments. Experiments were conducted to determine the important factors that are likely to be responsible for variation in early vigor among barley (Hordeum vulgare L.), bread wheat (Triticum aestivum L.), durum wheat (T. turgidum var durum), triticale (Triticosecale Wittm.), and oat (Avena sativa L.) cultivars. Variation among and within species in the following factors was examined: (i) time to emergence, (ii) efficiency of utilization of seed reserves, (iii) shoot biomass formed from seed reserves and attributed to a larger plumule relative to the radicle, (iv) embryo size, (v) size of leaf epidermal cells, and (vi) leaf expansion rate during the lower temperatures in winter. Seeds of a similar mass were used in all experiments to eliminate the effects of seed size. The superior vigor of barley and triticale compared with the two wheat species and oat previously found in field experiments were confirmed. The size of the embryo was the single most important factor to account for differences in vigor among the species. It accounted for [approximately equal to] 90% of the variation in vigor between the species. Earlier emergence in barley accounted for the remaining variation in vigor. The breadth of the first seedling leaf is suggested as an indirect estimate of early vigor that integrates embryo size and specific leaf area and could be used in a breeding program to increase the vigor of cereals.

Because of its relationship with water-use efficiency (W), carbon isotope discrimination in leaves (Delta) was proposed to be useful for identifying genotypes with greater water-use efficiency. In this study we examined the relationship between W and Delta in four peanut (Arachis hypogaea L.) genotypes. The genotypes were grown in and around mini-lysimeters embedded in soil and were subjected to two drought regimes, intermittent and prolonged water deficit conditions, by varying the irrigation timing and amount. Automated rain-out shelters prevented any rain from reaching the experimental plots during the treatment period. The mini-lysimeters allowed accurate measurement of water use and total dry matter (including roots) in a canopy environment. Water-use efficiency, which ranged from 1.81 to 3.15 g kg to the -1, was negatively correlated with Delta, which ranged from 19.1 to 21.8%. Tifton-8 had the highest W (3.15 g kg to the -1) and Chico the lowest (1.81 g kg to the -1), representing a variation in W of 74% among genotypes. Variation in W arose mainly from genotypic differences in total dry matter production rather than from differences in water use. It is concluded that Delta is a useful trait for selecting genotypes of peanut with improved W under drought conditions in the field. A strong negative relationship existed between W and specific leaf area (SLA, cm cubed g to the -1) and between Delta and SLA, indicating that genotypes with thicker leaves had greater W. SLA could therefore be used as a rapid and inexpensive selection index for high W in peanut where mass spectrometry facilities are not available.

Carbon isotope composition may be useful for selection in plant breeding. Theory predicts that C discrimination (delta) by leaves can be associated with the ratio of photosynthesis (as indicated by [CO.sub.2] assimilation rate, A) to leaf conductance (g) to diffusion of water vapor or [CO.sub.2]. In earlier studies with a cowpea (Vigna unguiculata [L.] Walp.) mutant and parent, drought-induced differences in [delta] were associated with expected differences in A/g based on theory, whereas genotypic differences in [delta] were not. Studies were conducted for 2 yr with a broader range of genotypes to determine whether these conclusions are generally valid. Eighteen cowpea accessions were grown under weekly irrigation or stored soil moisture in four randomized split blocks at Riverside, CA, in 1987. Twelve of these cowpea accessions were studied under similar conditions in 1988. Gas exchange measurements were made of A and leaf conductance to water vapor (g.sub.h), and leaves were sampled for [delta]. Significant drought-induced reduction in [delta] of 1.6 x [10.sup.-3] was observed and an increase in A/g of 42% which is similar to the value (36%) predicted based upon the change in [delta]. The drought-induced increase in A/g was due to substantial decreases in [g.sub.h], which more than compensated for a 59% reduction in A. Genotypic differences in [delta] of up to 2 x [10.sup.-3] were observed which were predicted to be associated with 43% higher A/ g; however, genotypic differences in A/g were small, and genotypic means of A/g and [delta] were not associated as expected based on theory. Genotypic differences in [delta] were more consistent than differences in A/g, A, or [g.sub.h] and should be easier to select in breeding, but the physiological basis for the genotypic differences in [delta] has not been elucidated for cowpea., The genotypic variation in gas exchange efficiency (A/g), photosynthetic rate as indicated by carbon dioxide assimilation (A) and total diffusive conductance to carbon dioxide (g) were investigated. These gas exchange attributes were also tested for possible associations with carbon-isotope discrimination (delta). The experiments were carried out on 18 cowpea accessions representing a broad range of genotypes over a two-year period. The results showed that the genotypic means of delta and A/g were not associated. Among the parameters investigated, delta showed the most consistency in genotypic differences, making it more ideal for selection.

Xylem-Tapping Mistletoes: Water or Nutrient Parasites? Mistletoes are obligate epiphytic parasites of higher plant species (1). While a few mistletoes derive substantial nutritional benefits by connecting directly to the host's [...]

ABSTRACT A mathematical model of stomatal conductance is presented. It is based on whole-plant and epidermal hydromechanics, and on two hypotheses: (1) the osmotic gradient across guard cell membranes is proportional to the concentration of ATP in the guard cells; and (2) the osmotic gradient that can be sustained per unit of ATP is proportional to the turgor pressure of adjacent epidermal cells. In the present study, guard cell [ATP] is calculated using a previously published model that is based on a widely used biochemical model of C[sub 3] mesophyll photosynthesis. The conductance model for Vicia faba L. is parameterized and tested As with most other stomatal models, the present model correctly predicts the stomatal responses to variations in transpiration rate, irradiance and intercellular CO[sub 2] . Unlike most other models, however, this model can predict the transient stomatal opening often observed before conductance declines in response to decreases in humidity, soil water potential, or xylem conductance. The model also explicitly accommodates the mechanical advantage of the epidermis and correctly predicts that stomata are relatively insensitive to the ambient partial pressure of oxygen, as a result of the assumed dependence on ATP concentration. [ABSTRACT FROM AUTHOR]

ABSTRACT A model has been derived for the enrichment of heavy isotopes of water in leaves, including progressive enrichment along the leaf. In the model, lighter water is preferentially transpired leaving heavier water to diffuse back into the xylem and be carried further along the leaf. For this pattern to be pronounced, the ratio of advection to diffusion (Péclet number) has to be large in the longitudinal direction, and small in the radial direction. The progressive enrichment along the xylem is less than that occurring at the sites of evaporation in the mesophyll, depending on the isolation afforded by the radial Péclet number. There is an upper bound on enrichment, and effects of ground tissue associated with major veins are included. When transpiration rate is spatially nonuniform, averaging of enrichment occurs more naturally with transpiration weighting than with area-based weighting. This gives zero average enrichment of transpired water, the modified Craig–Gordon equation for average enrichment at the sites of evaporation and the Farquhar and Lloyd (In Stable Isotopes and Plant Carbon-Water Relations , pp. 47–70. Academic Press, New York, USA, 1993) prediction for mesophyll water. Earlier results on the isotopic composition of evolved oxygen and of retro-diffused carbon dioxide are preserved if these processes vary in parallel with transpiration rate. Parallel variation should be indicated approximately by uniform carbon isotope discrimination across the leaf. [ABSTRACT FROM AUTHOR]

ABSTRACT The effectiveness of several leaf water models (‘string-of-lakes’, ‘desert river’ and the Farquhar–Gan model) are evaluated in predicting the enrichment of leaf water along a maize leaf at different humidities. Progressive enrichment of both vein xylem water and leaf water was observed along the blade. At the tip, the maximum observed enrichment for the vein water was 17.6‰ at 50% relative humidity (RH) whereas that for the leaf water was 50‰ at 34% RH and 19‰ at 75% RH. The observed leaf water maximum was a fraction (0.5–0.6) of the theoretically possible maximum. The ‘string-of-lakes’ and ‘desert river’ models predict well the variation of leaf water enrichment pattern with humidity but overestimate the average enrichment of bulk leaf water. However, the Farquhar–Gan model gives good prediction for these two aspects of leaf water enrichment. Using the anatomical dimensions of vein xylem overestimates the effective longitudinal Péclet number (P [sub l] ). Possible explanations for this discrepancy between the effective and the xylem-based estimate of P [sub l] are discussed. The need to characterize the heterogeneity of transpiration rate over the leaf surface in studies of leaf water enrichment is emphasized. The possibility that past atmospheric humidity can be predicted from the slope of the Δ [sup 18] O spatial variation of leaf macrofossils found in middens is proposed. [ABSTRACT FROM AUTHOR]

The objectives of this study were: (1) to quantify the genetic variation in foliar carbon isotope composition (δ13C) of 122 clones of ca. 4-year-old F1 hybrids between slash pine (Pinus elliottii Engelm var. elliottii) and Caribbean pine (Pinus caribaea var. hondurensis Barr. et Golf.) grown at two field experimental sites with different water and nitrogen availability in southeast Queensland, Australia, in relation to tree growth and foliar nitrogen concentration (Nmass); and (2) to assess the potential of using δ13C measurements, in the foliage materials collected from the clone hedges at nursery and the 4-year-old tree canopies in the field, as an indirect index of tree water use efficiency for selecting elite F1 hybrid pine clones with improved tree growth. There were significant differences in foliar δ13C between the nursery hedges and the 4-year-old tree canopies in the field, between the summer and winter seasons, between the two experimental sites, and between the upper outer and lower outer canopy positions sampled. This indicates that δ13C measurements in the foliage materials are significantly influenced by the sampling techniques and environmental conditions. Significant differences in foliar δ13C, at the upper outer canopy in both field experiments in summer and winter, were detected between the clones, and between the female parents of the clones. Clone means of tree height at age ca. 3 years were positively related to those of the upper outer canopy δ13C at both experimental sites in winter, but only for the wetter site in summer. There were positive, linear relationships between clone means of canopy δ13C and those of canopy Nmass, indicating that canopy photosynthetic capacity might be an important factor regulating the clonal variation in canopy δ13C. Significant correlations were found between clone means of canopy δ13C at both experimental sites in summer and winter, and between those at the upper outer and lower outer canopy positions. Mean clone δ13C for the nursery hedges was only positively related to mean clone stem diameter at 1.3 m height at age 3 years on the wetter site. The clone by site interaction for foliar δ13C at the upper outer canopy was significant only in summer. Overall, the relatively high genetic variance components for foliar δ13C and significant, positive correlations between clone means of foliar δ13C and tree growth have highlighted the potential of using foliar δ13C measurements for assisting in selection of the elite F1 hybrid pine clones with improved tree growth. [Copyright &y& Elsevier]