INDUCED POLYPLOIDY 1 57 



and evening for a period of 4 or 5 days. Care should be taken to reduce 

 evaporation from the treated area. 



Some notable advances with polyploids. A revolutionary practical ap- 

 plication has been made to watermelons by Kihara (1952) and his associates 

 (Yamashita, Kondo, Matsumoto, Nishida, and Nishiyama; see also Kihara 

 and Nishiyama, 1947; Kihara and Yamashita, 1947). Triploid seedless water- 

 melons, as true sterile hybrids, are a significant achievement. This new crop 

 with its novel features has been adapted successfully on a large scale. A 

 basic requirement for the seedless watermelon is an induced tetraploid parent 

 that provides "triploid seed" from which triploid plants are derived. 



Seedless fruits have special value in commerce, with premiums given to such 

 products. This is true for seedless watermelon, as well as bananas, grape- 

 fruit, grapes, pineapple, and oranges. Scientists have added the seedless water- 

 melon only recently. 



The general scheme for making triploids follows several necessary steps. 

 First, the tetraploid 4X seed parents are developed by treating diploids with 

 colchicine. Next, hybridizing the correct tetraploid with the best diploid 

 yields seed for seedless watermelon plants. Triploids are sterile because the 

 unbalanced number of chromosomes creates a meiosis, with three sets of 

 homologous chromosomes coming together in the pairing process. The unequal 

 chromosomal distribution causes ovular and pollen abortion; hence a seed- 

 less fruit develops from the triploid when pollinated with viable pollen. 

 Diploid pollinators are planted with a 1 to 4 ratio, that is, one diploid to 

 four triploids. Fruit yield is directly related to pollination. Tetraploid plants 

 may provide the fruiting stimulus; however, these strains have fewer viable 

 pollen because tetraploids have reduced fertility. If neither the tetraploid nor 

 the diploid is flowering when triploids blossom, no seedless fruit will be 

 formed. 



Under certain conditions and among specific triploid hybrids, the ovules 

 may develop into a hardened seed coat, of course, without embryo. Botanically 

 then such fruits are seedless, but horticulturally they cannot be classed as 

 seedless, since these hardened "seeds" are quite as objectionable as true 

 seeds. There is some tendency for "false-seed" formation among the first set 

 of fruit and perhaps again greater tendencies toward the end of the season. 

 In other cases, a particular combination of tetraploid and diploid parents will 

 always produce these false seeds at any part of the season. The prominence 

 of false seed has created unfavorable impressions among growers and caused 

 some disappointment when triploids were first marketed. As better cultivation 

 practices were developed and the selected specific hybrids were used that 

 eliminated this feature, better seedless watermelons have rapidly gained ac- 

 ceptance at the consumer level. 



The finer texture of flesh, probably a feature of the triploid cell, more uni- 

 form distribution of sugar from center to rind, an increase in sugar content, 



