Physiological traits for improving wheat yield under a wide range of conditions

A better understanding of relatively simple crop-physiological attributes that determine yield in a wide range of conditions may be instrumental for assisting future breeding. Physiological traits may be selected either directly or through the use of molecular-biology tools. Physiological and breeding literature frequently distinguishes between yield under potential, stress-free conditions and under the pressure of stress, mostly abiotic. Although the rationale behind the idea that the different physiological attributes contribute to yield under these contrasting conditions may be sound, in practice there is a large body of evidence pointing out the other way around. For instance, genotypes with physiological attributes conferring higher yield potential usually also perform better under stress conditions, at least when excluding extremely severe environments. As breeders normally need to release improved cultivars to be grown in different sites throughout several seasons and subjected to a wide range of management decisions, identifying physiological traits that may confer simultaneously high yield potential and constitutive tolerance to stress would be critical. These traits must allow the plants to capture more resources or to use them more efficiently. A well-known attribute conferring high yield potential and widely studied physiologically has been semi-dwarfism. Semi-dwarf cultivars normally yield better than tall ones in a wide range of stressful conditions (at least if seedling emergence is not a major inconvenience). This is because reducing height to a certain level does not alter the ability of the crop to capture resources, whilst improving markedly the efficiency with which the resources are used to produce yield. This trait is not further useful as modern cultivars possess already a stature within the ranges optimizing yield. Two other traits that may also be associated with improved performance in a wide range of conditions may be the discrimination against 13C (?13C) and a lengthened stem elongation phase at the expense of previous phases. Although more research is needed before conclusions may be robust, physiological evidence supports the hypothesis that increasing ?13C and lengthening the stem elongation phase would result in an improved performance over a range of environmental condition

Maize grain weight sensitivity to source–sink manipulations under a wide range of field conditions

Physiological causes for grain weight determination in maize (Zea mays L.) are not clear. Source–sink relationships during grain filling modulate grain weight, and there are controversies regarding the degree of source limitation that may exist during grain filling. We aimed to analyze likely causes of the responsiveness of maize grain weight to defoliation and degraining treatments imposed 15 d after silking, quantifying the responsiveness of grain weight to these source–sink manipulations in a large number of field conditions (52 background conditions in which source–sink manipulations were imposed). Grain weight was largely unresponsive to increases in source availability but was diminished by defoliations in six out of seven experiments. Interestingly, grain weight reductions due to defoliation were not hierarchical (grains from different positions along the ear responded similarly) and were not worsened by imposing a simultaneous heat stress. Heat affected the grain growth capacity directly, and indirect effects (through reducing source strength due to accelerated senescence) were not evident. The penalty imposed by heat was neither increased by defoliation nor diminished by degraining, and the reduction in grain weight was similar for grains with different potential size. Our study reinforced the concept that maize yield is limited by the sink strength during grain filling, even when grain weight may respond to reductions in the grain filling source–sink ratio.Raziel A. Ordóñez, Roxana Savin, C. Mariano Cossani, and Gustavo A. Slafe

Comparative response of wheat and oilseed rape to nitrogen supply: absorption and utilisation efficiency of radiation and nitrogen during the reproductive stages determining yield

We investigated the response of spring wheat and oilseed rape to nitrogen (N) supply, focusing on the critical period for grain number definition and grain filling. Crops were grown in containers under a shelter and treated with five combinations of applied N. Wheat and oilseed rape produced comparable amounts of biomass and yield when corrected for the costs of biomass synthesis (SC). From the responses of biomass and yield to late N applications and the apparent contribution of mobilised biomass to yield, it seems that the yield of oilseed rape was more source-limited during grain filling than that of wheat, particularly at the medium and high N levels. Both species recovered equal amounts of N from the total available N in the soil and had similar N use efficiencies, expressed as yield per unit of N absorbed. However, oilseed rape had higher efficiency to convert absorbed N in biomass, but lower harvest index of N than wheat. Oilseed rape had similar or lower root biomass than wheat, depending on N level, but higher root length per unit soil volume and specific root length. The specific uptake rate of N per unit root dry weight during the critical period for grain number determination was higher in oilseed rape than in wheat. In wheat, N limitation affected growth through a similar or lower reduction in radiation use efficiency corrected for synthesis costs (RUESC) than in the cumulative amount of intercepted photosynthetically active radiation (IPARc). In oilseed rape, lower growth due to N shortage was associated more with RUESC than IPARc, during flowering while during grain filling both components contributed similarly to decreased growth. RUESC and the concentration of N in leaves and inflorescence (LIN€decreased from flowering to maturity and were curvilinearly related. Oilseed rape tended to have higher RUESC than wheat at high N supply during the critical period for grain number determination, and generally lower during grain filling. The reasons for these differences and possibilities to increase yield potential are discussed in terms of the photosynthetic efficiency of the different organs and changes in source–sink ratio during reproductive stages

Genetic variability in duration of pre-heading phases and relationships with leaf appearance and tillering dynamics in a barley population.

The stem elongation phase seems critical in yield potential determination in barley (Hordeum vulgare L.). Extending its duration, without modifying total time to anthesis, has been proposed as a promising breeding tool. A prerequisite for its use is that the duration of phases before and after jointing (that including leaf and spikelet initiation, LS, and that of stem elongation, SE) should be under different genetic control. In addition, little is known about the implications of changes in the duration of LS and SE upon other developmental traits which could affect other aspects of yield generation, such as phyllochron and tillering. Thus, the objectives of the present work were to study the genetic variability in LS and SE, in traits related to leaf appearance and tillering, as well as their relationships, in a double-haploid (DH) population derived from the cross Henni × Meltan. DH lines and both parents were studied in four field trials. Despite the similarity in development between parents, there was significant genetic variability in duration of both LS and SE (i.e. considerable transgressive segregation was observed), with no major genetic correlations found between them. Although some significant genetic correlations were found between duration of phases and leaf appearance and tillering traits, it has been shown that modifying the duration of LS does not necessarily imply concomitant changes in traits that could be important for an early expansion of the crop canopy (i.e. phyllochron, onset and rate of tillering)