Here's one about quality improvement on grapefruits:


Fruits 60 (2005) 311-317
DOI: 10.1051/fruits:2005037
Effects of citrus rootstocks on fruit yield and quality of Ruby Red and Marsh grapefruit
Ali-Akbar Ramina and Ali Alirezanezhadb

a Department of Horticulture, College of Agriculture, Isfahan University of Technology, Isfahan 84156, Iran
b Shahid Chamran University, Ahwaz, Iran

(Received 21 March 2005 Accepted 15 August 2005; published online 10 January 2006)

Abstract - Introduction. Generally, in Iran, citrus cultivars are grafted on Sour orange rootstocks. However, susceptibility of this rootstock to tristeza (CTV) has required a research program to replace Sour orange trees with other rootstocks. Materials and methods. Yield performance and fruit quality of Ruby Red and Marsh grapefruits were assessed for 5 years (1999 to 2003) in the Agricultural Research Station in Dezful (Southern Iran) according to the eight different rootstocks used: Carrizo citrange, Citrus amblycarpa, Cleopatra mandarin, King mandarin, Sour orange, Swingle citrumelo, Troyer citrange and Volkamer lemon. Results and discussion. For the 5 years studied, cumulative yields per tree and yield per trunk cross-section area were the highest on Cleopatra mandarin, Sour orange, Swingle citrumelo and Volkamer lemon, whereas trees on Citrus amblycarpa, Carrizo citrange, King mandarin and Troyer citrange were the least productive. The results showed that individual fruit weight, size and skin thickness were significantly higher for both cultivars on Volkamer lemon, compared with the other rootstocks. Fruits on the Cleopatra mandarin rootstock had the lowest weight and fruit diameter. The rootstock type also affected the fruit juice amount and soluble solid content. Fruits from cultivars which were grafted on Sour orange rootstock had total soluble solids of 9.93% and 51% of juice content, whereas on Volkamer lemon rootstock, they had 7.81% of total soluble solids and 45.3% of juice content. Total acid of the fruits from cultivars grafted on Volkamer lemon had the significantly smallest amount (1.33%). In both cultivars of grapefruits, no significant differences were observed except in the size of the fruit, fruit juice and vitamin C contents. Interactions between rootstock and cultivar were significantly different. Conclusion. Our results indicated that the yield and fruit quality of grapefruit are strongly influenced by the rootstock used. Also, the interaction between cultivars and rootstocks may be an interesting strategy for the adaptation of cultivars to different climatic areas.

Résumé - Introduction. Généralement, en Iran, les cultivars d'agrumes sont greffés sur des orangers amers. Cependant, la sensibilité de ce porte-greffe à la tristeza (CTV) a motivé le lancement d'un programme de recherche destiné à remplacer l'oranger amer par d'autres porte-greffes. Matériel et méthodes. Le rendement et la qualité des fruits des pomelos Ruby Red et Marsh ont été évalués pendant 5 années (1999 à 2003) dans le centre de recherches agricoles de Dezful (Iran méridional) à partir de l'utilisation de huit porte-greffes différents : citrange Carrizo, Citrus amblycarpa, mandarinier Cleopatra, mandarinier King, oranger amer, citrumelo Swingle, citrange Troyer et Citrus volkameriana. Résultats et discussion. Pendant les 5 années d'étude, les rendements cumulés par arbre et ceux rapportés à l'épaisseur du tronc ont été les plus élevés lors du greffage des pomelos sur mandarinier Cleopatra, oranger amère, citrumelo Swingle et Citrus volkameriana, alors que les cultivars greffés sur Citrus amblycarpa, citrange Carrizo, mandarinier King et citrange Troyer ont été les moins productifs. Les résultats ont montré que le poids du fruit, ainsi que la taille et l'épaisseur de la peau des fruits des deux cultivars de pomelo étaient sensiblement plus élevés sur les arbres greffés sur Citrus volkameriana que sur ceux greffés sur les autres porte-greffes. Les fruits sur mandarinier Cleopatra ont eu les poids et les diamètres de fruits les plus bas. En outre, le type de porte-greffe a affecté la quantité de jus de fruit et la teneur en solide soluble. Les fruits des cultivars greffés sur oranger amer ont eu des taux de solides solubles totaux de 9,93 % et une teneur en jus de 51 %, tandis que, sur le porte-greffe Citrus volkameriana, ils ont eu 7,81 % de solides solubles totaux et 45,3 % de jus. L'acidité totale des fruits des cultivars greffés sur Citrus volkameriana a eu les valeurs les plus faibles (1,33 %). Pour les deux cultivars de pomelo, aucune différence significative n'a été observée, exceptés la taille du fruit, la teneur en jus du fruit et son contenu en vitamine C. Les interactions entre porte-greffe et cultivar ont été significativement différentes. Conclusion. Nos résultats indiquent que la production et la qualité des fruits du pomelo sont fortement influencés par le porte-greffe utilisé. En outre, l'interaction entre les cultivars et les porte-greffes pourrait être intéressante à considérer pour l'adaptation des cultivars aux différentes zones climatiques.

still another one:

ROOTSTOCK EFFECTS ON THE YIELDS, TREE AND FRUIT CHARACTERISTICS OF THE MANDARIN CULTIVAR ‘CLEMENTINE’ ON THE ISLAND OF RHODES
K. Tsakelidou a1, X. Papanikolaou a2 and E. Protopapadakis a3
a1 National Agricultural Research Foundation, Soil Science Institute of Thessaloniki
a2 Agricultural Research Station of Rhodes
a3 Subtropical Plants and Olive Trees Institute of Chania in Crete, Greece


Abstract

The effects of ten rootstocks on tree development, leaf nutrient levels, fruit internal quality and yield were studied. Plantings of the mandarin cultivar ‘clementine’ were established at the Agricultural Research Station of Rhodes on the rootstocks Cleopatra mandarin, Troyer and Carrizo citranges, rough lemon, Palestine sweet lime, Volkameriana, Citrus macrophylla, Rangpur lime and C. moi with sour orange as a standard for comparison. Data indicated that rootstocks significantly influenced tree development and fruit quality characteristics as well as foliar nutrient concentrations. The largest trees were on Volkameriana and the smallest on the two citranges. In terms of trunk cross-sectional area, the most vigorous trees were on C. moi and the least on Palestine sweet lime and the two citranges. Cleopatra mandarin, the two citranges, and Palestine sweet lime showed a tendency to biennial bearing. The highest cumulative yield of clementine trees was on C. macrophylla and Volkameriana and the lowest on Troyer and Carrizo citrange. C. macrophylla and Volkameriana produced the most efficient trees with the largest and heaviest thin-skinned almost seedless fruit. Fruit on C. macrophylla produced the highest juice content but also the lowest concentration of total soluble solids. Differences were observed in leaf nutrient contents between the rootstocks. These trials showed that C. macrophylla and Volkameriana could be used successfully as alternatives rootstocks to sour orange.
(Accepted February 4 2002)

Can't find the one results of field trials for California yet.

But the Volckameriana is most successful for dry desert areas than the tropical or humid desert areas.

That could be reason why the quality has been lousy in Florida and elsewhere.

Here are some notes in the desert areas of Egypt:
http://www.aucegypt.edu/academic/ddc/citrus.htm

Thanks for the references, Joe. But we may be saying the same thing in different ways -- your references say that Volk makes a big tree that produces big, abundant fruit. No argument there. And the only one of the citations that lists internal quality says Volk has the lowest solids (sweetness) and lowest acidity (the "umph" in citrus flavor) -- so as I said, they are more "dilute" tasting than on other rootstocks, and I consider that to be "lower" quality, overall.

Under desert conditions, acidity can be too high, so maybe the reduction would be seen as an improvement. But under our conditions, we want both high solids and high acidity.

I should add that under our conditions, Poncirus (including FD) produces very high solids and high acidity, similar to sour orange, and therefore arguably the "best" quality fruit. Somewhat below that, then, would be Swingle, then Carrizo, then rough lemon, then Volkameriana and macrophylla at the bottom of the list.

As with all my other fruits and rootstock combo, their actual mileage in my own yard is the only thing that matters most to me. Recommending to others would always have caveatus emptor as the results could be all over the place when all things are considered to specific situation. Even then, your own yard from year to year could have variations in the results from the same tree.

But it is interesting to note the results of others and try them out if it works for you. So we read scientific literatures, news and hypes, we share our ideas, we share results from our own experiences and experimentations, and encourage to share even the bad results to prevent the same mistake.

I have you to thank for, and so now I am thinking that probably I won't be using Volk in the Philippines on a large scale because of Florida's results but still I just have to try them out. When results differ, then there is more new knowledge to be gained. I tend to believe credible reports at their face value and then try to reconcile why results are different. By assuming that what we observe are mostly correct and then explaining why results are different, then truly it is very good way to gain more knowledge with pleasant results and camaraderie.

Yes we tend to forget things, make mistakes, be misquoted, or misinterpreted, but we can resolve those in a very friendly way. Unlike in some forums where personal attacks on credibility are always on the line

Joe and I have been discussing rootstocks quite a bit today and I came across this interesting information on Gold Nugget and Shasta Gold grafted onto Volkameriana.

From http://www.plantbiology.ucr.edu/documents/files_of_Roose/Gold_Nugget_fact_sheet_TR-Mar06.pdf
http://www.plantbiology.ucr.edu/documents/files_of_Roose/Shasta_Gold_fact_sheet_TR-June06.pdf
http://www.plantbiology.ucr.edu/documents/files_of_Roose/Tahoe_Gold_fact_sheet_TR-June06.pdf
http://www.plantbiology.ucr.edu/documents/files_of_Roose/Yosemite_Gold_fact_sheet_TR-June06.pdf

Quote:

Fruit from trees on Volkamer lemon and Schaub rough lemon generally have slightly lower solids and acid than those from trees on Carrizo citrange, C-35 citrange, Swingle citrumelo or Rich 16-6 trifoliate orange, but this effect is less noticeable than with oranges (emphasis mine) and does not preclude the use of Volk or Schaub as a rootstock with trees used for fresh fruit marketing.

I also read from those same sources that C-35 doesn't seem to dwarf the mandarins as much as sweet oranges.

Results of ‘Fallglo’ Trials for Citrus in Arizona - 20011
Glenn C. Wright and Marco A. Peña
Department of Plant Sciences, U. of A., Yuma Mesa Agriculture Center, Yuma, AZ

Abstract
A ‘Fallglo’ mandarin trial was established at the Yuma Mesa Agriculture
Center in 1995. Results suggest that trees on C. volkameriana rootstock, rough lemon rootstock, and, in 2001-02, Gou Tou orange rootstock had the greatest yields. There was little effect of rootstock upon fruit size or juice quality.

http://ag.arizona.edu/pubs/crops/az1303/az1303-9.pdf

I would expect Volk. to perform well in a hot and dry state such as Arizona. Volk. is rated as being good in high pH soils, and good under drought. It is poor against freezes but that is not much of a problem n Arizona. Its over all rating for tree yield has always proved high with large fruit. It is a rootstock almost custom made for Arizona. - Millet