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Ozone on Long Island and its Impact on Plants

Ozone continues to be an important air pollutant in the atmosphere.  It is often referred to as ambient ozone, ground-level ozone, or tropospheric ozone to distinguish it from the ozone layer in the stratosphere that is beneficial.  Every summer on Long Island ozone reaches high enough concentrations to cause visible foliar injury in sensitive plants as well as be a concern for human health.  While regulations have resulted in a reduction in vehicular emissions, which are an important source of precursors for the formation of ozone by the action of uv radiation, there has been an increase in the number of vehicles.  In addition to motor vehicle exhaust, industrial emissions and chemical solvents are the other major human-generated sources of these chemicals.  The agricultural area of Long Island being downwind of a large urban area means it is in a prime spot for episodes of high ozone.

Data on ozone concentrations at several locations in NY is obtained by the DEC Air Quality Monitoring program and is available at http://www.dec.ny.gov/chemical/8406.html.  One location is LIHREC in Riverhead.  At the webpage under ‘Important Links’ on the right is ‘Current Air Quality Measurement Data’.  Click on the link to get a map with current conditions.  Tables with high ozone values for days during current year and pdf files for previous years are at http://www.dec.ny.gov/chemical/38377.html.  Ozone concentration is considered high when greater than the National Ambient Air Quality Standards (for humans), which is currently average of 70 ppb over 8 consecutive hours in a day.  There is always some ozone in the air.  Typically the concentration is below 30 ppb.  Any day that the average was above 70 ppb at any location, the highest average for all other locations are also included in the table.  Riverhead average is often one of the highest, even compared to the other more urban NYC metro area monitoring sites.  This is actually not surprising.  Ozone concentrations tend to decline more quickly in urban areas because vehicles and other sources of precursors for ozone formation are also sources of compounds involved in breaking down ozone.

Ozone is more toxic to plants than other common air pollutants.  And plants generally are more sensitive to ozone than people.  Injury includes stippling (white to tan spots) and bronzing, which can lead to leaf death.  Type of injury varies with the plant.  Stippling occurs with spinach and cucurbit crops.  Bronzing occurs with beans, potato and tomato.  There are photographs posted at http://blogs.cornell.edu/livegpath/gallery/.  Ozone injury is especially devastating with spinach because a crop can be rendered unmarketable.  Injury often appears all of a sudden and is widespread in a planting, reflecting leaves being damaged at the same time.  And often one age group of leaves is most severely affected, which are the leaves that during the time of exposure were most actively transpiring (stomates wide open allowing ozone to enter).  Leaves without acute (visible) injury may also die prematurely because ozone induces accelerated senescence of leaves that involves many of the genes involved in natural senescence.

Occurrence of injury to leaves due to exposure to ozone varies from year to year reflecting temporal variation in when ozone is high, susceptibility of crop and variety, growth stage of plant when exposed, and other environmental conditions at the time.  Extensive bronzing was observed in some potato crops in 2017.  Potato varieties are known to vary in susceptibility.  In mid-June there were 4 days (June 10-13) when ozone was high.  The eight-hour ozone averages were 69, 72, 89 and 88 ppb, respectively.  Potato plants at that time on Long Island typically are in an actively-growing, susceptible phase.  And environmental conditions likely were good for growth with adequate soil moisture (rain occurred 4-5 days before the start of this high ozone episode) and solar radiation was high while ozone was high.  There have been times on Long Island when limited injury developed following a high ozone episode because it occurred during a dry period when plants would have closed stomates.

Presumably injury and senescence induced by ambient ozone affects the productivity of plants; however, determining this impact is challenging.  Assessing impact necessitates having plants not affected by ozone to compare with plants that are affected.  The main method that has been used entails growing plants outdoors in specialized chambers with charcoal-filtered air next to plants grown in similar chambers with non-filtered air.  A disadvantage of this system, aside from the cost, is the fact that the environment inside is different from that outside the chamber where the plants normally grow and this could have a confounding effect.

An alternative method for investigating ozone impact has been identified that entails comparing two lines or clones of a plant that differ in sensitivity to elevated ozone but have similar productivity when ozone levels are low.  A system with snap bean was developed for assessing ozone impact.  The two bean clones are grown outdoors in the ground.  Both fresh market and mature yield are measured by removing pods from some plants every week as they reach size for fresh market consumption and removing pods once they have dried from the other plants.  Since beans reach maturity in just 12 weeks, two to three successive planting times are needed to cover the entire summer growing period.

Ambient ozone on Long Island has been demonstrated to have an impact on plant productivity using the bean system at LIHREC.  This work was conducted over several years.  During growth periods when ozone levels measured at this location were low, which was sometimes in the spring and other years during fall, the ozone-sensitive and tolerant plants did not differ significantly in the number or weight of bean pods produced.  This documents these pairs do produce similarly when ozone is low, providing validity to the system. Leaves of the sensitive bean have exhibited bronzing, which often has been sufficiently severe to result in the leaves drying up and dropping off the plant.  Images of the injury are posted at http://blogs.cornell.edu/livegpath/gallery/beans/ozone-injury/.  Impact of episodes of high ozone on productivity of bean has been extremely high.  Weight of beans harvested immature for fresh market was reduced as much as 62%.  There were up to 56% fewer bean seeds in mature pods.  And the average weight of those seeds was up to 42% lower.

A simple relationship has not been found between ozone concentrations that plants were exposed to and the subsequent impact measured.  This partly reflects the fact the dose of ozone that gets inside of plants depends on stomatal conductance and other aspects of flux.  If plants are water stressed when ozone is high, stomates will be closed, and thus ozone dose will be lower than for well-watered plants.  Sometimes an acute exposure (several days of very high ozone concentrations) can result in severe leaf injury that is more detrimental to plant productivity than moderate high ozone levels on all the other days during the growth period.  For example, ozone exceeded 80 ppb on 6 of 7 days during 15 – 21 July 2007 with hourly average reaching 120 -128 ppb three times.  Very severe ozone injury was observed on 25 July, which was 6 days before the first harvest of fresh market pods in the second planting that year.  High ozone events in Riverhead have varied from year to year since 1996. The yearly highest 1-hour ozone concentration recorded has ranged from 104 ppb to 168 ppb.  The date that this has occurred has varied from 7 June to 9 August.  Ozone has been at least 80 ppb for as few as 40 hours on 8 days during a growing season and as many as 184 hours on 31 days.

 

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