The effect of root geometry on the yield of wheat growing on stored water

Abstract

It is suggested that, when wheat is growing predominantly on stored water, it is an advantage for the plants to have root systems of high hydraulic resistance, so that they will conserve water during early growth and thus have more water available while filling their grain. This paper describes pot experiments in which, with wheat growing entirely on stored water, an attempt was made to increase the resistance of the roots by forcing plants to rely entirely on one seminal root. The treatment was successful in that single-rooted plants did use substantially less water before anthesis than did normal plants, and they produced substantially greater grain yields. The conservation of water by the single-rooted plants appeared to be due to their having extraordinarily large pressure drops in the root xylem: calculations showed that the flow rate in the xylem reached 800 mm sec^-1 with a concomitant pressure gradient greater than 0.1 bar mm^-1. The corresponding values in the normal plants were much less, although still surprisingly high (250 mm sec^-1 ; 0.03 bar mm^-1).

The xylem of the seminal roots is dominated by one large vessel whose diameter probably determines the hydraulic resistance. It may be possible to breed high root resistance into existing cultivars by breeding for smaller vessels.