Apparently irrigation is not important. UC Davis professor of viticulture Larry Williams made the bold, brash and shocking statement at the end of his presentation at the annual Recent Advances in Viticulture and Enology (RAVE) research conference held on March 19, 2009 at UC Davis. Shocking, yes, because Williams said it with only a partial grin on his face. I know Larry well, and he is a friend of mine. He has a dry sense of humor that I appreciate, and he often makes off-hand remarks just to get a reaction. But the smirk on his all-telling, not-very-poker face usually gives away his intent. Nevertheless, in chatting with him after the conference, he didn't completely back off his statement, rather softening it by saying that fine tuning specific vineyards' irrigation practices using an advisor or consultant, such as myself, still has a place. Maybe he was trying to appease me. But, my friend Larry should know that one cannot make statements in public when I'm in the room and not expect to be challenged in this here column!

Now, Williams did make some good points, and I was even able to absorb a few of them, though not all of them since his talk was quite fast-paced. He talked about some trials he conducted where he did some deficit irrigation treatments while also doing some leaf removal treatments in a factorial experiment. He wanted to see if the differences in fruit composition from different irrigation regimes were primarily due to the water deficit stress or to the increased exposure that often accompanies greater water stresses. Williams did these experiments in a California Central Valley Merlot vineyard on a California sprawl trellis system. Perhaps because of the dense shade that a sprawl can create in the fruit zone, he found that leaf removal treatments had a greater effect on fruit light exposure than did the irrigation treatments. No surprises there. He also found that sun-exposed clusters were about 12° to 13°F warmer than shaded clusters. Again, no surprises. He placed whole clusters in a pressure chamber to measure cluster water potential and found that the exposed clusters had lower water potential (that is, more stressed) than shaded ones. Yet again, not a surprise since exposed fruit will transpire water more rapidly.

Williams also found that fruit chemistry was altered more by exposure than from water deficit treatment. This finding was mimicked in the wine composition. As for timing of the leaf removal, he found that later (veraison) leaf removal had a greater impact on increased color density of the wine (perceived, not measured) while the late leaf removal was not able to reduce the vegetative aromas in the wine to the extent that early (post-set) leaf removal did. On the subject of veggie, he found that the most vegetative wines were from the high irrigation treatment while there was no perception of vegetal aromas for the lowest irrigation treatment (40 percent of full crop ET), regardless of leaf pulling regime.

So Williams' conclusion was that there were irrigation effects on wine composition and flavor but that fruit exposure effects overshadowed those of irrigation. Well, in a dense California sprawl canopy I can imagine that this would be the case. Mitigation of a poor fruit environment outweighed the more subtle effects of water status. I can accept that. Score one point for Williams.

Williams then gave brief results from some coastal wine trials that he had conducted in the past, to give some legitimacy to his proclamation about the unimportance of irrigation. He spoke about a deficit irrigation trial he had done in Napa Carneros on Chardonnay. He found that deficit irrigation reduced yields in Chardonnay but did not elicit any changes in wine flavor. I'll give Williams another point for that but take it right back for doing an irrigation trial in Chardonnay. Water status could have an effect on a white variety (it probably does in Sauvignon Blanc), but the water status effect would be most prominently observed in reds, in the form of changes to anthocyanin levels and tannin quality.

Williams moved on to conducting red variety irrigation trials in Napa as well as the Central Coast near Paso Robles. All of those trials were done on VSP trellises, so the sprawl factor was not in play. During the RAVE presentation, he ran through the results fairly quickly, so I cannot repeat them at this time. However, he showed that preference for wines did not follow any pattern. Sometimes wines with higher levels of irrigation were preferred and sometimes just the opposite. Sometimes the same set of wines would get ranked one way by one panel and seemingly flipped around by the next. Mind you, most were not controlled tastings nor were they descriptive analyses. It seemed that it was these results that prompted the ill-stated (in my opinion) conclusion of his talk. I'm going to take another point away for basing decisions on hedonic (preference) rankings. Taster preferences are all over the map, even with wine professionals, so it is nearly impossible to get consistent results that way.

I know that well, having conducted many of my own irrigation trials. Rather, I learned more by doing descriptive sensory analyses on wines made from irrigation trials. One practice that I often espouse is the periodic water deficit strategies, originally experimented with and published by Matthews and Anderson¹, where water deficit stress is induced either between fruit set and veraison or between veraison and harvest. Some refer to this as regulated deficit irrigation (RDI). This is in contrast to Williams, who has been experimenting primarily with what he calls "sustained deficit irrigation" or "SuDI." In SuDI, a particular level of irrigation, based on a percentage of crop ET, is applied throughout the season regardless of phenological state. The periodic deficit method reduces the application of water during one of the two periods and gives a higher irrigation level during the other. In practice, leaf water potential is used to modulate the level of stress.

Without going into detail, I have found in such trials that one can manipulate wine style using either of the two different irrigation strategies. Inducing some water deficit stress before veraison reduces berry size appreciably. Wines from this practice have high tannin levels, sometimes being very astringent. Those wines are "big and bold." On the other hand, water deficits after veraison tend to stimulate the "flavor ripening" processes. This is hormonally-induced and is triggered at many levels, including gene transcription, which stimulates production of specific sets of enzymes. I have found that the style of post-veraison deficit wines varies greatly, depending on overall level of water deficit--but they tend to have riper, sometimes jammy fruit flavors and well-developed tannin structure. Actually, I feel that, since grape physiology changes so dramatically at veraison, beginning the imposition of water deficit should occur a couple of weeks before veraison is anticipated.

I would give myself a point for that discussion, except that I am not going to provide any specific evidence in this column. So you will have to trust me on that, and I'll call a draw with Williams. Except that I have the last word… Maybe irrigation isn't always important. For instance in dry farmed vineyards and in regions with summer rainfall, irrigation is not that important. But, vine water status is important everywhere, and in most of California vineyards, irrigation is indeed important.

Vigor and Methoxypyrazines

Clearly, Williams' presentation got my dander up, but there were other presentations at RAVE, including a rather involved talk on flavor compounds in grapes by UC Davis professor Susan Ebeler. Yet, I only have room to discuss one more presentation, which was from Alan Lakso, professor in the Department of Horticultural Sciences at Cornell University. Lakso talked about how the "bell pepper" compound 2-methoxy-3-isobutylpyrazine (IBMP) develops and diminishes in fruit of particular varieties. As Ebeler had earlier mentioned, the precursors and mechanism of degradation are not understood but they do diminish in fruit with exposure to sunlight. Lakso showed that IBMP increases in the fruit before veraison, reaching a peak concentration right before veraison and then declines through ripening.

Lakso ran experiments to help determine whether vine vigor had a direct effect, via shoot length, on fruit IBMP concentration or if it was primarily due to fruit exposure. The result surprised me. Lakso pruned vines to a bilateral system on a VSP trellis but left only six shoots after suckering on one side of the vine. The other side had varying numbers of shoots retained. The shoot number of the vine affected the shoot elongation rate and extent on both sides of the vine. In this manner, he was able to get a wide variation of shoot length on the six-shoot side, yet all fruit had similar exposure since shoots were vertically-positioned.

Lakso measured final shoot length and compared those measurements against the corresponding maximum levels of IBMP. He found that the IBMP concentrations were positively correlated with shoot length (i.e., higher shoot length corresponded with higher IBMP concentration). The preliminary conclusion is that IBMP may be higher in more vigorous vineyards but due to the shoot elongation and not necessarily due to fruit shading. That means that IBMP may indeed originate in the leaves and be translocated into the fruit. This contrasts with a 2008 RAVE presentation² from UC Davis professor Mark Matthews, who concluded that IBMP may not be transferred from parent vine to the cluster. Matthews did that using green grafting of "non-veggie" variety clusters to "veggie" variety parent plants and vice-versa. In speaking with Lakso, there might have been a problem with the reciprocal grafting as the IBMP transfer might have occurred before the grafting exchange.

While IBMP appears to be independent of cluster exposure, cluster exposure was found to be clearly related to reduction of the IBMP pool in the fruit at veraison. Leaf removal, if done, should be done earlier in the season to achieve the desired reduction in IBMP. Lakso found that leaf removal near fruit set reduced IBMP more than leaf removal near veraison.

Finally, I'd like to tie Williams' and Lakso's talks together as there is a connection. An important aspect of water management involves avoidance of irrigation too early in the season. That means restricting vineyard vigor by abstaining from irrigation until the vines slow down or stop growing. Williams does that by waiting until the vines get to -10 bars of midday leaf water potential. That makes sense, especially from a scientific experiment standpoint. However, watching shoot growth early in the season is of fundamental importance and a great way to decide irrigation initiation in the spring or summer--and no pressure chamber is needed. Get this right and you don't need to worry so much about those pyrazines. wbm

1    Matthews, M.A. and M. M. Anderson. Fruit Ripening in Vitis vinifera L.: Responses to Seasonal Water Deficits. Am. J. Enol. Vitic., Dec 1988; 39: 313 - 320.

2    Greenspan, M. UC Davis' RAVE Continues to Deliver. Wine Business Monthly. June 2008.