PHYSIOLOGICAL-RESPONSES OF BANANA (MUSA-AAA, CAVENDISH SUBGROUP) IN THE SUBTROPICS .1. INFLUENCE OF INTERNAL PLANT FACTORS ON GAS-EXCHANGE OF BANANA LEAVES

Gas exchange measurements on photosynthesis rate (A), transpiration rate (E), stomatal conductance (g(s)) and internal CO2 concentration (Ci) of banana leaves were taken to describe the physiological responses of the-banana plant to internal plant factors under subtropical climatic conditions. Field measurements were made with portable photosynthesis measuring equipment on the proximal, medial and distal part of the leaf. The leaf area enclosed in the chamber (6.2 cm(2)) was exposed to direct sunlight and records were taken when steady state conditions were reached. Soil moisture was always returned to field capacity the day before measuring. Standard plants of +/- 1.1 m height in the vegetative growth phase showed highest A, E and g(s) from leaf 2 to leaf 5, with leaves 6-9 showing progressively reduced activity. No differences in A, E and g(s) were measured between the proximal, medial and distal third of banana leaves 2-5. Overcast conditions in summer reduced A of leaves 1-9 by 34% compared with sunny conditions. The stomatal density on the abaxial and adaxial leaf surfaces was +/- 130 and +/- 54 stomata mm(-2) respectively during summer and winter. The physiological activity of both leaf surfaces was related to stomatal density, being higher on the abaxial than on the adaxial surface. The level of A was reduced by unsuitable climatic conditions more drastically on the adaxial leaf surface, thereby increasing the Ab/Ad ratio for A during winter and spring. Highest physiological activity of banana plants was measured in the vegetative growth phase during late summer, since in this growth phase the most efficient leaves 3-5 were replaced every month, and temperature conditions allowed highest assimilation potential. Also in winter, plants growing vegetatively had a higher physiological activity than those in the reproductive phase, but lower temperatures led to an overall reduction in A, E and g(s) during this season. Plants still growing vegetatively in spring-and the second summer (summer planting date of January 1991) could renew their physiologically old leaf area, being able to use favourable climatic conditions more efficiently by showing the highest A, E and g(s) during the second summer. The cessation of leaf production after flowering, and the ageing of the current leaf area were responsible for the progressively lower A, E and g(s) after flowering and especially during the last two months of bunch development.