Effect of acorn size on survival and growth of Quercus suber L. seedlings under water stress

While the effects of seed size on survival and early seedling growth have been demonstrated in Quercus spp., there is a knowledge gap regarding these effects under water limiting conditions. To address this need, we studied cork oak (Quercus suber L.) seedling development among treatments including three acorn classes (small: < 4 g, intermediate: 4–6 g), and large: ≥ 6 g) and two water regimes (well-watered versus water stressed). The following hypotheses were proposed: the reserve effect (larger seeds retain a larger proportion of reserves after germinating), the metabolic effect (seedlings from larger seeds have slower relative growth rates), the seedling-size effect (larger seeds produce larger seedlings), and that plant tolerance to water stress increases with increasing seed size. Acorn size had no effect on duration and rate of acorn germination nor on seedling survival, but it significantly affected almost all growth parameters, excepting the number of growth unit and leaves and leaf area. Seedlings coming from large acorns always showed the highest values for the affected growth parameters, followed by those coming from acorns of intermediate size; seedlings coming from small acorns always exhibited the lowest values. Water stress significantly affected survival rate (88.2% for well-watered seedlings versus 64.7% for water stressed seedlings) and biomass of fine roots and stems, but independently of acorn size. Our results are in line with both hypothesizes of the reserve effect and seedling-size effect, but not for the metabolic effect nor the plant tolerance to water stress increases with increasing seed size hypotheses. In general, our results show that large acorns had no advantage in survival or seedling growth under water stress comparatively to intermediate or small acorns, at least under the conditions of our experiment.