Regulation of soil water threshold on tomato plant growth and fruit quality under alternate partial root-zone drip irrigation

Increasingly limited global freshwater resources coupled with low water use efficiency (WUE) increases the need for water-saving irrigation technologies in agricultural production. Numerous studies have investigated the emerging water-saving irrigation technology of alternate partial root-zone drip irrigation (APDI) on crop yield and WUE. This study investigated the effects of lower soil water thresholds for alteration of the irrigation (LTAI) to the previously dry root-zone on tomato plant growth, yield, and irrigation water use efficiency (IWUE) under APDI in year 2016 with field plot area of 6.48 m². There were six APDI treatments coded as AS80, AS70, AS60 and AD80, AD70, AD60, indicating APDI under sufficient irrigation (S) or deficit irrigation (D) with LTAI controlled at 80 %, 70 % and 60 % field water capacity (FC) in the previously dry root-zone, respectively. In addition, conventional sufficient (CSD) and deficit (CDD) drip irrigation treatments were added as the controls. The results showed that lowering the irrigation amount significantly reduced tomato plant growth, yield and harvest index, but improved fruit quality in terms of reducing sugar, soluble sugar, vitamin C, and sugar-acid ratio. Compared with that of conventional drip irrigation, the promotion effect of APDI, especially with LTAI at 70 % FC, on root growth was observed in the 0−20 cm top soil layer where more than 90 % of the plant roots were distributed irrespective of irrigation amounts. Under sufficient irrigation, tomato shoot growth and the harvest index showed no significant difference among treatments, but tomato yield and the root to shoot ratio increased significantly in AS70, while fruit quality was improved in AS70 and AS60. Under APDI with deficit irrigation, when LTAI decreased from 80 % FC to 70 % FC in the previously drying root-zone, the yield and IWUE in AD80 and AD70 increased by 8.6 % and 17.4 %, respectively, and thus increased the harvest index compared with CDD. The AD70 treatment saved 31.2 % irrigation water without significant yield reduction, but increased IWUE by 44.9 % compared with the CSD. However, the harvest index and the yield decreased significantly when LTAI decreased from 70 % FC to 60 % FC (AD70 Vs. AD60). In addition, the yield showed no significant difference between AD60 and CDD. Fruit quality increased under APDI as compared with CDD under deficit irrigation, with the best quality appeared in the AD70 and AD60 treatments. These results indicated that the effects of APDI on tomato growth and fruit quality were modulated by LTAI and irrigation amount. APDI with LTAI controlled at 70 % FC was the optimal choice for better yield and fruit quality, especially under deficit irrigation. Unfavorable LTAI could diminish and even eliminate the beneficial effects of APDI on yield and IWUE as compared with conventional drip irrigation.