Yield per tree (Figure 2) roughly correlated 

 with tree size, with the largest trees producing 

 the most fruit and the smallest trees producing 

 the least. It is more important, however, to 

 compare potential yield relative to tree size, i.e. 

 more smaller trees can be planted per acre, 

 which may or may not result in more overall 

 5deld. Yield efficiency is a somewhat accurate 

 assessment of relative jdeld potential. It pre- 

 sents 5deld per trimk cross-sectional area. Fig- 

 ure 3 gives cumulative 3rield efficiencies for trees 

 in this planting. The rootstocks break clearly 

 into three groups. The most efficient trees were 

 on P. 16, P.2, B.9, or C.6. The least efficient were 

 on A.313, P. 18, B.490, MAC.l, or seedling. 

 Trees on M.26 EMLA, P.22, M.7 EMLA, P.l, 

 M.4, or MAC. 39 were intermediate in efficiency. 

 A less accurate method for assessing potential 

 3deld uses estimates of planting density based 

 on tree spread (Table 1). In this planting, more 

 containment pruning was used for the largest 

 trees than for the smallest, so potential planting 



Table 1. Projected spacing and tree density of 

 Starkspur Supreme Delicious on various 

 rootstocks in the 1984 NC-140 Cooperative 

 Planting in Massachusetts. 



Spacing 



densities were very rough estimates, particu- 

 larly for the largest trees. Multiplying density 

 by yield per tree gives potential 3deld per acre. 

 Figure 4 plots )deld per acre by year from 1987 

 through 1993. Figure 5 gives potential yield per 

 acre on a cumulative basis over the seven fi*uit- 

 ing years of these trees. The results were similar 

 to those obtained when comparing yield efficien- 

 cies among rootstocks. The highest yields per 

 acre may be expected from trees on C.6, P.2, 

 P.22, B.9, or M.26 EMLA. The lowest may be 

 expected from trees on P. 18, A.313, B.490, 

 MAC.l, or seedling. 



Fruit Ripening 



Knowledge of the effects on finiit ripening is 

 a critical component of rootstock evaluation. 

 The potential for advancement or delay in ripen- 

 ing must be known so that harvest can be 

 managed appropriately. If the delay or ad- 

 vancement is predictable, it may be beneficial to 

 use it to expand the harvest season. 



To assess the effects of rootstock on ripening, 

 internal ethylene, soluble solids (sugars) con- 

 centration, watercore development, and starch 

 loss were measured in fruit fi"om this planting. 

 Ethylene is a gaseous hormone present in all 

 plants, but is very important to ripening in a 

 number of fruits. Ethylene is a trigger of rip>en- 

 ing and during the course of ripening, it in- 

 creases many fold in apple fruit. It is possible to 

 track ripening of apples by measuring ethylene 

 concentration in the core cavity. Table 2 reports 

 the date in 1989, 1990, 1992, and 1993 when the 

 average internal ethylene concentration 

 reached one ppm. Results were not entirely 

 consistent from year to year, but a few 

 rootstocks were consistently either in the lowest 

 or highest category. Specifically, fi-uit from 

 trees on B.9, MAC.39, M.7 EMLA, M.26 EMLA, 

 or P. 16 consistently were among the first few to 

 reach one ppm internal ethylene. Fruit from 

 trees on seedling, M.4, B.490, P. 18, or A.313 

 consistently were among the last to reach one 

 ppm internal ethylene. 



Internal ethylene is one of the most accurate 

 measures of the progress of ripening; however, it 



14 



FruH Notes, Summer, 1994 



