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The requirement for simultaneous increases in stover as well as grain yields in pearl millet in arid zone environments means that conventional selection for grain yield improvement through increased harvest index (HI) is not applicable to such environments. In addition, there is a need to retain the adaptive traits present in local landrace germplasm, so that new cultivars for the arid zone do not trade increased productivity for reduced yield stability and increased risk of crop failure. This research was designed to test the hypothesis that it will be possible to meet these requirements by exploiting heterosis for overall biomass production in topcross hybrids (TCH) made with adapted, landrace-derived pollinators and dual purpose male-sterile seed parents, which partition the extra biomass of their hybrids equally to grain and stover. General combining ability (GCA) estimates for seven landrace-derived populations/varieties, derived from multi-environment tests in arid zone environments, indicated that selection history played a large role in determining GCA for both biomass and HI, with prior selection (for grain yield) favoring GCA for HI at the expense of GCA for biomass. A similar analysis of a set of male-sterile seed parents indicated a wide range of GCA for both grain and stover yields, with a similar tradeoff of GCA for one trait against GCA for the other. It was, however, possible to identify several parental lines with a positive GCA for biomass, achieved by a positive GCA for growth rate, and neutral GCA for HI, resulting in positive/neutral GCA for both stover and grain yields. A limited test of the ability of parental GCA to predict heterosis in TCH indicated that heterosis for stover yield was closely related to pollinator GCA for stover yield, and heterosis for grain yield was related to both pollinator and A-line GCA for HI. The same test confirmed the original hypothesis that crosses of parents with positive GCA for biomass/growth rate and neutral GCA for HI could produce TCH with positive heterosis for grain yield without an off-setting negative heterosis for stover yield. The frequency of such parental lines was limited, however.

Pearl millet landraces from Rajasthan, India, yield significantly less than improved cultivars under optimum growing conditions, but not under stressed conditions. To successfully develop a simulation model for pearl millet, capable of capturing such genotype×environment (G×E) interactions for grain yield, we need to understand the causes of the observed yield interaction. The aim of this paper is to quantify the key parameters that determine the accumulation and partitioning of biomass: the light extinction coefficient, radiation use efficiency (RUE), pattern of dry matter allocation to the leaf blades, the determination of grain number, and the rate and duration of dry matter accumulation into individual grains. We used data on improved cultivars and landraces, obtained from both published and unpublished sources collected at ICRISAT, Patancheru, India. Where possible, the effects of cultivar and axis (main shoot vs. tillers) on these parameters were analysed, as previous research suggested that G×E interactions for grain yield are associated with differences in tillering habit. Our results indicated there were no cultivar differences in extinction coefficient, RUE, and biomass partitioning before anthesis, and differences between axes in biomass partitioning were negligible. This indicates there was no basis for cultivar differences in the potential grain yield. Landraces, however, produced consistently less grain yield for a given rate of dry matter accumulation at anthesis than did improved cultivars. This was caused by a combination of low grain number and small grain size. The latter was predominantly due to a lower grain growth rate, as genotypic differences in the duration of grain filling were relatively small. Main shoot and tillers also had a similar duration of grain filling. The low grain yield of the landraces was associated with profuse nodal tillering, supporting the hypothesis that grain yield was below the potential yield that could be supported by assimilate availability. We hypothesise this is a survival strategy, which enhances the prospects to escape the effects of stress around anthesis.

Tillering is an important adaptive feature of pearl millet (Pennisetum americanum [Pennisetum glaucum]) to the unpredictable growing conditions of dry areas of the semiarid tropics. Yet, this feature has largely been ignored in the development of simulation models for pearl millet. The objective of this paper is to parameterize and validate a leaf area module for pearl millet, which dynamically simulates crop leaf area from the leaf area of individual axes through simulating inter-axis competition for light. To derive parameters for the model, four cultivars (BJ 104, WRajPop, HHB 67 and RCB-IC 911), contrasting in phenology and tillering habit, were grown under well-watered and well-fertilized conditions across a range of plant densities in three experiments at two locations in India (Patancheru, Andhra Pradesh and Jodhpur, Rajasthan) during 1996 and 1997. For selected plants, observations on the number of primary basal tillers and on the number of visible, fully expanded, and senesced leaves on each axis were made twice a week throughout the growing season. Occurrence of panicle initiation (PI) was observed in two experiments only, but data were complemented by published and unpublished data, obtained for comparable cultivars. Parameters were obtained for the time from emergence to PI as a function of daylength, leaf initiation rate, ate of leaf and tiller appearance, and the leaf senescence rate. Parameters for leaf size were determined in a previous paper. Our parameter estimates compared well with published data and were, with the exception of time to PI and leaf size, mostly independent of cultivar, axis and density. Genotypic effects on productive tiller number could be attributed to differences in main shoot leaf size. Validation of the leaf are module showed that the module adequately reproduced the effects of density, photoperiod and genotype on the leaf area of individual axes and on productive tiller number. This was despite the fact that the reduction in leaf area of non-productive tillers was achieved in the module through a reduction in leaf size, whereas the crop reduced leaf area through a reduction in leaf number. Our results indicate that leaf area index (LAI) of a tillering crop can be simulated adequately by simulating LAI from individual leaf area and incorporating the effects of competition for light

Pearl millet (Pennisteum americanum L.) is an essential crop in farming systems of the dry areas of the semi-arid tropics and its tillering habit is an important adaptive feature. This is the first paper in a series aiming at developing and validating a pearl millet simulation model that recognises tillers as functional entities, analogous to intercrops. The objective of this paper is to quantify the effects of total leaf number per axis (TLN), cultivar, plant density and axis number on parameters that are used to simulate potential leaf area per plant. Four cultivars with different phenology and tillering habit were grown under well-watered and well-fertilised conditions at two locations in India, covering a range of daylengths and plant densities. For selected plants, the area of fully expanded leaves was measured non-destructively. A bell-shaped function adequately described the relationship between individual leaf area and leaf position on an axis. Its shape was determined by the position (X0) and area (Y0) of the largest leaf and by the breadth and skewness of the leaf area profile curve. TLN affected all four parameters, although the association with Y0 was weak. Cultivar only affected Y0, suggesting that parameterising new cultivars is straightforward. The observed density effect confirmed that competition for light between axes started during stem elongation. The results highlighted the consistent differences between leaf area profiles of main shoots and tillers. For a high-tillering crop like pearl millet, modelling leaf area dynamics through individual leaves is justified, as this approach can potentially deal with cultivar and environmental effects on tillering.

This paper quantifies the effects of N-stress on development and growth of sorghum by identifying critical values for stover N% (SNC) and specific leaf nitrogen (SLN) for a range of physiological processes (rates of leaf appearance, leaf expansion, leaf area increase, and biomass accumulation). It also compares the merits of the SNC and SLN approach for implementation in the N-routines of crop growth simulation models. Data from experiments covering a range of nitrogen treatments, grown at three contrasting locations in Australia, were used in the analyses. For the rate of biomass accumulation, the critical and minimum SLN were constant for most of the growing season, except for early stages prior to approximately panicle initiation. For SNC, however, the critical and minimum values continued to decline throughout the growing season, although the decline was limited at later development stages. During the stage of leaf growth, the critical N content was similar for all processes considered. Although critical SLN was less sensitive to development stage than SNC, identification of a critical SLN was hampered if environmental effects on specific leaf area were present. The merit of each approach in simulation models will depend upon their ability to capture genotypic differences in N-dynamics. (C) 2001 Elsevier Science B.V. All rights reserved.

Stay-green in sorghum [Sorghum bicolor (L.) Moench] is an important component of post-flowering drought tolerance. This research was conducted to describe (i) stay-green as post-flowering green leaf area duration (GLAD) and its components [green leaf area (GLA) flowering, timing for onset of senescence, and senescence rate] and (ii) the expression of heterosis for stay-green in terms of heterosis for its components. The study was conducted during the 1992 to 1993 and 1993 to 1994 post-rainy season at ICRISAT headquarters near Hyderabad, India. It involved a nine-parent complete diallel, in two experiments differing in soil-water availability after flowering. Weekly estimations of % GLA were made on 36 leaves per plot. Relative (%days) and absolute (m2 days) GLAD and their components were derived from a fitted logistic function. The variances in both relative and absolute GLAD were each fully (R2 > 0.96) accounted for by their components. In spite of significant genotype × environment interactions for the component traits, the expression of heterosis for non-senescence as related to the stay-green trait was stable across experiments. The inheritance of the onset of senescence was additive, but a slow senescence rate was dominant over a fast rate. Consequently, a large relative GLAD (slow senescence) was partially dominant over a small relative GLAD. Because of the dominance of a large leaf area at flowering, the partial dominance in relative GLAD translated into overdominance for a large absolute GLAD. These results offer an opportunity for improving drought tolerance of sorghum in environments with post-flowering drought stress.

Grain yields in drought-affected environments are often characterized by large genotype-by-environment (GE) interactions, caused by a highly variable timing and intensity of stress occurrence. The aim of our study was to assess the effectiveness of simple meteorological environmental descriptions in providing an understanding of the environmental contribution to the GE interaction of pearl millet (Pennisetum glaucum (L.) R. Br.) in the semi-arid tropics. The experiment comprised fourteen genotypes, grown across 24 environments in India. Data on water availability throughout the season and maximum temperatures during grain filling were used to describe the environments. Escape from drought and heat stress were the major factors determining the GE interaction. Both the timing and intensity of stress occurrence were important. Knowledge of the environmental causes of the GE interaction may facilitate the identification of adaptive plant traits, and may also enable a more rational choice of test locations for breeding programs.

Throughout much of the semi-arid tropics, fluctuations in grain yield can largely be attributed to differences in timing and intensity of drought stress. Since seasonal rainfall in these environments is often poorly related to grain yield, the aim of this paper was to establish a relationship between water availability and grain yield for pearl millet (Pennisetum glaucum (L.) R. Br.), grown across 24 semi-arid tropical environments in India. We used a simple soil water budget to calculate a water satisfaction index (WSI) throughout the season. The cumulative WSI at maturity explained 76% of the variance in grain yield. This was three times as much as explained by actual rainfall, because WSI accounted for differences in water losses and pan evaporation. A classification of environments into four groups of water availability patterns explained 75% of the environmental sum of squares for grain yield. For a subset of 13 environments, environmental differences in grain number could also be explained by water availability patterns, whereas differences in grain mass were related to both water availability and temperature. Our results indicate that cumulative WSI, which is an integrated measure of plant-available water, can provide an adequate estimation of the environmental potential for yield in environments where grain yield is mainly limited by variable availability of water.

SUMMARYGrain yield of barley (Hordeum vulgareL.) in northern Syria is limited by water stress and extremes of temperature. The present study compared the grain yield of two barley cultivars, Harmal (spring type, cold-sensitive, early heading) and Arabi Aswad (winter type, cold-tolerant, medium early heading), under varying rainfall and temperature. Grain yield was obtained from three sites in northern Syria for seven seasons (1984/85 to 1990/91), resulting in 18 site × season combinations, here called environments. Multiple regression models, containing one rainfall and one temperature variable, were used to quantify yield responses to environmental fluctuations.Total seasonal rainfall was the variable most strongly correlated with the grain yield of Harmal, accounting for 62·8% of the variance. For Arabi Aswad, rainfall from November to January gave the best fit, accounting for 61·8% of the variance. December and January rainfall had the highest contribution to the yield of both cultivars; the contribution of March rainfall tended to be negative. The overall yield response to seasonal rainfall was 11·89 kg/ha/mm for Harmal and 8·57 kg/ha/mm for Arabi Aswad; the expected grain yield at the driest site wasc.1270 kg/ha for both cultivars. The addition of a temperature variable gave a better fit, accounting forc.80% of the variance in grain yield for both cultivars if winter rainfall was combined with number of night frosts in spring. It reduced the expected yields at the driest site toc.986 kg/ha. Arabi Aswad had a lesser response to both rainfall and frost than Harmal.In environments where low yields are due to both water and temperature stress, farmers are advised to grow Arabi Aswad because its lesser sensitivity to environmental fluctuations will ensure a better yield stability.

Selection for yield of barley (Hordeum vulgare L.) in unfavorable Mediterranean environments will be more efficient if plant traits can be identified that are related to yield under stress. The aim of this study was to assess whether, in fall-sown barley, plant traits measuredin favorable environments can be used as criteria for indirect selection for grain yield in unfavorable environments. Thirty-six two-row barley genotypes were grown in 15 Mediterranean environments. The plant traits measured were plant ideotype in winter (WPI, a principal component of winter growth habit, winter plant color, and cold tolerance) and heading date. Values were averaged across three favorable environments. Winter type barley had high values for WPI; spring type barley had low values

In the Mediterranean region, progress in selection for yield in harsh environments is hampered by large environmental variation between seasons and locations. This study analyses the genotype-by-environment (GE) interaction of 36 two-rowed genotypes of barley, grown in 14 environments in Syria and North Africa. It assesses the effect of growth type (winter or spring type) and heading date on the GE interaction and determines whether or not high-yielding (HY) environments are representative of low-yielding (LY) ones. Average grain yield per environment ranged from 7 to 513 g m-2. Genotypes and environments were classified by a cluster analysis and the interaction was analysed with an additive main effects and multiplicative interaction model. Genotypes were classified into four clusters, related to their growth type and earliness of heading. Environments were clustered into HY and LY groups; this classification was related to seasonal rainfall and temperature. Medium-early heading winter types had a positive interaction with LY environments and a negative interaction with HY environments, whereas late heading genotypes (spring and winter types) had the opposite interaction pattern. Early heading spring types had above-average mean yields; the highest-yielding among them tended to have a low interaction with environments. High-yielding environments did not discriminate well between genotypes with high or low yields in LY environments, and may thus have limited value for yield selection for LY environments. For a breeding programme aimed at improving yield in environments where favourable conditions are rare, selection for yield should be done in representative less-favourable environments

In a barley/livestock farming system of northern Syria, high biomass production in addition to high grain yield is desirable. The aim of this study was to assess the effect of environment and phenology on growth and yield of barley in northern Syria. Leaf area duration (LAD), specific leaf area (SLA), crop growth rate (CGR) on a thermal time basis, and biological and grain yields were compared for entries representing three contrasting development patterns: early heading spring types (pattern A), medium early heading winter types (pattern B), and late heading spring types (pattern C). The experiment was conducted in 1988/89 at two sites: Tel Hadya (TH, 250 mm precipitation) and Breda (BR, 180 mm). Cold damage occurred in winter and, especially at BR, drought stress developed in spring.

In the low-rainfall environments of the Middle East, genetic progress in grain yield through direct selection is slow. This study was conducted to identify a combination of traits or plant ideotype in barley (Hordeum vulgare L.) conducive to adaptation to terminal drought-stressed low-rainfall Mediterranean environments. Thirty-six two-rowed barley entries, ranging from local landraces and breeding lines to European cultivars, were grown at different sites in northern Syria. Observations were made on growth habit, plant colour, growth vigour, ground cover, cold tolerance, and heading date. Good ground cover and vigour in spring and early heading were related to higher yield under terminal drought. However, this good performance in spring was associated with two contrasting plant ideotypes in winter. The first ideotype, characteristic of landraces from Mediterranean environments with cold winters, was based on a moderate vernalization requirement. This assured appropriate cold tolerance, associated in winter with prostrate growth habit, dark plant colour and poor early vigour. It enabled heading early enough to avoid terminal drought stress. The second ideotype was based on avoidance of terminal drought stress through early heading and good early vigour. The associated higher vulnerability to low-temperature stress was compensated by an ability to recover from cold damage. This ideotype was characteristic of entries from Mediterranean environments with mild winters. Since environmental stresses in the Middle East are variable, individual traits can only successfully be incorporated into a breeding program if they are considered within the entire plant ideotype.