Photosynthetic Physiological Mechanisms Responsible For Metaxenia Effect In Cathayensis Fruits

Hickory（Carya cathayensis Sarg.）and pecan（C.illinoensis K.Koch）are economic trees in Carya widely planted in the world.The hickory with thick shell and unique good flavor comes from china,the pecan with larger fruit and thinner shell originated from the United States.Previous studies have found that metaxenia effect occurred in hickory fruits.The fruits exocarp became green,dry matter of fruit and kernel significantly increased after pollinated with pecan pollen.However,there was no significant different stripe found between offspring and mother in term of moleculer marker,indicating that variation of fruits is not caused by genetic changes.This study was conducted to reveal the photosynthetic characteristics of leaves and fruits in hickory and pecan during fruit development,prove that fruit photosynthesis in both hickory and pecan play an important role for the fruit growth.To elucidate the photosynthetic physiological mechanisms responsible for metaxenia effect in hickory fruits,we also compared the differences photosynthetic characteristics of fruits between pollinated with hickory pollen and pecan pollen,and clarified the relationship between fruit photosynthesis and dry matter accumulation.The main results showed as follows:（1）The hickory and pecan fruit development process can be divided into two stages according to the dry/fresh mass ratio:the rapid fruit enlargement period（hickory fruit from fruit setting to 59DAP;pecan from fruit setting to 112DAP）and kernel mature period（hickory fruit from 59 to 121DAP;pecan fruit from 112 to 155DAP）.The photosynthetic rate in hickory leaves decreased by 48.5%from 50 to 76DAP,while the photosynthetic rate in pecan leaves decreased by 32.3%from 88 to 123 DAP.The gross photosynthetic rate in hickory fruit was significantly greater than that of the leaf during 88 to 123 DAP.Pecan fruit had a significantly higher photosynthetic rate than leaves during ontogeny.Using the photosynthetic rate/respiration rate and dry matter at different fruit development stages,we calculated the contribution of fruit photosynthesis to its fruit carbon requirement at each fruit growth stage.The contribution of fruit photosynthesis to pecan carbon requirements was significantly greater than that of hickory.Thus,fruit photosynthesis in both hickory and pecan significantly contribution to the carbon requirements of late growth stage.（2）Compared with the hickory fruits pollinated with hickory pollen（hp）,the light saturation point（LSP）、the maximum photosynthetic rate(P_（g,max）)and the CO₂ re-fixation rate was significantly higher in hickory fruits pollinated with pecan pollen（pp）during from 50 to 85 DAP.The chlorophyll content and the total nitrogen in fruits pollinated with pp were significantly higher than those in fruits pollinated with hp during from 50 to 103 DAP.The Yield,ETR and qP in fruits pollinated with pp were significantly higher than those in fruits pollinated with hp at PAR of 1265μmolm^-2s^-1.In addition,the fruits pollinated with pp had significant higher the maximum potential relative electron transport rates(rETR_max)than fruits pollinated with hp.Thus,we suggested that the higher photosynthetic rate in fruits pollinated with pp might related with the higher chlorophyll content,nitrogen content and Yield.（3）The dry matter of fruit and kernel significantly increased with greener exocarp after pollinated with pp pollen,which is called metaxenia effect.However,there was no significant difference in the photosynthetic rate of leaves from corresponding fruits（pollinated with hp and pp）.A further analysis suggest that,there was a significant positive correlation between the chlorophyll content,nitrogen content and photosynthetic rate,between the increment of photosynthetic rate and the increment of dry matter,respectively.Except for the early stage of the seed development（59-75DAP or 60-73DAP）,the contribution of fruit photosynthesis to fruit carbon requirement in fruits pollinated with pp was significant higher than that in fruits pollinated with hp.In addition,The Y（NO）in fruits pollinated with hp significantly increased from 103 to 120DAP,while there was no significant changes in fruits pollinated with pp.Therefore,we suggested that the higher dry mass in fruits pollinated with pp was closely related to its higher fruit photosynthesis and the better adaptability of high-light.