Horst W. Caspari

Total soil water content accounts for augmented ABA leaf concentration and stomatal regulation of split-rooted apple trees during heterogeneous soil drying

A split-rooted containerized system was developed by approach grafting two, 1-year-old apple (Malus×domestica Borkh. cv ‘Gala’) trees to investigate the effect of soil moisture heterogeneity and total soil moisture content (θv) on tree water relations, gas exchange, and leaf abscisic acid (ABA) concentration [ABAleaf]. Four irrigation treatments comprising a 2×2 factorial experiment of irrigation volume and placement were imposed over a 30-day period: control (C) [>100% of crop evapotranspiration (ETc)] applied to both containers; PRD100 (>100% ETc) applied to one container only; and two treatments receiving 50% ETc applied to either one (PRD50) or both containers (DI50). Irrigation between PRD (partial rootzone drying) root compartments was alternated when θv reached ~35% of field capacity. Maximum daily sap flow of the irrigated roots of PRD100 exceeded that of C roots throughout the experimental period. Pre-dawn water potential (Ψpd) was similar between C and PRD100; however, daily water use and mid-day gas exchange of PRD100 was 30% lower. Slightly higher [ABAleaf] was observed in PRD100, but the effect was not significant and could not explain the observed reductions in leaf gas exchange. Both 50% ETc treatments had similar, but lower θv, Ψpd, and gas exchange, and higher [ABAleaf] than C and PRD100. Regardless of treatment, the container having the lower θv of a split-rooted system correlated poorly with [ABAleaf], but when θv of both containers or θv of the container possessing the higher soil moisture was used, the relationship markedly improved. These results imply that apple canopy gas exchange and [ABAleaf] are responsive to the total soil water environment. Abbreviations:A assimilation[ABAleaf] leaf ABA concentrationBd bulk densityDI deficit irrigationDOY day of yeardw dry weightE transpirationETc crop evapotranspirationFC field capacitygs stomatal conductanceLA leaf areaPAR photosynthetic active radiationPRD partial rootzone dryingΨpd pre-dawn leaf water potentialθv volumetric soil moisture contentθw , gravimetric soil moisture contentTCA trunk cross-sectional areaTDR time-domain reflectometryWUE water use efficiency. ©2012 The Author(s).

Deficit Irrigation and Partial Rootzone Drying Compared in Fuji Apples: Fruit Yield, Fruit Quality and Soil Moisture Trends

Increase in competing demands for water from cities, recreational users, and environmental groups are prompting the need to find new approaches to irrigation management that maintain yield, save water and reduce environmental degradation. The objective of this study was to evaluate the impact of Deficit Irrigation (DI) and Partial Rootzone Drying (PRD) on apple yield and size as compared to a well-watered control (CI). Fuji apple trees received 609 mm, of irrigation in the CI; 385 mm in the DI; and 357 mm in the PRD per year averaged over the 2001, 2002 and 2003 growing seasons. Fruit weight per tree was reduced by 10% in the DI treatment as compared to the CI and PRD treatments. Also, the individual fruit size was reduced by 9.0% in the DI and 4.0% in the PRD as compared to the CI treatment. However, the only statistically significant difference was observed in 2002 in which the CI’s fruit weight per tree was found to be greater than the DI. As for fruit quality, the deficit irrigated treatments produced higher soluble solids and greater firmness than the control treatment. Finally, soil water content monitoring showed that more water was preserved in the soil profile under PRD than under DI regime in 2001 and 2003 while the soil moisture in 2002 was nearly equal at the end of the growing season. This project demonstrated that a 35 to 45% reduction in irrigation resulted in minimal apple yield and size reduction when implementing DI and PRD irrigation strategies.