Perspectives on Applied Physiology in Support of Viticulture

September 2017
by Dr. Alan Lakso

Developing a long-term physiology research program that has relevance to practical fruit production requires an integrated approach of studying the physiology of practical problems as well as generating fundamental knowledge of physiological principles that underpin practice. My program emphasized understanding of the physiological principles of carbon and water physiology, especially integrated with growth and development, environmental responses and interactions with cultural practices. As such, we have used many different research methods, from measuring growth, organ and whole-plant gas exchange, soil and plant water relations, and root growth and development.

The emphasis on understanding physiological principles is informed by a quote from Ralph Waldo Emerson: “The value of a principle is in the number of things that it will explain.” This can be manifested in not only explaining past results, but also in being able to better predict results under different conditions. To help with both of these, I incorporated modeling into my program, integrated with measurements.

Modeling has been an extremely valuable research tool for several reasons. Models can help integrate measurements taken only at intervals, evaluate the state of knowledge of our systems, elucidate the key processes regulating system behavior, help understand system dynamics and predict performance under differing conditions. Of course, as great simplifications of complex systems, they are rarely precise. However, the usefulness for specific questions is more important than absolute accuracy. Finally, it is important to understand that models do not produce answers, only quantitative hypotheses to test.

I have addressed many issues in 40 years of research, but three examples indicate how physiology can be useful to practical farming.

Apple thinning
I developed a simplified, dynamic, daily time-step, carbon supply-demand simulation model with versions for grapes (VitiSim) and apples (MaluSim) to estimate plant photosynthesis, respiration and organ growth. This model is used to examine seasonal patterns of carbon supply-demand balances and look for carbon deficits or excesses at critical periods of fruit development. It was found that in one growing season there were major fluctuations in carbon supply versus demand in both grapes and apples, and that they helped explain key growth and development processes.

The MaluSim model was developed to integrate occasional measurements of leaf function, shoots and fruit growth and organ respiration through the season, and to see if there were major supply-demand variations related to productivity. Compared to measurements, the model has given very good estimates of dry matter production and realistic behavior.

The model identified a period about one to three weeks after bloom, when an apple tree would be under carbon deficit due to the extreme demand of the excess number of young fruitlets. Weather such as cloudy, warm periods of several days at that time, which can lead to natural fruit drop and strong response to chemical thinners, caused clear carbon deficits.

Collaboration with Terence Robinson, a management specialist on apple thinning, demonstrated that the carbon balance of an apple tree could be a useful tool to assess the tree sensitivity to chemical thinners. We currently have an online system for growers to run the model simulation and make adjustments in thinners to obtain more precise crop adjustment. This has been very useful as a decision-support tool for apple growers and an example of how understanding physiological processes of importance to fruit production can lead to useful management tools.

Effects of pruning on grapes
The Concord juice industry in New York has not seen significant increases in price paid per ton of grapes in more than 40 years. This has led to great efforts to reduce production costs, mechanize and increase sustainable yields.

One of the key practices has been to reduce pruning costs with mechanical or minimal pruning. A 12-year study of normal versus minimal pruning effects on Concord grapevines revealed minimal pruning generally led to higher yields and lower Brix, suggesting over-cropping. Yet minimally pruned vines gave more sustainable and consistent cropping year-to-year (always 10 tons per acre or more) than normal pruning.

A great deal of data was collected on yield as well as seasonal canopy development and fruit growth, gas exchange and water relations. This data allowed us to integrate all these factors into the VitiSim model. The model is able to point out that the very rapid canopy development in minimally pruned vines gives a more rapid attainment of canopy photosynthesis and much lower demand from shoot growth by bloom. The improved carbon balance coincided with the period of fruit set and flower bud development and likely explains the ability to maintain high yields over many years.

In contrast, by véraison normally pruned vines had developed similar canopies giving similar vine carbon supply. However, the larger crop on minimally pruned vines then led to lower Brix at harvest. The practical solution for minimal or mechanical pruning, developed by colleagues Bob Pool and Terry Bates, has been to wait until fruit set is done and 50% of final berry size is achieved before estimating potential yield and then mechanically thinning the vines to a crop level that can be ripened. This practice takes advantage of the benefit of minimal pruning for yield but overcomes the limitation of excess crop during ripening.

Monitoring of grapevine water status
The importance of water status for regulating vine growth, fruit composition, wine character and expression of terroir has been recognized. Optimal patterns of water stress at key times to improve wine quality are evolving, but we do not have the tools to monitor the dynamic behavior of vine water status in the field. The pressure chamber gives good results but has limited value as a manual tool giving only spot readings.

Cornell engineering colleague Abraham Stroock and I collaborated to develop an electronic microchip version of a tensiometer for soils and also to insert into the trunk of vines to monitor stem water potential continuously. In initial tests in our lab and the vineyard, we found that t he sensor is effective and the results correlate with the standard pressure chamber estimate of stem water potential. We believe this will give the grower much more information to optimize water management for grape quality. It is being commercialized by a spin-off company from our research called FloraPulse.

Lessons learned
Many years of research and learning from colleagues have taught me some lessons:
• Be careful of interpreting correlations as cause and effect. Even those that seem logical may be correlations due to both factors being correlated to a third factor.
• Grapevine roots do not grow in consistent patterns and can vary each growing season. Despite many years of detailed studies, we could not relate patterns of root growth to behavior of the top of the vine, which has very consistent behavior. The reasons why are not clear, and the lack of coordination is puzzling.
• The effects of stress (low light, water stress, foliar pests, etc.) are generally stronger if the crop load is larger. Heavily cropped vines already struggle to ripen a crop, so any additional stress will have a strong effect.
• The corollary is that the effect of crop load on vegetative growth is stronger if there are other stresses. In the absence of stress, the grapevine can support a surprisingly large crop and strong vegetative growth.
• Nelson Shaulis (the late Cornell University viticulture professor) emphasized that we need to be much more precise in our thinking and language than we normally are, especially about important but often poorly defined concepts such as “competition,” “crop load,” “quality,” “vigor,” “resistance versus tolerance” and “vine balance.” A great question to improve precision is, “What are the units of these terms?”
• In research we often have to accept that we were wrong and must try again. The words of Sir Winston Churchill are apt: “Success is the ability to go from one failure to another with no loss of enthusiasm!”
• We tend to treat our hypotheses like our babies. They must be protected and defended. Too often experiments are conducted or data selected to support our hypotheses, when instead we must be strongly skeptical and do experiments to disprove our hypotheses. Philosopher Karl Popper beautifully described science: “The method of bold conjectures along with ingenious and severe attempts to refute them.” If we do our very best to find any possible condition or experiment to disprove our hypothesis and cannot, then the hypothesis is likely much more sound than the other way around. Be your harshest skeptic.
• Complex problems require team research. Find excellent team members with enthusiasm and expertise and treat them as equal colleagues.

The future
• Understanding crop physiology to improve practice will continue to be important. Models will be an increasingly important tool for environmental footprints for the industry and climate change estimation. 
• Genomic information generally does not translate directly to field performance. Better integration of genomics and whole-plant physiology is needed as a link between genomics and viticulture.
• Physiology will be needed to:
      • Ground-truth remote sensors,
      • Develop new measurement parameters for sensing,
      • Provide models to integrate impacts from sensed data with geographic information systems (GIS) to display outcomes for management use. 

Dr. Alan Lakso completed a Ph.D. at the University of California, Davis, in 1973 on “Effects of Temperature on Malic Acid Metabolism in Grapes” with Dr. Mark Kliewer. He moved to Cornell University’s New York State Agricultural Experiment Station in Geneva, N.Y., to begin a program on grape and apple physiology and retired in 2014 and is a Cornell professor emeritus.

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