Does alternation increase water productivity when applying partial root-zone drying to tomato?

In developing countries, horticultural production relies on smallholder farmers who cannot access advanced irrigation technologies. In Ghana, tomato production does not match the increasing demand for this crop, especially during the dry season so there is a need to increase irrigated tomato production. Optimal application of low cost irrigation techniques can increase water productivity and therefore the irrigated acreage. Partial root-zone drying (PRD) is a water-saving irrigation technique that applies sub-optimal irrigation to half of the root-zone, while adequately irrigating the remainder. PRD increased water productivity compared to homogeneous drying in different crops including tomato. Although root-to-shoot signaling studies predict that the drying of the irrigated zones need to be alternated to maintain PRD effects, studies on the effects of alternation are scarce. To investigate the optimal PRD application technique in tomato we carried out a field trial in Kumasi (Ghana) on an improved bush-type cultivar (Petomech) where both alternated and fixed PRD alternation at 50% of ETc (crop evapotranspiration) were compared with full irrigation (100% ETc). Irrigation treatments did not affect fruit yield, increasing water productivity. Plant physiological responses to alternation were also assessed in a controlled environment experiment in Lancaster (UK) on a miniaturised, containerised PRD system (75% of ETc) using the dwarf cultivar Micro-Tom. PRD decreased fruit yield by ca 15% regardless of whether it was alternated or not. Whole-plant gas exchange and stem diameter variations did not differ between alternated or fixed PRD, even under high vapor pressure deficit, either during the drying cycles or upon re-watering. Stem growth was reduced early after applying PRD and was not affected by alternation. Contrary to previous reports, these results suggest that alternation has little impact on tomato yield, and so fixed PRD may be more readily assimilated by smallholder farmers.