Page 57 - Grapevine July-Aug 2020

Around The Vineyard

As soils dry out, the level of plant water stress is I proposed a leaf temperature-based water stress
higher during the morning and in the afternoon. index, which is in Table 1 (Below). Measurement
However, one must be careful to distinguish the is suggested in the early afternoon, or when it is
effects of soil moisture from those of higher sun- sunny and air temperature reaches its maximum.
light, temperature, wind speed and lower humidity. Mid-shoot leaves well exposed to the sun are test-
It is challenging to take spot measurements with ed. I suggest pressing the leaf blade between fin-
the pressure bomb during the day to predict soil gertips and palm and quickly sensing leaf tempera-
moisture conditions. Direct measurement of soil ture on the palm. One must take an instantaneous
moisture profiles is preferred, which are much less impression of leaf temperature, as holding a leaf
variable over the day. will quickly bring its temperature to that of your
hand!
Use of Plant Appearance
The reader might be thinking, “Why not use an
I had almost side-by-side vines with different infrared thermometer to measure air temperature
soil moisture conditions in an irrigation trial I con- as we have seen used recently to indicate fore-
ducted, and I soon learned how the appearance of head temperature with Covid-19 virus detection?”
vines change as they develop water stress. One of Indeed, such devices are now quite cheap, portable
the most obvious symptoms is that shoot tips stop and accurate, but be careful always to measure
rapid growth, and eventually, they stop growth leaves with the same angle to the sun.
altogether. This symptom relates to moisture stress
over several week’s duration. Conclusion

Another of the visual effects of water stress is on Leaf temperature will give an instantaneous mea-
leaf inclination. Initially, the petioles droop a little, sure of vine water stress. In contrast, leaf inclina-
and as stress continues and becomes worse, the tion and shoot tip growth assessment will indicate
leaves first hang vertically and then begin to cup water stress over the previous two weeks or lon-
by folding inwards along the main vein. When very ger. Therefore, leaf temperature can give a better
stressed, you will see the backs of several leaves indication of water stress, and so, irrigation needs,
if you look along the row. I was working with the while shoot growth will advise how effective the
Shiraz (Syrah) variety, and the leaf backs are hair- irrigation has been.
ier than the front, so they are easy to distinguish.
These symptoms may take several days or a week The clever vine irrigator might believe these visual
to develop. guides more than those of randomly taken pressure
bomb tests to manage vineyards to optimize wine
Leaf Temperature Assessment quality. Modern irrigation monitoring is developing
systems based on thermal images, either close up
This assessment relates to present vine water or remote, related to my simple system of using
stress. When vines are water-stressed, stomata one’s hand!
(leaf pores controlling water loss on the leaf under-
side) partially or fully close, and so the loss of water Table 1. Assessing temperature of sunlit leaves.
from the leaf ceases. Transpiration (like evapora-
tion) acts to cool leaves. So a sunlit leaf will have a Quickly touch a fully sun-exposed leaf, in early
temperature not so different from that of the air, afternoon—sample several leaves.
perhaps a little warmer or cooler. However, when
the vine is water-stressed, sun-exposed leaves are 1. Water stressed, leaf hot to touch, more than
noticeably hotter than air temperature because about 4-6 ⁰C above air temperature.
the stomata close, and shade leaves are around air 2. Slightly stressed, leaf warm to touch, less than
temperature. about 2-3 ⁰C above air temperature.
3. Not stressed, leaf at about air temperature.

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