Issue link: https://read.dmtmag.com/i/314286
8 JUNE 2014 GOOD FRUIT GROWER www.goodfruit.com physiologists called together, as their research careers are nearing their end, to talk about what they had learned. Light distribution Apple yield is controlled by fruit numbers, which are determined early in the season, Lakso said. "At that time, fruit is supported by spur leaves, so orchard yield depends on light interception by spurs." Later in the season, shoot leaves contribute more, but early on, they contribute virtually nothing. From that fundamental fact, the shape of modern fruit orchards has emerged. Growers know they have to prune to assure that light can penetrate to all the spurs for the fi rst month after bloom. Trees need to have adequate gaps in the canopy. Trees should be oriented in north- south rows to minimize shading on the north side. Trees should be slender so light can penetrate the canopy. Trees also need to be tall if maximum yields are to be achieved. Lakso's fundamental work contributed greatly to the tall spindle system developed at Cornell by Dr. Terence Robinson and Steve Hoying. Fruit set and drop Lakso's physiological research also laid important groundwork for the Cornell MaluSim carbohydrate model now being used by growers in New York, Mich- igan, Pennsylvania, and other eastern fruit-growing states. Robinson has been the lead researcher adapting the model for practical use, which growers can access through the Northeast Regional Climate Center. The model integrates how weather affects the carbo- hydrate supply versus demand. This helps predict how effective fruit thinning chemicals will be, depending on upcoming weather conditions. The usefulness of the model depends on having accurate predictions of tem- perature and cloudiness, and the New York researchers have worked closely with the Northeast Weather Net- work to help growers have better predictions of weather on their farms. The model is based upon basic principles of carbon production and partitioning that Lakso sought to under- stand. If the carbon supply is limiting, then the relative strengths of the sinks of different tree organs are used to partition carbon, he said. Trees partition carbon to leaves fi rst, then to fruit, and fi nally to roots and tree wood. "Fruit drop if there are too many competing fruit and if they can not continually grow. When early season fruit growth is reduced for any reason, drop occurs when fruit have too low growth rates. Low light causes fruit drop," Lakso said, especially when it occurs with high temperatures, especially at night. "Weather effects on drop and thinning response seem to be consistent with carbohydrate supply/ demand balance," he said. Pests and pruning Over the years, growers have fought their battles against European red mite and foliar diseases that reduce the photosynthetic ability of trees to size fruit. Lakso did experiments in collaboration with entomol- ogists to determine how much effect they had. He found that whole tree photosynthesis, and thus carbohydrate supply, is reduced by leaf damage, by shading of leaves, and by summer pruning to remove some foliage to improve fruit color. The effects on fruit size, however, depend on the level of crop (and carbohydrate demand), with heavily cropping trees being especially sensitive to reductions in carbohydrate supply. "Reduced tree photosynthesis can affect return bloom and future fruit set as well," he said, and also can lead to weak fl owers. "Ideally we need to prune to have open canopies so we don't need to summer prune so much," he said. Water stress Lakso also spent many research hours studying water stress on apples and grapes. "Woody plants are often on variable soils and gener- ally have large, deep, but relatively low-density root sys- tems, unlike annuals," he said. "The water status of trees and vines strongly depends on the weather as well as the soil. It is very dynamic with changes in the weather." Last year, Lakso, working with chemical engineer Abe Stroock and graduate student Vinay Pagay, developed a microsensor than can be embedded in a tree or vine where it can measure water stress directly. Like minia- turized tensiometers that cost around $75, the microsen- sors are low cost because they are chips that can be easily manufactured. They have been tested in the lab and are very accurate over a large range. They were described in an article in Good Fruit Grower in the December 2013 issue. Lakso said the microsensor was a dream come true. He's been working to develop such a thing for more than 15 years, and, on the eve of his retirement, it's within reach. The device is being patented. • Alan Lakso used "full canopy balloon chambers" to measure carbon dioxide removal from air to study whole tree photosynthesis and later used them to study water stress on trees. PHOTO COURTESY OF ALAN LAKSO "Ideally, we need to prune to have open canopies so we don't need to summer prune so much." —Dr. Alan Lakso