Waching daily Jun 1 2017

Frost

can affect your crop at all stages of

development reducing yield and quality. Here

Dr. Ben Biddulph Research officer with

DAFWA explains how a frost occurs and

the contributing environmental factors

that will determine the severity. When we

have a wet canopy we get a lot more

frost damage than if the canopy was dry

and there's two main reasons because

of this. One is because of the wetness the

actual water. The other because of the

ice nucleators which happening. So

frost is not just freezing damage.

there's three main processes which

happen with frost. The first two are

cold and desiccation and most of

our frost event that's what we get.

So the cold is basically that

temperature response to the diagonal

stress of actually going from quite a

warm daytime temperature to a low

overnight minimum and then back up

to a warm temperature following a frost

event. So that cold effect essentially

the plants have to change their

membrane fluidity and their energy

balance and the process around that just

to cope with that cold stress. So frost

has initially has that effect of

cold the next effect is desiccation so

when we get a frost event we get ice

the dew forms on the canopy and then that

dew freezes after that it then starts

to suck water out of the air and so it

will freeze water from out of the air

but then it will also start to freeze

water out of the plant mesophyll

cells and out through the stomata itself

so it'll actually start to desiccate the

plant tissue and the flag leaf tissue

often after a frost event you can go out

the morning and when you see the flag

leaves defrost you'll see them wilt as

well and that's because they've lost all

that water from inside the tissue to

the outside so the desiccation effect.

So there's these first two processes and we get

that for most frost events. The other

effect we get is ice formation

and freezing damage. Now plants of

wheat will normally supercool to about

minus 10 degrees out in the paddock but when

we add ice nucleators they will start to

freeze at warmer temperatures. Now

ice nucleators are just like you know when

you leave your beer in the freezer and you

forget to take it out. You know when you

first take it out if you don't bang it

hard or you don't open the beer won't freeze

but soon as you open it and break that

seal it will freeze straight away and

that's because you basically had an ice nucleator

come in there.

Same thing happens with your wheat out in

the paddock. Essentially

when it's at minus 10 degree or minus 4 or 5

degrees it will super cool down to those

temperatures and the plant tissue won't

freeze. So you only get the cold and

desiccation effects. What happens with

ice nucleators they will actually make your

canopy freeze at warmer temperatures.

So normally in the paddock, we do

have ice nucleators in the form of

dust and different bacterial spores on

the canopy and these normally raise

the freezing point of those canopies to

around minus 5 degrees but when we have

a rainfall event which goes through and

actually wets the canopy those rain

droplets also bring in with

them ice nucleators and so a lot of

those ice nucleators which come with that

rainfall event raise the

freezing point to around minus 2C. So when we

have a wet canopy with a lot of these

ice nucleators which have been

introduced by the rainfall and the rain

itself we end up with a lot more

freezing damage because the plant

tissue will freeze at minus 2C doing and won't

super cool anywhere near as much so

compared to a dry frost, a frost when

you've got a wet canopy always a lot

more damage.