In field source-sink manipulation of early and late harvest nectarine varieties changes photosynthetic responses due to stomatal closure and carbohydrate accumulation in source leaves

Abstract

Sink/source balance is determinant in carbon partitioning within the plant. Thinning is an 18 agronomic practice that consists in changing the ratio between adult leaves and fruits, source and sink 19 organs respectively. Decreasing fruit load on the tree is supposed to lead to an increase in fruit diameter, 20 weight and soluble solids content in the remaining fruits. However it has been reported that a reduction in 21 sink organs may result in decrease of photosynthetic process. The aim of this work was to study the effect 22 of sink/source balance in physiological parameters of modern Prunus persica varieties with distinct times 23 of fruit development and harvest. Differential thinning was performed in two nectarine varieties, 24 ‘Magique’ and ‘Red Pearl’, early and late harvest varieties respectively. Fruit development kinetics as 25 well as physiological parameters as chlorophyll fluorescence, gas-exchange, and chlorophyll and sugar 26 composition of leaves were determined during the whole season. Photosynthesis rate was decreased in 27 thinned trees associated to stomata limitation and sugar accumulation in leaves. The most abundant sugar 28 detected in leaves was sorbitol that is the main sugar exported by leaves of Prunus persica. On the other 29 hand, most fluorescence parameters and chlorophyll content remained constant in leaves of both varieties 30 with different thinning treatments in the whole season. We suggest that increasing sink/source ratio by 31 thinning leads to stomata closure, and this is associated to an excess of sugar that are not exported and 32 remains accumulated in leaves of thinned trees. In these modern varieties thinning impacted 33 photosynthesis parameters, but had no impact in final fruit size. Decrease of sink strength by fruit thinning 34 allowed the development of other sink organs as vegetative suckers probably as an acclimation process to 35 maintain source-sink balance within the tree. 36 37 Key words: modern varieties, peach, suckers, thinning, sugar, photosynthesis, Prunus persica