Those are conclusions reached thus far by Nick Dokoozlian, University of California Extension viticulturist, in his continuing studies on the synthetic cytokinin, including its interaction with gibberellic acid (GA) and ethephon used to enhance grape quality.
CCPU, the acronym of the Valent Agrosciences compound’s long chemical name, is going through its paces under an experimental use permit, which expires after the 2003 season. The EUP allows the product to be applied to 3,500 acres of grapes annually.
Dokoozlian brought growers up to date on his research during a recent grape day at the Kearney Agricultural Center at Parlier.
Data has been gathered on Thompson Seedless and Flame Seedless, and this season he is concentrating on the seeded Redglobe and the GA-sensitive Melissa and Crimson Seedless.
"One problem with CPPU is, since it does increase berry size and delay maturity, it reduces berry color, particularly with colored varieties such as Flame Seedless and Redglobe.
"It also alters berry shape. Gibberellic acid elongates berries, but CPPU at higher rates causes them to become more round. And it increases fruit set, so you have to be very careful about application timing," he said.
Research over the past 10 years with the product shows it performs with three to 12 grams per acre on white varieties and lesser rates on red varieties. Although more data is being collected, effects on black varieties are thought to similar to those of reds.
"The lower range of three, four, or five grams per acre gives a minimal fruit-sizing effect and stem thickening. Beyond six to eight grams you start to see severe inhibition of color and if higher rates are timed precisely you may see maturity delayed as much as three weeks on Thompson Seedless."
Typically, about a week after fruit set, CPPU is applied to intersect with cell elongation for maximum effects on berry size. If applied too early, Dokoozlian said, it can bring on increased fruit set, but CPPU timed at the optimum time will retain berries and make clusters more compact. For most cultivars that means adding CPPU with GA in the second sizing spray.
"Now we are looking at applying CPPU later in the season, at fruit softening when color is starting to accumulate, with the idea of delaying maturation without affecting berry size and -- more importantly -- without affecting berry color."
Going further into the relationship between berry size and CCPU, he said CCPU won’t replace GA, mainly because of costs. A CCPU treatment of seven to eight grams per acre for sizing costs about $300 per acre, vs. GA at commercial rates for about half that amount. Using lesser amounts of both products creates a synergistic effect and is more economical.
And what about the color reduction said to be associated with CCPU? Dokoozlian said CPPU at very low rates indeed promotes color. However, at rates of four to six grams, it can cause irreversible decrease in color, defying even high rates of ethephon to enhance color.
"CPPU greatly increases berry firmness and increases cap-stem removal force. So fruit goes into storage firmer, and with the greater stem removal force required we have less shatter."
Questions have been asked about CPPU and potential water loss, but Dokoozlian said he found no significant difference in water loss between treated and untreated fruit in cold storage.
He reminded growers that even if berries are enlarged and pack out with more boxes per acre, color will still be key in how much of the crop is marketable.
And, he added, he doesn’t expect high (12 gram/acre) rates of the product to be used in California. Rather, the more common practice would be three- to six-gram treatments, depending on the vineyard and the grower’s harvest objectives.
"One place you might be able to use high rates is with a white variety like Thompson Seedless where you are trying to delay maturation. But it can alter appearance, leaving the fruit looking a little green even when fully ripe and the stems looking gnarled. Consumer acceptance of fruit treated with very high rates is questionable."
Dokoozlian noted that CCPU was first applied to table grapes in Chile, where it is no longer widely used. Fruit treated with high rates met with negative comments on appearance upon its arrival in the U.S.
No effects on vegetative growth of vines has been observed, he said, and CPPU does not reduce fruitfulness of either seeded or seedless cultivars in the year following application.
However, it has been reported to alter berry flavor and texture in Flame Seedless and Redglobe, and it has been shown to reduce seed number and seed size in seeded grapes.
According to Dokoozlian, Japanese scientists discovered CCPU about 25 years ago. Early tests showed it have greater biological activity on plants than naturally occurring cytokinins.
During testing on several crop plants from tobacco to grapes later in the 1970s, researchers found that five to 15 parts per million of the compound applied at fruit set would double the size of natural Thompson Seedless berries. In the same experiments the synergistic effect of CPPU and GA was observed.
CPPU was registered for use on table grapes in South Africa, Chile, and Mexico in the mid-1990s, and in 1998 Abbott Laboratories, predecessor of Valent Biosciences, bought the marketing and distribution rights for the product in California. Valent received the EUP for it early this year.
CCPU is also known as KT-30, the original name given by the Japanese researchers who synthesized it.