Water relations and carbohydrate partitioning of four greenhouse-grown olive genotypes under long-term drought

Olive plants of broad-leaved 'Minuta' (MN) and 'Nocellara del Belice' (NB) and narrow-leaved 'Passulunara' (PA) and 'Biancolilla Siracusana' (BS) were studied to evaluate their responses to drought. In a greenhouse, two-year-old rooted cuttings were irrigated to field capacity (WW) or with 20% of WW evapotranspiration (DS) for over three months. Subsequently, all pots were rewatered to field capacity for 20 days. Gravimetric soil water content (SWC), leaf relative water content (RWC), stomatal conductance (gs), and leaf carbohydrates, percentage of leaf drop and shoot elongation were determined throughout the trial. In WW, SWC fluctuated between 80 and 100% of field capacity, whereas in DS, SWC decreased sharply reaching a minimum level around 30-35% of field capacity after two months of drought. At this time, drought induced a significant reduction of: (a) RWC in PA and BS, (b) gs in MN, NB, and PA, and (c) shoot elongation (-23%) in PA. Conversely, drought increased leaf drop in all genotypes, especially in MN and NB. RWC and gs levels were mostly restored after rewatering. Initially, drought induced an increase of mannitol and total carbohydrates in MN and a decrease in NB. At more advanced drought stages, mannitol and total carbohydrates decreased in PA and BS. NB exhibited a general increase of the (mannitol + glucose)/sucrose ratio in response to drought. The two broad-leaved genotypes (MN and NB) maintained similar leaf hydration levels in DS and WW plants proving to be generally intolerant to dehydration, whereas the two narrow-leaved genotypes (PA and BS) tolerated a fair degree of dehydration.