HI again,
I have a few grapefruit and navel orange trees with small fruits on them that are coming along nicely. They are in containers and are outside enjoying the nice warm weather. I am in SW Ontario(zone5-6?) and will obviously have to bring them in sometime in early October I guess. I was reading that sweeter citrus need lots of heat to really sweeten the fruit. Does that mean they need it in the summer or do they need the heat in the fall and winter when they are just about ripe? I keep them in my basement under growlights all winter so if in fact they need the heat to sweeten in the fall/winter what is the best way to provide that heat?
The other part of that question is: if the grapefruit which ripen in the winter need heat to sweeten the fruit, how do I provide the chill hours they need throughout the winter if in fact they might still have fruit on them that requires heat for sweetening? Man I hope that made sense to anybody!!

Thanks,
Chris

I am not an expert, but I do know that it is the cool weather that brings on the sweetness in satsumas. I believe the summer heat helps in generating the acid (flavor), without which the fruit is insipid. You can search this forum for the effects of climate --there was a good post or link on that a while back.

Different Skeeter!
Heat brings Sweetness, Cold bringt acidity and color...
So Florida Oranges are rich and sweet, but nearly inedible because of sugar.
So you need water to make it more liquid, as enjoying that sirup, also the typical orange flavor is missing.. to less acidity..

So in my mandarines I get much sugars because of heat, but often I miss the fresh flavor, because cold is missing, and et green fruit.. Look:



So in biology: in Summer much sugar is produced by photosynthesis, and stored in the fruits, also much minerals and water is stored in the fruits.. in Winter, the sugar is used by the plant and replaced by acids... and that's the point of harvest...

Skeet is correct about cool weather and the sweetness of citrus.. To develop maximum sugar acid balance content (the taste you like) citrus requires a span of cool/cold nights. Commercial growers welcome a run of cooler weather knowing the sugar content is rising, which is how they are paid for their fruit. - Millet

Sorry Millet,
but far from my knowledge Citrus gathers suggar during warm and sunny days, while cooler temperatures onset acids.
So dessert climates with cold nights and very high heat in daytimes brings well colored fruit, high in sweetness and well balanced in flavour, by even well set good acids.
That's also in mediterran climates, but here usually more acids compared to fruit from tropical climates, were cold is missing...

So one can find following text in "Biology of Citrus", by Spiegel-Roy and Goldschmidt from the Cambridge University Press, page 100 to page 101:

"Internal quality is also affected by climate. Fruit development in hot, tropical climate tends to have a high total soluble solids content, which is an advantage for processing industry. On the other hand, these fruits are low, often very low, in acid, resulting in poor edible quality. Thus, the somewhat cooler, subtropical areas are preferable for production of oranges and mandarines for the fresh fruit market."


It's the cold, what brings acid to the fruit, and makes this more tasty and palateable. And it's not the cold, what brings the sugar to the fruit.
Nearly the same one can find in "Citrus - crop production science in horticulture" by Davis and Albrigo, CAB International Press and even on "Citrus Industry, Vol. 3 - chapter 9" by Walter Reuther from the University of California Press.
So fruits from the tropic are rich in TSS, but often insipid by the lack of acididy, because the fruits are low in TA (Citric and ascorbic acid), so have a high sweetness, but are not tasty.
So giving the fruit cool temperatures will rise the acid content, and make the fruit more palateable and tasty.

The following is from:

http://flcitrus.ifas.ufl.edu/UF%20IFAS%20Short%20Course%20Proceedings/CitrusFlowering-duplex/citrusflowering1-4high.pdf

Low Tropics. The low tropics are typified by a hot humid climate during most of the year (wet
tropics), or by distinct wet-dry seasons in some areas( wet-dry tropics). There is little diurnal or
seasonal variation in temperature or day length. Rainfall and relative humidity are high either
throughout the year( wet tropics)or seasonally (wet-dry tropics). Generally, citrus production in this
area is low and fruit are used primarily for local consumption resulting from the adverse effects of
hot, humid climates on fruit quality of most important citrus cultivars. Disease, pest and weed
pressures are especially severe throughout most of these regions.
Flowering usually occurs rather continuously throughout the year with peak levels occurring
when rainfall occurs following periods of dry weather. Flowering intensity is controlled primarily by
availability of water (wet-dry tropics). As a result, there may be several different stages of fruit and
flower development on the same tree, which may cause difficulty in determining fruit maturity and
harvest time.
Fruit growth rate is rapid, but fruit quality for oranges and mandarins is poor. Peel color is
typically green and juice color pale, light yellow. Moreover, total soluble solids (sugars) and acids
tend to be low due to high average temperatures throughout the year which cause respiratory
metabolism of sugar and acids. On the other hand, grapefruit and limes develop characteristic peel
and juice color and low acid which produce fruit with high internal quality. For example, even' Star
Ruby' grapefruit develops a deep red color at sea level in this region. External fruit quality (peel
blemishes)is a problem for fruit from the low tropics. The high average temperatures and relative
humidity are very conducive to disease and pest development.

So apparently heat causes lower acid and sugar. In other articles I have read about satsuma flavor it shows acid increasing until August, sugar increasing with cool temperatures in the fall all the way through Dec.

Well,
according the the book "Citrus - Crop science in Horticulture" Satsuma Mandarines are something special and are not benefical for tropical reagions, because do not develop a good TSS and TA Ratio in hot reagions.
But now compare to grapefruit, which will develop an inferior TSS:TA ratio, and will only a develop a palateable quality if grown in areas with high anual heat units.
If not, grapefruit will by high in acids and have a very bad internal coloration.

So, even in mandarines there a significant differences, that's why I am not with satsumas, but have good mediterran mandarines and sure most Florida Tangelos will work well.
In oranges many typical mediterran oranges won't be benefical, but Rhode Red and CaraCara which do well in the tropics will be good.
I have friends, the harvest impalateable "Star Ruby" Grapefruits and very tart oranges, because due very cold wintering periodes, but: His Satsumas and most mediterran oranges come in best quality.

So, I still guess: Sugar is developed due heat, together with TSS, but the fruit will be insipid, if the variety isn't well fit to year round high temperatures, low TSS and low TA contants make the fruit not very high quality.
So do I grow a fruit to cold, I get low TSS and high TA contents, but for many varieties of mandarines and oranges, a good potion of cold is needed, to buffer the high sugar content, and make the fruit palateable and of a very good taste.

So sugar comes still not be cold, sugar comes by heat, for many varieties. But as mentioned: Without cold, many fruits won't develop a high quality because of the inferior taste for us humans. We need acids in the fruit and like a balanced flavour.

I think the basic fact that most of the commercial citrus in the world is not produced in Hot climates without cool fall temperatures suggest that heat is not the key to good flavor in citrus. "Heat" is a relative term. Some heat is required for growth, but again, it is the excess heat above mid 90s F that I have read will reduce both acid (flavor) and sugar (TSS). Satsumas are perfect for our climate in N FL-- they achieve flavor during the warm summer months but are too tart until it starts to cool (Owari is edible but tart in Oct (temps in 70s), by end of Nov (temps in 60) is good, just on the sweet side, and by the end of Dec (temps in 50s) is excellent--sweet and juicy full of flavor.

Here is a bit more from the link I posted looking at citrus from various regions:

Subtropical Growing Regions
Subtropical regions that produce citrus encompass the area from 23.5 north-south latitude to
about 400 north-south latitude depending on local climatic conditions, in particular minimum
temperature, of the region. For example, 400 north latitude is near Philadelphia on the east coast of
the United States which is of course too cold to grow citrus, while citrus can be grown commerciaily
at the same latitude in Europe. Subtropical climates have at least one month with average temperature
is less than 65°F, with 8 or more months averaging more than 50°F. These regions are also subject to
frosts or in some cases severe freezes. The subtropical regions account for a large part of the world's
citrus production. They can be further subdivided in arid or semi-arid and humid growing regions.
Climatic differences among these regions have significant effects, particularly on fruit quality and
ultimately on whether the fruit is marketed as fresh or processed.
Dry: Subtrogical Reiions. These regions, which also include Mediterranean-type climates, are
characterized by hot, dry days and cool nights. In addition, the low relative humidity (RH) contributes
to large diurnal fluctuations in temperature. Average rainfall is low in these areas (generally, 15 to
30 inches/yr) and generally falls in the winter. Relative humidity is also very low, often being less than
20%. Some of the major producers in the region include the Mediterranean area (Spain, Italy,
Morocco, Greece, Turkey, Israel, Egypt), California, northwest Mexico, Australia, and northern
South Africa among others. Within this regions, there are also arid regions such as those of the
southern California or Arizona deserts which often average less than 10 inches of rain/yr.
Flower induction and flowering are regulated by temperature in subtropical regions. Flower
induction occurs as temperature decreases in the fall and winter and growth rate decreases
substantially or growth ceases. This "non-apparent" growth period is important for flower induction
and the transition from vegetative to flowering shoots. Flowering occurs the following spring as
temperatures again increase. There is generally one major flowering period, although a second much
less intense bloom may also occur a few months later (termed the "June" or off-bloom). The duration,
intensity and type of flowering is also regulated by temperature prior to or during bloom and will be
discussed in subsequent papers.
Fruit quality is excellent for the fresh fruit market due to the excellent peel and juice color that
develops as temperatures fall below 55-60°F. Moreover, peel blemishes are less intense than in
tropical regions due to the lower relative humidity and rainfall. The low relative humidity and average
temperatures also reduce pest and especially blemishes caused by fungi. Consequently, the major fresh
fruit producing countries in the world are located within the Mediterranean-type (semi-arid) climates.

In SE Texas the satsumas sweeten up considerably after we have a few days or nights below 50 egrees F.

I have to take issue with the comment

So Florida Oranges are rich and sweet, but nearly inedible because of sugar.

I don't think you can find a better eating orange than a Page, although it is not really an orange. But, we call them oranges and they sure are good to eat.

But when do So FL oranges get ripe-- in the winter when nights are cool.

Satsumas will grow in So FL, but they never get as sweet as they do here in NW FL and they never develop color. Gainsville climate is very similar to ours, in fact there were several times last winter when Gainsville got colder than here. I agree that Page and Ponkan mandarins are good--they are among my favorites, but I don't have much preference for one over the other. I will have both from my yard this winter.

Skeeter,
my madarines grow indoors and the last I harvest rippen from last December zu today, and they are still green, but rich and sweet in flavour.

So, why they are sweet, without any cold, espacially during the last stage of ripening?

So if cold plays a rule, most of my mandarines and oranges had to be inedible sour and acid, but that's not the case...
Any guess, why getting sweet citrus without cold?

Cold is a relative term--60s are cold for citrus. We usually have some 50s (F) by Nov sometimes even 30-40s which makes them even sweeter.

Sweetness is also a balance between acid and sugar--citrus fruits are considered sweet when the sugar/acid ratio is 10 or higher--the higher the acid the higher the sugar content must be to make the fruit taste sweet.

Taste is also subjective and relative to experience.

I have now bought clementines from 4 different countries, Spain, Morocco, US (CA) and recently So. Africa. The ones from Spain are the best --sweet with lots of flavor, CA is about the same as So. Africa--to me they do not have the flavor of the Spanish clementines, but they are good. The ones I had from Morocco were a little tart, but different years bring different conditions so next year may be different.

Skeeter,
guess something is realy misunderstood here, let's see, two quotings about Citrus and temperature:

Quote:

Photosynthesis is essential for sugar accumulation so too cold retards sugar buildup, but hot temperatures accelerate maturation and result in less time for sugar accumulation. Cool to cold temperatures reduce respiration and acid metabolism, therefore acidity declines slower in a Med. climate. Hot temperatures accelerate respiration and acid breakdown. This again is part of the shorter maturation time. Under fast maturation, cell senescence and juice loss occur rapidly. Valencia matures in 14 to 16 months in Med. and in less than 9 months in a tropical climate.

and

Quote:

Basic physiological processes are responsive to heat. As heat increases, the rate of the function like photoassimilation increases, but only up to a point after which respiration exceeds metabolism. In general, citrus fruit increase in sweetness as the temp raises, but cultivars respond differently. Grapefruit do better in heat, oranges less so.

So, as you can see, these two explainations are not made by myself, these two quotings come from other people I asked, usually citrus horticulturists...
So cold won't bring up sugar... It's heat what brings up sugar, and it's how this heat will bring up the sugar, if to quick... well, not much time for the fruit to get sweet enough... To cold isn't what brings up the sugar...
Sorry, that I was unable to explain it the ways, as the two did above, my english isn't that well...

http://books.google.com/books?id=hF8bpk7fbsQC&pg=RA1-PA171&lpg=RA1-PA171&dq=Citrus+sugar+acid+ratio+chart&source=web&ots=uLTyO4ScaN&sig=9aNOTxNa6P8P2rkNzRXKVehc0Q8&hl=en&sa=X&oi=book_result&resnum=3&ct=result#PRA1-PA170,M1

This is a link to a book--page 170 and 171 both talk about the cooler subtropical climates producing fruit with more acid and sugar than warmer Mediterranean climates. I have looked for the chart I found one time that showed sugar and acid content over time but can't find it---it clearly showed sugar content increasing most from Sept to Jan while acid content increased during the summer. If I find it I will post it.