Effects Of Crop Load On Aerial And Root Growth In 'Fuji' Apple

Crop load significantly affects the growth of apple trees.High crop load inhibits the vegetative and reproductive growth of apple trees,resulting in a series of problems,such as reducing growth potential,inhibiting of floral bud initiation and the deterioration of fruit quality;Low load leads to the excessive vegetative growth and reduction of yield.However,few report is about the relationship between crop load and root growth.In this study,the potted apple and field apple plants of Tianhong No.2/SH40/Crabapple were used as test materials.The growths of fine roots under different load were observed by minirhizotrons,and the physiological and biochemical indexes related to root growth were measured.The relationship between the growth and physiological parameters of aerial part under different crop load were also studied.The results are as follows:1.By setting different crop load gradients on field and potted apple,the effects of different crop load on fruit weight,vertical diameter,horizontal diameter,soluble solid content and titratable acid content were studied.The results showed that the fruit quality was mainly affected by the crop load level of the current year,and the increasing crop load level reduced the weight of single fruit,the content of soluble solids,the vertical diameter and the horizontal diameter of fruits.The fruit shape index decreased with the increase of crop load.The fruit growth of high crop load treatments was restricted significantly in late August.The cluster amounts decreased significantly with the increase of crop load.2.The growth of shoots of each treatment showed bimodal curve in the growth period,which appeared in June and September,respectively.The shoot growth of each treatment was restricted by the increase of crop load.The results showed that the net photosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate and intercellular C02 concentration varied under different crop load.The Pn,Gs and transpiration rate of leaves increased with the increase of crop load,while the differences of Ci between different crop load were less obvious.3.The dynamic changes of root length density and root turnover rate of apple under varying field treatments were studied by minirhizotrons for three consecutive years.The results showed that there were two growth peaks in the growth of roots under all crop loads in a year,and the fine roots growth appeared in summer and autumn,while the dead fine root peak is in summer as well.The root length density decreased with the increase of crop load at the root growth peak,the root growths of low load treatments were higher those in high crop load treatments.The annual turnover rate of roots increased with the increase of the crop load.The root vigor under different crop load in the whole observation period correlated with the growths of roots and shoots.With the increase of crop load level,there was no obvious changing trend of root vigor,but its dynamic showed bimodal curves.4.The dynamic changes of IAA,GA3,ZR,DHZR,IPA and ZT in the roots of potted apple from 50 DAFB to 170 DAFB were determined by HPLC.CTKs and GA3 showed double peak curve during the observation period,and the peak value appeared from 70 DAFB to 90 DAFB and 130 DAFB to 150 DAFB,respectively.The contents of IAA in roots only peaked on 130 DAFB;in the peak value of hormone level,high load treatment significantly reduced the level of endogenous hormone in roots.On August 10,the endogenous hormone levels of all treatments were at the lowest level with no significant difference in the observation period.5.Under different crop load levels,the relative expression of MdCRF2 and MdCRF6 decreased with the increase of crop load,while MdCRF7 showed opposite behavior;the relative expression of MdARRO-1 up-regulated with the increase of crop load;the relative expression of MdARF5 up-regulated with the increase of crop load;the relative expression of MdIPT1b in different crop load levels showed little difference.