Gala Leaf Surface Area Requirements per Apple

The importance of leaf function, nitrogen and their relationship to fruit size can be quantified. Leaf surface area requirement is an interesting concept to discuss from the point of view that in Ontario we now have the bulk of our plantings on fully dwarfing or semi dwarfing rootstock. The implication here is that these trees on dwarfing rootstock are capable of sending most of the carbon that the tree fixes to the crop at 70% and more.

When comparing trees on size controlling rootstocks to fully vigorous or seedling rootstocks we find that the bulk of the carbon these systems fix goes into growing wood and less than half of the fixed carbon goes to the actual crop. This is one of the outstanding advantages we have in growing fruit on M9 or M26 for example.

Researcher Dr. Lailiang Cheng at Cornell University in New York has studied the relationship between leaf surface area, nitrogen and fruit size in Gala apples under completely controlled conditions. Bearing Gala trees in their 5^th leaf on M26 rootstock grown in containers were studied in sand culture after being fed with various concentrations of nitrogen using Hoagland’s nutrient solution.

The high rate of nitrogen given to the trees was equivalent to about 50 lb/ac of actual N. This rate of nitrogen also produced the greatest bourse shoot growth and best rate of photosynthesis on a whole tree basis.

On M26 rootstock, approximately 70% of the dry matter that the tree accumulated ended up in the fruit and nitrogen appeared to be sufficient at 2.0 to 2.2% for leaf analysis taken in late July. These same trees produced fruit weighing 180 grams on average, which translates into a 113 count size or 3-inch diameter apples.

Dr. Cheng also found that each apple of the larger size category required a leaf surface area of 80-90 square inches or 550 square centimeters. This was based on a crop load maximum of 8.2 fruit per square centimeter of trunk cross sectional area. Mature Gala leaves should be a dark green colour, according to Dr. Cheng.

I would estimate that a typical mature leaf of Gala in good form would be about 8-10 square inches in surface area (Figure 1). That would mean that each apple on the tree finished to 180 grams would need between 8 and 10 mature leaves to get the desired result.

Figure 1. Mature leaves of Gala should be dark green for best rates of photosynthesis and their surface area would typically be 8-10 square inches.

This study helps to explain why sustained large yields of count sized Gala require an adequate leaf surface area to accommodate not only the crop but the tree in general. A thousand bushel crop of Gala grown to an average fruit size of 180 grams would then require a leaf surface area of 8,000,000 square inches, 55,555 square feet or 5,500 square meters per acre.

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