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Abstract. There is little consensus on whether having a large root system is the best strategy in adapting wheat (Triticum aestivum L.) to water-limited environments. We explore the reasons for the lack of consensus and aim to answer the question of whether a large root system is useful in adapting wheat to dry environments. We used unpublished data from glasshouse and field experiments examining the relationship between root system size and their functional implication for water capture. Individual root traits for water uptake do not describe a root system as being large or small. However, the recent invigoration of the root system in wheat by indirect selection for increased leaf vigour has enlarged the root system through increases in root biomass and length and root length density. This large root system contributes to increasing the capture of water and nitrogen early in the season, and facilitates the capture of additional water for grain filling. The usefulness of a vigorous root system in increasing wheat yields under water-limited conditions maybe greater in environments where crops rely largely on seasonal rainfall, such as the Mediterranean-type environments. In environments where crops are reliant on stored soil water, a vigorous root system increases the risk of depleting soil water before completion of grain filling.Additional keywords: root biomass, root length, root length density, root system size, water capture.

Preanthesis stored C and N in wheat (Triticum aestivum L.) are important in a mediterranean climate because grain filling frequently depends on the remobilization of preanthesis assimilates. We determined the effect of the rate of development of postanthesis water deficits on the remobilization of C and N to the grain using stable isotopes of C and N accumulated in the plant during the vegetative phase. Plants were grown in pots with adequate water and under similar temperature and humidity conditions until anthesis, and then were transferred to two temperature and humidity regulated greenhouses, and watering was stopped. One greenhouse was maintained at minimum relative humidity of 80% and the other at 40%. Within 6 d of anthesis the rates of development of plant water deficits became different and for the first 19 d after anthesis they were 0.10 and 0.18 MPa d−1 for the high and low humidity regimes, respectively. Total grain C with fast development of water deficits was reduced by 24%, relative to the slow rate, because postanthesis C assimilation was reduced by 57%, while remobilization of preanthesis stored C was increased by 36%. Total grain N was not affected by the rate of development of water deficits because there was a greater retranslocation of preanthesis N with fast relative to slow development of water deficits and because there was a smaller loss of preanthesis N with fast development of water deficits. Fast development of water deficits reduced losses of preanthesis N from 25% to 6%. The absolute contributions of preanthesis C and N to the grain were 449 and 35 mg plant−1, respectively, with fast development of water deficits. These contributions accounted for 64 and 81% of the total grain C and N, respectively. The gain in grain 13C and 15N in the mainstem and Tiller 1 of plants exposed to rapid development of water deficits, arose not only from remobilization from the straw of those shoots, but also seemed to be supplemented by C and N remobilized from Tillers 2 and 3.