Background and aims Crop tolerance to waterlogging depends on factors such as species sensitivity and the stage of development that waterlogging occurs. The aim of this study was to identify the critical period for waterlogging on grain yield and its components, when applied during different stages of crop development in wheat and barley. Methods Two experiments were carried out (E1: early sowing date, under greenhouse; E2: late sowing date, under natural conditions). Waterlogging was imposed during 15-20 days in 5 consecutive periods during the crop cycle (from Leaf 1 emergence to maturity). ResultsThe greatest yield penalties occurred when waterlogging was applied from Leaf 7 appearance on the main stem to anthesis (from 34 to 92 % of losses in wheat, and from 40 to 79 % in barley for E1 and E2 respectively). Waterlogging during grain filling reduced yield to a lesser degree. In wheat, reductions in grain number were mostly explained by reduced grain number per spike while in barley, by variations in the number of spikes per plant. Conclusions The time around anthesis was identified as the most susceptible period to waterlogging in wheat and barley. Exposing the crop to more stressful conditions, e.g. delaying sowing date, magnified the negative responses to waterlogging, although the most sensitive stage (around anthesis) remained unchanged.

The effect of environmental conditions immediately before anthesis on potential grain weight was investigated in wheat at the experimental field of the Faculty of Agronomy (University of Buenos Aires, Argentina) during 1995 and 1996. Plants of two cultivars of wheat were grown in two environments (two contrasting sowing dates) to provide different background temperature conditions. In these environments, transparent boxes were installed covering the spikes in order to increase spike temperature for a short period (c. 6 days) immediately before anthesis, i.e. between ear emergence and anthesis. In both environments, transparent boxes increased mean temperatures by at least 3·8 °C. These increases were almost entirely due to the changes in maximum temperatures because minimum temperatures were little affected. Final grain weight was significantly reduced by higher temperature during the ear emergence–anthesis period. It is possible that this reduction could be mediated by the effect of the heat treatment on carpel weight at anthesis because a curvilinear association between final grain weight and carpel weight at anthesis was found. This curvilinear association may also indicate a threshold carpel weight for maximizing grain weight.

Worldwide wheat yields have been only slightly, and non-significantly, increasing during the 90's, suggesting that they may be levelling off. Considering that there is consensus that large new growing areas will be not introduced and management improvements will be increasingly harder to obtain, genetic improvement would play a more important role to keep rising wheat yields in the future than in the past. In this scenario, the use of physiological bases as a complementary tool to identifying alternative ways for breeding seems to be crucial for breaking the apparent barriers in wheat yields. In this presentation, we attempted to envisage, from published and recent unpublished evidences from our lab, using studies carried out under both controlled and field conditions, some opportunities to manipulate the rate of crop development during the late reproductive phase. This phase has been recognised as critical in terms of yield generation, and the idea of manipulating its response to photoperiod as a tool for increasing yield potential in wheat is reviewed.

Although individual grain weight is an important source of variation for grain yield, there is still poor understanding of the causes determining final grain weight. Almost all studies conducted for understanding the determinants of grain weight have been focused on the post-anthesis period. However, there is important evidence that preanthesis conditions could also modify final grain weight. Three experiments including different sowing dates, genotypes and temperature regimes between booting and anthesis, were carried out in Argentina and Mexico to analyse the effect of temperature and associated traits during the pre-and post-anthesis periods on grain weight under field conditions. In these experiments final grain weight could not be explained by average or maximum temperature during the post-anthesis period. However, average temperature between booting and anthesis was closely related to the observed grain weight differences, probably as a consequence of the effects of this factor on carpel growth. Differences in grain weight between genotypes and grain position were successfully explained by differences in carpel weight at anthesis. These results suggest that our knowledge to determine grain weight could improve if the immediately preanthesis period conditions were taken into account.

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desiccants or windrowing) without reducing wheat yield, thereby providing better returns in double-crop systems. Estimation of the time of physiological maturity could be beneficialThe most precise method of determining the time of to avoid yield penalties due to lodging, sprouting, hail, and other harvest risks. The aim of our study was to evaluate a simple empirical physiological maturity in crops is to follow kernel dry relationship (model) to determine physiological maturity by simulta-matter accumulation after anthesis. By weighing ovenneously analyzing the water and dry matter dynamics of wheat kernels.dried samples after physiological maturity, a regression An experiment was conducted in which two cultivars with different model can be determined that calculates when that stage kernel mass potential were sown on four dates. Fresh and dry kernel was actually reached. Following this procedure, we can mass from different positions in the spike were measured twice weekly. normally tell when physiological maturity is reached noTo validate the regression model, measured and calculated data from earlier than 2 wk after it actually occurred, which does different cultivars, growing seasons, and kernel positions were comnot allow us to decide on the earliest possible harvest pared. A negative linear relationship between kernel dry matter (relatime. In addition, this methodology requires weighing tive to final kernel mass) and kernel water concentration was deterequipment that is not commonly available on most mined. This showed that in wheat, physiological maturity is reached at 37% of kernel water concentration. Validation of the regression farms. model was done using data from field experiments in Argentina and Development of alternative, simple methods to deter-Mexico, and from controlled-conditions experiments reported in the mine with a single measurement if a crop is at physiologiliterature. The regression model successfully simulated results from cal maturity would be useful. For example, the use of field experiments (r ϭ 0.98; P Ͻ 0.001). In addition, data from conthe milk line and black layer are an indirect but fretrolled-conditions experiments showed the same negative linear relaquently used indicator of physiological maturity in tionship between relative kernel dry matter and kernel water concenmaize (Muchow, 1990). In wheat (as in other crops), tration (slope ϭ Ϫ0.4), and the model achieved a good fit for measured there are no simple early visual morphological signs data (r ϭ 0.96; P Ͻ 0.001). This regression model is proposed for use strongly correlated, for a wide range of environmental by farmers and crop managers, who can simply measure grain humidity conditions, with the cessation of kernel dry matter accuwith grain moisture meters. mulation. The kernel water concentration (KWC; i.e., percentage of water in kernels) is a potentially useful trait. Farmers and crop managers can simply estimate 895