Vegetables include a versatile group of crop plants. Tomato, eggplant or Capsicums are botanically fruits but traditionally they are treated as vegetables and with due respect to the convention, we have included them in Volume 5 dedicated to vegetables. On the other hand, we could have included crops such as cowpea, pea and potato in this volume; however, they found place in Volume 3 representing pulses, sugar and tuber crops. Beets could also have appeared in Volume 3 for they include sugar beets, contributing to one quarter of the sugar production of the world. But, in this volume deliberations are needed on other beets. This is also true for Brassica rapa. The vegetable B. rapa includes some crops widely used as vegetables, however genetically they hardly differ from the oilseed B. rapa crops and have highly homologous genomes to other oilseed Brassica species. This is the main reason for detailing genomics and breeding of this species in Volume 2 on oilseeds, and presenting only a brief on molecular works but depicting the various vegetable Brassica rapa crops in detail in this volume. Green papaya and plantain are highly popular items in many curries, particularly in Asian countries and could have been included in this volume. However, papaya has been included independently as a fruit crop, and plantain has been discussed alongside banana in Volume 4 devoted to fruits and nuts.
The relative attention drawn by a crop of the scientific community depends not on its relative global importance but on its acreage in the developed nations. Tomato is a prime vegetable crop in the United States and the leading one slightly ahead of lettuce. It could win a place in several laboratories and has been well supported by funding agencies for works on genome mapping, molecular breeding and advanced genomics. The International Tomato Genome Project is progressing nicely and a stupendous amount of sequence data has already been accumulated. Tomato has a significant place in classical as well as modern genetics as a model plant. It can boast of its ranking among a few plant species possessing the earliest chromosome maps based on morphological, cytological, biochemical and/or molecular markers. Tomato is the proud crop plant to witness emergence of many concepts and strategies including molecular mapping of a gene; gene isolation through positional cloning and transposon tagging; comparative mapping; Mendelization, fine-mapping and cloning of QTLs; besides the homely terms of chromosome landing or reverse genetics amongst an array of others. Recent welcoming of the tomato into the Solanum genus has facilitated concerted genomic efforts along with other vegetable crops such as potato, Capsicums and eggplant.
Lettuce is grown and consumed in limited countries of the world but witnessed significant progress due to its horticultural importance in developed countries particularly in the United States. The most popular among all single genetic strategies practiced today, bulked segregant analysis, was devised originally for this crop.
Brassica oleracea includes many globally grown vegetables and appreciable progress has been made in this species. Again this species could have been better dealt with alongside other Brassica species in Volume 2 from a genetic perspective. The conventional classification of crop plants based on their utility was the deciding factor for the placement of this Brassica species in Volume 5.
Extensively grown crops like eggplant, Capsicums or Alliums have attracted relatively less attention for advanced genomics works. We feel Cucurbits should have been addressed more since they are used so widely in the countries with the highest populations on Earth. Included are the popularly used vegetable crops such as melon, watermelon, squash and cucumber with considerable progress, but "orphan crops" like bitter gourd, sponge gourd, snake gourd, bottle gourd, spine gourd, pointed gourd, pumpkin, etc., have no molecular story to tell.
People in developing nations do not consume a large amount of salad in their diets, and fewer pages on radish, beets or carrot could therefore have been justified. Still, appreciable progress has been made in these crops due to their popularity in developed countries.
Some "poor man's vegetable crops" with modified roots and stems, such as Alocasia and Colocasia, fruits like ladies' finger, country bean, drum stick, fig and jackfruit must be discussed in the near future. Several herbs consumed in many countries under the genera Spinacea, Corchorus, Ipomoea, Amaranthus, Chenopodium, etc., also deserve attention.
Considerable progress has been made with other vegetables, particularly asparagus and faba bean and should have been included; this is also true of some leafy vegetables. We are hopeful to see a sea-change in the table of contents of this volume in future editions.
The authors of two chapters of this volume, lettuce and onion, took the pain and pleasure to draft their chapters single-handedly. The lead authors of the remaining chapters have arranged themselves into multi-lab or even multi-national groups. Our thanks and gratitude go to all of them and all the co-authors for their useful contributions.
I am grateful to Springer for the work done in achieving this volume, like the preceding ones. All the shortcomings and flaws related to the content and its presentation are fully mine, and I will be looking forward to suggestions on how to address them in the future.
Pennsylvania, 18 June 2006
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