The diurnal and seasonal water relations, and composition, of `Braeburn' apple fruit under reduced plant water status

Abstract

Water relations of the apple fruit have been under-studied compared to leaves. We therefore studied diurnal and seasonal fruit water relations and fruit composition of `Braeburn' growing in drainage lysimeters under three irrigation treatments. The treatments were: control (C) which was fully irrigated, early deficit (ED) which was deficit irrigated from 55 days after full bloom (DAFB) until rewatering at 100 DAFB and late deficit (LD) which was deficit irrigated from 105 DAFB until harvest (177 DAFB). Periodically throughout the season, leaf water potential (Ψl), fruit water potential (Ψfw), and fruit osmotic potential (Ψfs) were measured at predawn, midday and dusk. Compared to C, the ED and LD trees had reduced Ψl at all measurement times during the day throughout their respective deficit periods. Size was reduced in the ED fruit during the stress period, despite turgor maintenance. The LD fruit showed no change in fruit water relations, composition (titratable acidity and the concentration of sugars and minerals), or size during the stress period when compared to C. These data indicate that fruit water relations, composition, and size are modified if stress is induced early in the season but unaffected under a late-season water deficit. Additionally, fruit water relations showed minimal diurnal fluctuations irrespective of treatment, but Ψl showed a large diurnal variation in all treatments.