AS INFLUENCED BY POTASSIUM NITRATE SPRAYS AND SHOOT DECAPITATION.

Potassium nitrate (KNO3) is an effective flower promotor in polyembryonic mango (Mangifera indica L) cul tivars. In addition, shoot decapitation suppresses apical domi nance and stimulates axillary growth of terminal mango shoots. Experiments were performed to evaluate the indi vidual and combined effect of a single KNO3 spray (80 g/liter) and shoot decapitation on flowering and fruit set of 'Haden' (monoembryonic) and 'Manila' (polyembryonic) mangos. Ma ture vegetative shoots were treated in January, 1985, during flowering for both cultivars. Percentage of flowering shoots was highest in both cultivars when they were sprayed only, but spraying and decapitating combined produced triple and nearly double the number of panicles/shoot in 'Haden' and 'Manila' respectively, as compared with untreated controls. Fruit set in 'Manila' was markedly superior to that of 'Haden' in all cases. Spraying of 'Manila' alone with KNO3 produced twice the number of fruit/shoot or per panicle in relation to the rest of treatments. Fruit size and quality were not affected by increased fruit set. Decapitation alone resulted in the least flowering in both cultivars, and in no fruit set for 'Haden'. Mangos are cultivated on nearly 4000 ha in Colima, on the Pacific coast of central Mexico. About 75% of this area is planted to 'Haden' and 'Manila' which have monoem bryonic and polyembryonic seeds, respectively. Despite favorable ecological conditions in the area (7), these cul tivars frequently flower poorly and consequently produce low yields. Low flowering intensities are often associated with low numbers of mature terminal shoots on a tree. However, the problem is also observed on trees with a high number of shoots old enough to flower. Flowering of 'Haden1 and 'Manila' trees begins between December and January and generally ends in March. Due to the existence of apical dominance in mango, panicles generally develop from apical buds of shoots produced in the previous season, and only rarely from axillary buds or on older wood (10,12). Induction of axillary panicles, how ever, has been achieved in the 'Haden' mango (9,10) and in other Indian cultivars by shoot decapitation. (11). More recently, potassium nitrate (KNO3) sprays have been found very effective in inducing normal and out-ofseason panicle formation in polyembryonic Philippine mangos (1,2,3). This product is an effective flower-inducer in trees which remain vegetative but are already in a bearThe author appreciates the help of Manual Alvarez and Sergio Lopez, technicians, in collecting data, and of Adriana Rojas in typing the manu script. Financial support from the government agency CONACYT is also acknowledged. ing age (2,3). Concentrations ranging from 10 to 160 g/liter induced flowering in 100% of treated 4.8to 8.5-monthold shoots, only 2 to 3 weeks from spraying, while all un treated shoots remained vegetative. Sprays were applied in August, October, and November. Initial investigations on the usefulness of KNO3 as a mango flower-inducer in Mexico were conducted on 'Manila' trees (6). It was found that 7-month-old shoots, sprayed in November with 20 to 80 g/liter KNO3, were more responsive than younger ones. Flowering intensities, however, were not as high as those reported in the Philippines. Besides increasing flowering rates, KNO3 can also ad vance the flowering and fruiting periods of mango by up to several months (3), a characteristic of great commercial interest. The present experiments were carried out to study the flowering and fruiting behavior of 'Haden' and 'Manila' shoots in response to apical bud pruning (decapi tation) and/or a single KNO3 spray. Materials and Methods Experiments were conducted on bearing, 9-yr old 'Haden' and 'Manila' mango trees growing on a sandy and well-drained soil at Tecoman, Colima on the west coast of central Mexico. Trees were planted at 10 x 10 m and their cultural management consisted of irrigation, weed and pest control, and light annual pruning; management did not include fertilizer application. Climatic and other gen eral characteristics of the area have been described previ ously (7). Five single tree replicates were used for each cultivar. Experimental units consisted of terminal vegeta tive shoots, 6 months of age or older, with crispy, dark green leaves and woody stems. Twenty-five shoots, 5 in each tree, were selected from the periphery of the tree and the following treatments applied to each cultivar: un treated control; decapitation with pruning shears of the apical bud and surrounding 5 to 7 leaves; a single spray of 80 g KNO3/liter in water; and combined decapitation and KNO3 spray. Preliminary tests were performed to select KNO3 dosage. Concentration was chosen as to not cause v leaf burn or defoliation but to effectively induce panicle formation. KNO3 solutions were applied with a hand sprayer to all leaves of corresponding shoots until run-off. Treatments were applied in mid-January, 1985, during the natural flowering period in each cultivar. Percentage of flowering shoots was recorded at 0, 7, 14, 21, and 28 days from treatment application to evaluate flowering rates. New flowering did not occur after 28 days from beginning the experiments. Flower initiation was considered as the stage in which apical or axillary buds were distinctly swollen, measured more than 1 cm \n length, and had bracts beginning to open. The number of panicles/shoot was also recorded. Final fruit set was determined as the number of fruit harvested divided by the initial fruit set (number of fruits after 60 to 65 days from treatment; fruit at this stage mea sured 1 to 3 cm in diameter). Final fruit set values and percentages of fruit retention were transformed to Vx+ 1 Proc. Fla. State Hort. Soc. 98: 1985. 179