Method for inhibition of head formation in lettuce

This invention relates to the use of catechol or substituted catechol half esters of beta-haloethylphosphonic acid to inhibit head formation in lettuce. Increased pigmentation of the head leaves is achieved as a result of the application.

CROSS-REFERENCE TO RELATED APPLICATIONS 
Not applicable. 
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
Not applicable. 
BACKGROUND OF THE INVENTION 
This invention relates to the use of catechol or substituted catechol half 
esters of beta-haloethylphosphonic acid to inhibit head formation in 
lettuce. 
Commercially important varieties of crisphead lettuce (Lactuca sativa) form 
heads, as they mature. Heads occur when the leaves curve over the center 
of the plant and fold over on each other tightly such that the exposure of 
the interior leaf tissue to sunlight is inhibited. Typically, these leaves 
are not well-pigmented and lack an abundance of chlorophyll in comparison 
with leaves that have been exposed to the sun. Though consumers enjoy the 
crisp nature of head lettuce, the lettuce lacks another highly valued 
consumer trait, darker green pigmentation, with a resulting higher 
.beta.-carotene content. 
Some phosphonate compounds have found use in the agricultural art, for 
example, for herbicidal purposes as described in U.S. Pat. Nos. 2,927,014 
and 3,223,514. Compound 2-chloroethylphosphonic acid has been described in 
U.S. Pat. No. 4,240,819 to be useful in inhibiting plant growth. It is 
currently marketed for use in tomatoes, cherries, grapes, apples, walnuts, 
peppers, blackberries and cantaloupes and is advocated for the use of 
promoting early coloration and maturity of tomatoes, grapes, apples and 
pears. In the case of cherries, walnuts and apples, it loosens the fruit 
for earlier, more efficient harvest. The compound promotes fruit 
abscission in cantaloupes and accelerates ripening and loosening of 
blackberries. 
SUMMARY OF THE INVENTION 
This invention provides a method for increasing the green pigmentation of 
head lettuce leaves and increasing the nutritional content while retaining 
the crisp leaf quality of these varieties. We have discovered quite 
surprisingly that catechol or substituted catechol half esters of 
beta-haloethylphosphonic acids such as 2-chloroethylphosphonic acid have 
the effect of inhibiting head formation in lettuce by causing a vertical 
orientation of the leaves such that leaves usually found in the head are 
exposed to the sun. This permits a highly desirable greater degree of 
green pigmentation of the leaves and increased nutrition. 
More particularly, the invention is a method for inhibiting head formation 
in a head-forming Lactuca sativa plant which comprises applying a catechol 
or substituted catechol half ester of beta-haloethylphosphonic acid to the 
plant after leaf formation. Such head forming lettuce plants include 
crisphead lettuce and butterhead lettuce. Preferably the 
beta-haloethylphosphonic acid is applied to a crisphead plant after about 
15 to 20 leaves have formed on the plant. The beta-haloethylphosphonic 
acid is applied preferably in a soluble form by spray application at a 
rate of 50 to 3,000 mls per acre. Head lettuce varieties of Lactuca sativa 
plants treated with the formulations described herein are also claimed.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention presents a surprising use for catechol or substituted 
catechol half esters of beta-haloethylphosphonic acids in inhibiting head 
formation in head-forming lettuce plants. The application of one of these 
compounds after true leaves have developed results in a decrease in the 
formation of the head such that interior leaf tissue is more exposed to 
sunlight. Chlorophyll concentration in the leaves is increased and the 
resulting leaves have a greater green color which is attractive to the 
food consumer. It is expected that such increase in chlorophyll formation 
results in an increase in nutritional value of the plant. The application 
of these compounds make the lettuce particularly desirable for those 
preparations in which the leaves are to be used and sold in pre-packaged 
salads or where the leaves are to be sold separate from the intact plant. 
The application of catechol or substituted catechol half esters of 
beta-haloethylphosphonic acids such as 2-chloroethylphosphonic acid may be 
made to any head-forming lettuce including the crisphead or iceberg types. 
Head lettuce or head-forming lettuce refers to those varieties of lettuce 
in which the leaves grow in a dense rosette. There are generally two 
subcategories of head lettuce, crisphead (commonly known as iceberg) and 
butterhead (the Bibb and Boston varieties). Crisphead lettuces are those 
lettuce plants which in the absence of the treatment proposed herein, form 
loose or compact heads with leaves that occur after about 10-15 true 
leaves have formed. The head leaves fold over or otherwise grow over each 
other such that the inner leaves are shaded from the sun. 
The catechol or substituted catechol half ester of beta-haloethylphosphonic 
acid used in the method of the present invention is soluble in water and 
can be applied to plants in aqueous solutions composed wholly or partially 
of water. Partial solutions include those formed of water and, for 
example, acetone or methylethylketone so long as they are non-toxic to the 
plant in the final solution. Any liquid medium thus may be used, provided 
that it is not toxic to the plant. The 2-chloroethylphosphonic acid is 
marketed commercially by Rhone-Poulenc AG Company, Research Triangle Park, 
N.C. under the trademark Ethrel.RTM.. The preparation of catechol or 
substituted catechol half esters of beta-haloethylphosphonic acids is 
disclosed and described in U.S. Pat. Nos. 3,531,549 and 3,551,528, which 
are both incorporated by reference herein. 
The catechol or substituted catechol half ester of beta-haloethylphosphonic 
acid such as 2-chloroethylphosphonic acid may be applied in any manner 
such that it is preferably directed to the leaves of the plant. Though the 
beta-haloethylphosphonic acid described here is generally soluble in 
water, it may also, if desired, be absorbed onto solid carriers, such as 
vermiculite, attaclay, talc or other suitable carrier for application via 
granular vehicle. Application of water-thinned solutions or solids is 
accomplished using conventional equipment that is well known in the art. 
Although the preferred method of application of the compound used in the 
process of this invention is directly to the foliage, the compound may be 
applied to the soil in which the plants are growing, such that the 
compound can be root-absorbed to a sufficient extent so as to result in 
inhibition of head formation in accordance with this invention. 
It is preferred that the compound used in the present invention be applied 
to crisphead foliage after about 15 to 20 true leaves have formed and, in 
the case of butterhead foliage after about 10 to 16 true leaves have 
formed, and before formation of the head. The timing of the application 
will necessarily vary depending upon the variety of interest and the 
climate, as is known and appreciated by those in the art. For example, 
when cold weather varieties are grown in hot weather climates, the heads 
tend to be looser and the concentration used to inhibit head formation 
could be less in those cases. Climate conditions may affect how much of 
the compound actually comes into contact with the plant, as is well known 
in the field. A single application is generally sufficient to accomplish 
inhibition of head formation though multiple applications are also 
acceptable. For example, if a second application is to be utilized, it is 
preferred that the application be applied about 3 to 10 days after the 
first application, depending upon growth conditions. 
The catechol or substituted catechol half esters of 
beta-haloethylphosphonic acids such as 2-chloroethylphosphonic acid may be 
applied to the foliage in an amount sufficient to obtain the desired 
inhibition of head formation. For example, where Ethrel.RTM. is chosen, it 
is supplied in a 21% solution which is applied in a range of about 50 to 
about 3000 ml/acre, preferably in a range of about 200 to about 2000 
ml/acre and most preferably in a range of about 300 to about 3000 ml/acre. 
Alternatively, the compounds described here may be applied to an acre at a 
rate of about 100 to about 2400 ppm, more preferably at a rate of about 
200 to about 1200 ppm. Typically on a lettuce crop, one spray application 
would be made at one of the suggested rates. The total volume of solution 
sprayed would be at the discretion of the applicator taking into 
consideration the equipment being used and the weather conditions. 
Inhibition of head leaf formation can be readily observed. Typically, the 
leaves found in the head are in contact with or immediately adjacent the 
leaves above and below them. Inhibition of head formation results in 
leaves that do not significantly contact each other and tend toward a more 
vertical growth orientation. 
Typically, crisphead leaf lettuce will have wrapper leaves which are the 
first leaves to emerge and are those leaves which are not involved in head 
formation and have a greater green pigmentation. Generally a mature 
lettuce plant will have about 15 wrapper leaves. If one counts from the 
outside of the plant toward the center past the wrapper leaves, the next 
leaves are considered the head leaves. Beginning with the oldest head 
leaves and working in, the oldest head leaf being designated as head leaf 
1, one can count the head leaves of two plants to be compared. Differences 
between head leaves numbered 5-10 of a plant treated with 
2-chloroethylphosphonic acid, for example, compared with head leaves 
numbered 5-10 of a control plant can be observed. Leaves 5 through 10 that 
have been treated with 2-chloroethylphosphonic acid will exhibit a 
substantially more planar and perpendicular growth orientation relative to 
an unsprayed control. A visible and measurable increase in green 
pigmentation of the inner leaves can be observed when a corresponding leaf 
from a treated plant is observed against a control plant of the same 
variety grown under the same conditions without treatment. See the figures 
for illustration purposes. 
The following example is provided for illustrative reasons and is not to be 
used to limit the invention as claimed. 
EXAMPLES 
The following examples are provided for the purposes of illustration and 
are not to be considered limitations on the invention. 
Example 1. A crisphead or butterhead variety of lettuce is planted. When 
the plants have on average about 15 leaves, a water-based preparation of 
2-chloroethylphosphonic acid at 800 ppm is sprayed on the plants with a 
conventional sprayer in a solution such that it is applied in a final rate 
of about 60 gallons/acre (about 227 liters).