Partial root drying effects on biomass production in Brassica napus and the significance of root responses

Partial root drying (PRD) has been shown to stimulate stomatal-closure response and improve water-use efficiency and thus biomass production and grain yield under water deficiency. While most studies focus on above-ground responses to PRD, we examined how root responses contributed to effects of partial root drying. In particular, in two experiments with oilseed rape (Brassica napus L.) we investigated whether roots were able to forage for patchily distributed water, and how this affected plant growth compared with uniform watering and alternate watering (in which different parts of the roots receive water alternately). The first pot experiment was carried out in the greenhouse and the second outside under a rain-shelter in which also the watering amount was varied. The results indicate that B. napus roots were able to forage for fixed water patches by selective root placement. In the first experiment with small plants, root foraging was equally effective as enhanced water-use efficiency under alternate watering. Both treatments resulted in about 10% higher shoot biomass compared with uniform watering. Alternate watering generally outperformed uniform watering in the second experiment, but the success depended on the time of harvest and the water supply level. Measurements indicated that only the alternate watering regime effectively reduced stomatal conductance, but lead to a higher shoot biomass only under more severe (50%) rather than under milder water deficiency (70% of a well watered control). Water deficiency strongly reduced leaf initiation rates and leaf sizes in B. napus, but for a given level of water supply the supply pattern (uniform control, fixed patchy or alternate watering) hardly influenced these growth parameters. Although also in the second experiment, the plants selectively placed their roots in the wet parts of the pot, root foraging was not as effective as in the first experiment. Possible reasons for these discrepancies are discussed as well as their implications for the application of PRD effects for crop growth.