Editor's Note: This article is part of a semi-regular series of research briefs sponsored by the Department of Viticulture and Enology at UC Davis. For more summaries of research sponsored by UC Davis Department of Viticulture and Enology, see http://wineserver.ucdavis.edu/trellissummary_categories.php.

When it comes to formation of clusters, light is everything. For this reason, it is a pleasure to write about an article that has received the recognition as "Best Paper in Viticulture" in 2006 from the American Society of Enology and Viticulture. I am talking about "Bud microclimate and fruitfulness in Vitis vinifera L." authored by Luis Sanchez and Nick Dokoozlian in American Journal of Enology and Viticulture, 56:4, 2005. What I like best in this paper are the many questions that the authors tried to answer and how they went about recreating the precise set-ups that would allow them to obtain each answer, one at a time, like unfolding acts of a play.

Definitions

Vine buds are compound buds comprised of one primary bud--the one which normally forms the shoot--and two secondary buds, which do not normally push. Inflorescence primordia are miniscule undeveloped clusters inside either the primary or the secondary buds. The term for the singular is primordium. Their size is related to the number of flowers with potential to transform into a berry and, therefore, to final cluster size. Potential fruitfulness is fruitfulness detected before budbreak. This contrasts with observed fruitfulness, which is fruitfulness after budbreak, that is, clusters that we are able to visually count.

We can express potential fruitfulness in two ways: as 1) percent bud fruitfulness (number of buds that contain one or more inflorescence primordia) or as 2) number of inflorescence primordia per node. Even though these two terms are very similar, if you pay attention, you will notice that when we express fruitfulness in the first way, a bud containing one inflorescence would count the same as a bud containing two or more inflorescences. This is the reason why the second definition came about. But this second definition is not free of bugs. This is because a tiny cluster primordia present in the smallest of the secondary buds would be counted the same as the large primordia present in the primary bud even though they will give rise to clusters of very different weight. This is why the current authors propose yet a third way to express potential fruitfulness: 3) integrated fruitfulness index (IFI) or the sum of the diameters of all inflorescence primordia per bud, which captures both the number and size of the inflorescence primordia present in any given node. Finally, observed fruitfulness is expressed as number of clusters per node. I think it is important that you feel comfortable with these terms--or else most of the paper's magic will be lost.

Stellar Experiments in the Field and in the Growth Chamber

Let's now look at the ingenious experimental set-ups the authors designed. To be able to attain different levels of light in the field, the authors "borrowed" the neighboring vines' shoots and trained them over the experimental vines in different ways until they were able to create varying levels of light exposures. They did this for each of the four varieties they wished to study: two table grapes [Thompsom Seedless (TS), Flame Seedless (FS)] and two wine grapes [(Chardonnay (CH) and Cabernet Sauvignon (CS)].

To study how the different light regimes they had created affected bud fruitfulness, the authors dissected the buds at each of the five basal nodes of representative shoots and counted the inflorescence primordia. (For TS, known to have unfruitful basal buds, they dissected nothing short than the first 15 buds--a lot of dissecting!) By placing a minute photodiode next to each bud, the authors were able to measure photosynthetically active radiation (PAR)--how much "quality" light was reaching the buds in a continuous fashion. And with the help of another tiny instrument, a hypodermic thermocouple inserted into the bud, they also measured diurnal temperature.

In the second part of the article, the authors tackled the experiments in the growth chamber. Here, they used thermostats to recreate three different temperature regimes (18, 25 or 32°C; 64, 77 or 90°F). Then, to be able to superimpose different light levels to the various temperatures, they did once again something original. They attached the pots containing the experimental vines at different heights on the wall--increasing their distance from the lamps--until they received 7, 21, 35 and 50 percent of light. They also allowed only one shoot from each vine to grow horizontally, so light exposure was the same for all the buds along the shoot. Let's now take a look at the results.

Effect of Light on Number of Clusters

For all cultivars studied, gradual increases in light exposure caused a gradual increase in bud fruitfulness. In general, wine grape cultivars (CS, CH) were more fruitful than table grape cultivars (TS, FS). Within each category, CH was more fruitful than CS, and FS was more fruitful than TS. The authors were also able to confirm that the three or four most basal nodes in TS were extremely unfruitful. This explains why we normally prune this variety to canes instead of spurs.

But why this increase of inflorescences with increased light? Much of the above effect was due to the fact that increased light exposure caused an important increase in the number of inflorescences in secondary buds. Once again, TS behaved a little differently, and the contribution of secondary buds to fruitfulness in this cultivar was very small. As Winkler already noted in 1933, this is the reason why TS does not have a good reservoir of inflorescences arising from the secondary shoots that replace dead primary shoots in the event of a frost.

As the authors noted, these results are in agreement with previous studies, which had found that light and temperature are the most important climatic factors for inflorescence induction and differentiation. For instance, one author had shown that optimum temperatures for bud fruitfulness are higher than for vegetative growth (35°C or 95°F was optimum in Muscat of Alexandria). Another Australian author showed that the climatic factor that best correlated with bud fruitfulness in TS was "hours of sunshine during a period of 20 days in the spring."

Effect of Light on Size of Clusters

The diameter of inflorescence primordia in primary buds--those contributing the most to final crop--increased proportionally with light exposure for all cultivars. In contrast, light did not have an effect on the size of inflorescence primordia of secondary buds, which were much smaller and uniform. To recapitulate, the contribution of secondary buds to increased fruitfulness during higher light regimes was due to an increase in the number of primordia, not an increase in primordia size. In contrast, both number of primordia and diameter of primordia in primary buds tended to increase with light.

Next, the authors asked themselves if there was such a thing as a light saturation point? They observed two types of response, depending on the cultivar. CS and TS seemed to reach maxima at about one-third of full sunlight. In contrast, fruitfulness in CH and FS (previously shown as the most fruitful of the four cultivars studied) did not saturate at any point for the range of irradiance values tested. As the authors noted, this suggests that trellis decisions and canopy management aimed at distributing sunlight evenly throughout the canopy may be more critical in some varieties than in others. That is, varieties that do not readily saturate would have more room for improvement.

Potential Fruitfulness vs. Observed Fruitfulness

It would certainly be nice if, by increasing canopy light, we were able to increase yields by two- or three-fold, just as we do with potential fruitfulness. But, how much of the change in potential fruitfulness actually translates into observed fruitfulness? The reality is that, as expected, potential fruitfulness was much higher than actual observed fruitfulness. And the higher the light level, the higher the deviation. As the authors explain, this is no surprise if we consider that potential fruitfulness expressed as Integrated Fruitfulness Index (IFI) included primordia in secondary buds. Secondary buds do not generally emerge in the spring, so these buds do not have a chance to contribute to observed fruitfulness and, in turn, to yield.

Not only did light affect the fruitfulness of secondary buds, but high (and medium-high) light exposures were also able to increase the amount of secondary shoots that pushed--that is, shoots arising from secondary buds. Both effects--more fruitfulness and more secondary shoots--happened at different rates in the different varieties, thus increasing more or less the Integrated Fruitful Index. Let's see an example. Even though potential fruitfulness was higher in CS than in CH, observed fruitfulness was actually higher in CH. The explanation is that CH pushed more shoots from secondary buds. That is, even though many potential flowers in CS did not have a chance to contribute to the total inflorescence pool--the shoots that carried them never pushed, the initially shy CH was the one that "walked the talk" as far as pushing shoots go.

The above example illustrates why IFI is not a good indicator of final crop, unless, as the authors point out, it is broken down into its primary bud and secondary bud components. Then, the contribution of primary buds to IFI would be expected to be a better crop predictor than percentage fruitfulness. This is because, as we saw, the latter would be insensitive to the presence of one or multiple primordia in the primary bud. In contrast, by integrating both number and size of primordia in a node, IFI is an excellent index of potential fruitfulness--that is, of the capacity of the node to differentiate inflorescence primordia.

Field Response vs. Growth Chamber Response

In a growth chamber, or greenhouse, researchers can do away with the variation inbuilt in any field experiment. By extending the studies in the field to the growth chamber, the authors were able to explore in detail the effect of different temperatures and light exposures on two of the above table grape cultivars: Thompson Seedless (TS) and Flame Seedless (FS).

In agreement with the field studies, they found that both temperature and light had a significant effect on bud fruitfulness. When three chamber temperature regimes were compared, both TS and FS performed best--that is, had higher percent fruitfulness--under the middle of the three temperature regimes tested (25°C or 77°F). TS coped with lower temperatures (18°C or 64°F) better than FS. FS was, in turn, the variety best equipped to withstand higher temperatures (32°C or 90°F). In brief, the ideal temperature to maximize fruitfulness depended on the variety.

As for the impact of light in a chamber, when four light regimes were compared, higher shoot light exposure increased bud fruitfulness up to a point. In agreement with the results found in the field, TS required less light to reach maximum fruitfulness than FS. The authors note here the very different light conditions of the chamber compared to the field. Even though maximum attainable irradiance in the chamber was only half that of a real field at noon, interception in the chamber was constant for almost 16 hours per day. The overall result was that vines in the growth chamber were able to intercept three times more daily irradiance than the vines in the field. For the same reason, photosynthesis would have been much higher than in field conditions, a factor that would likely have affected the total fruitfulness observed.

But the authors were also wondering, is it light on the shoot or light on the individual bud that matters? By studying the correlations between light and fruitfulness focusing either on individual buds or on all the buds present on a given shoot, the authors were able to arrive at the interesting conclusion that bud fruitfulness depended on the light intercepted by the whole shoot that carried the bud rather than on the light intercepted by the bud itself.

Finally, increasing light exposure not only increased fruitfulness, but it also significantly increased the diameter of shoot internodes. Thus, the authors found that internode diameter was the visual parameter which best correlated to bud fruitfulness. Does this mean a grower could be using internode diameter as a guide to select for fruitful canes? Unfortunately, pruning for larger diameters has its limitations. As we know, shoots of excessive diameter have also excessive vigor. Too much vegetation or, even worse, uneven growth can easily cancel out the benefits of a potential larger crop.

Conclusions

In this awarded paper the authors were able to develop a "fruitfulness road map" that can be used as a guide to the potential fruitfulness of every bud position, under each of four light levels, for the four most important grape varieties in California. Based on what they observed, they were able to conclude the following:

1. Higher light exposures--something we can achieve by opening up the canopy and spreading shoots evenly to sunlight--improved the amount and size of inflorescences of the primary bud in all of the varieties studied.

2. Higher light exposures increased the ability of secondary buds to push as well as the ability of those buds to carry crop.

3. The new index proposed by the authors--Integrated Fruitfulness Index (IFI)--is the best indicator to study flower differentiation (potential crop) because it integrates the number and the size of cluster primordia.

4. Percentage fruitfulness continues to be a better predictor for final yield (actual crop). wbm