High temperature is usually one of the most important stresses during grain filling affecting both yield and quality in barley crops. In the present study, an attempt was made to assess in the field the effects of short periods of high temperature, using transparent boxes covering only the spikes, with thermostatically controlled electric resistance for increasing the temperature. Treatments consisted of 2 malting cultivars and 5 heat treatments of high temperatures (8�C above the environmental temperature for 6 h/day for 5 consecutive days) over different periods during grain filling. Final grain weight was reduced by 2–14%, depending on the timing of heat stress and the genotype. There was a significant increase in grain nitrogen percentage in both cultivars, and grain β-glucans decreased with high temperatures in Logan and were unchanged in Beka. The resulting malt extract was reduced with exposure to high temperatures, depending on the cultivar, implying that even mild heat stress may change malting performance.

Under field conditions the occurrence of brief periods of moderately high (30–32°C) and very high temperatures (>35°C) is quite common during grain filling in small-grain cereals. These events occur under a wide range of different management and environmental conditions, such as different nitrogen supplies and source–sink ratios after flowering. The objective of the present work was to study whether the effect of a brief heat stress is modified by resource availability for the growing grains. We subjected spikes of barley 10 days after flowering to a heat treatment in factorial combination with different nitrogen availabilities and source–sink ratios during post-flowering to determine effects on grain weight and major malting quality attributes. Grain weight and screening percentage (proportion of grains <2.5 mm) were reduced by the mild heat stress. However, the magnitude of the effect was dependent on the nitrogen fertilisation and the source–sink treatments in which the heat stress was imposed. Grain protein and β-glucan percentages were increased by both nitrogen fertilisation and heat stress. Again, the magnitude of the increase was dependent upon the availability of resources. There was a trend to reduce malt extract in all treatments with respect to the control, but the reduction was only statistically significant with heat stress.

Temperature is one of the most important factors affecting grain quality in the majority of cereal growing areas. It has been shown that brief periods of high temperature during grain filling can reduce grain weight and quality. However, it is not known whether breeding for increased yields has altered the sensitivity to high temperatures after flowering. Four malting barley cultivars, chosen from previous studies to represent a series of successful breeding releases in Argentina, were sown under field conditions. The high temperature treatments consisted of increasing the spike temperature by about 5 • C for 6 h/day (from 10 am to 4 pm) for 20 days starting nine days after flowering (DAF), or for two periods of 10 days each, between 9 to 19 DAF or from 20 to 29 DAF. In all cultivars, grain weight and screening percentages decreased significantly with the treatments. In general, the two more modern cultivars (Q. Pampa and Q. Ayelén) were more sensitive to heat stress than the older ones (Heda and Unión). A similar pattern was found for malt extract. Thus it appears that breeding for higher yields has concomitantly increased sensitivity of grain weight (and yield) and quality to the episodes of high temperatures during the grain-filling period in barley.

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