Long days?

[philcooper]

A while ago, Hugh corrected me (I don't feel slighted as I'm not the only one to benefit from this!), about the advantages of early planting of potatoes other than 1st earlies. This was based upon the start of tuber forming in potatoes (tuberisation) being triggered by "long days". He advised me to read papers on the subject, several of which I have read.

All of them talk about "long days" and their effect on tuberisation. As other plants are said to be long day plants and today is the first day when daylength > nightlength - what is a long day??

[philcooper]

[Ceri]

can't find the book I got it from (bob flowerdew) but I remember reading about planting something or other in autumn as soon as the day is less than 14 hours - so conversely, does 14 hours make a long day

perhaps a 'long day' should be defined as 'any day that is dry and sunny when you are at work and can't get to the lottie'

[philcooper]

I would agree that 14 hours on the lottie = a long day

However as Autumn starts in September when all days <14 hours (of daylight) the advice seems a bit odd

[Ceri]

sorry if I've confused, I wasn't trying to answer the query, just that I'd read advice to plant whatever the plant was when it was less than 14 hours daylight each day, in the autumn - must find the book and re-read what it actually said as its driving me mad now!  I was just thinking that <14 hours advice given might support a definition of >14 hours being a 'long' day.

[Hugh_Jones]

A while ago, Hugh corrected me (I don't feel slighted as I'm not the only one to benefit from this!), about the advantages of early planting of potatoes other than 1st earlies. This was based upon the start of tuber forming in potatoes (tuberisation) being triggered by "long days". He advised me to read papers on the subject, several of which I have read.

All of them talk about "long days" and their effect on tuberisation. As other plants are said to be long day plants and today is the first day when daylength > nightlength - what is a long day??

I`m glad you don`t object to my correcting you phil, because I`m about to correct you again:

Firstly:  If you refer again to my posting of the 5th March you will see that I did not refer to “potatoes other than first earlies”, I referred to “maincrop potatoes” which, by definition, also excludes second earlies

Secondly: I did not say (if that is what you are inferring) that tuberisation was initiated by long days. I said it was initiated  as the days commenced to shorten after midsummer.

Thirdly: if your implication is that all the papers you have read indicate that tuber initiation results from long days, and that potatoes are long day plants (and I assume that is what you are implying, although your posting is somewhat ambiguous)  then I suspect you have either not properly read or have misunderstood those papers.

While it is impossible within the confines of this board to summarise a long list of  such studies, here is a representative sample which I have selected for their simplicity and the sheer impossibility of misunderstanding them.

(1)Wheeler & Tibbits (1996) (after listing numerous studies from 1923 to 1991) “For the most part these studies have shown that short days (i.e. long nights) promote tuber initiation and growth, while suppressing shoot growth.

(2) Martinez-Gargia, Virgos-Solez & Prat (2002). “ …and as day length shortens in the fall, some plants respond by flowering and others, like the potato, by tuberizing [short day (SD) plants] “

(3) Stephen D. Jackson (Horticultural Research International, Warwick) 1999

“What is a potato tuber? It is not formed from a root, as is often supposed, but from an underground stem called a stolon. In conditions that are noninductive for tuberization, e.g. LD (long day), the stolons often grow upward and emerge out of the soil to form a new shoot. In tuber-inducing conditions, e.g. SD (short day), however, the stolons grow underground until the tip of the stolon swells to form the tuber……………………………
The potato is a short-day plant, although the critical night length for tuberization and the strength of the photoperiodic response varies with different genotypes (Snyder and Ewing, 1989 ). Potato species such as S. demissum and S. tuberosum ssp. andigena are qualitative short-day plants that require daylengths of 12 hours or less to tuberize.”

There are, of course, many more such studies, but as most are devoted to finding methods of defeating photoperiodic response by the use of artifically induced LD and SD methods combined with UV to Infra Red light conditions and chemically altered environments, they have to be read with extreme care

The one misapprehension I may unintentionally have caused was in referring to tuberisation being initiated “when the days start to shorten after midsummer”.  In fact tuberisation is probably not initiated in most maincrop varieties until the middle of July or later.

As to your enquiry  about long days, these are days when daylight length exceeds night length, and is still increasing. Short days are those where daylength is decreasing.