PHYSIOLOGICAL-RESPONSES OF BANANA (MUSA-AAA, CAVENDISH SUBGROUP) IN THE SUBTROPICS .2. INFLUENCE OF CLIMATIC CONDITIONS ON SEASONAL AND DIURNAL-VARIATIONS IN GAS-EXCHANGE OF BANANA LEAVES

The response of banana plants to subtropical climatic conditions was investigated by measuring photosynthesis rate (A), transpiration rate (E), stomatal conductance (g(s)) and internal CO2 concentration (Ci) during various seasonal diurnal time courses and correlating these with climatic factors. Measurements were conducted hourly from early morning until late afternoon on standard plants of ca. 1.1 m height and mean functional leaf area of 4.8 m(2). Measurements were made with an ADC portable photosynthesis measuring equipment model LCA3. The leaf area enclosed into the leaf chamber was always exposed to full sun and data were recorded when steady state conditions were reached. Soil moisture potential was brought back to field capacity the day prior to measuring. Highest A, E and g(s) were measured during the morning of summer months and lowest A, E and g(s) were measured in winter after a night with a minimum temperature of 6.2 degrees C. Whereas diurnal peak A and g(s) occurred almost always before noon, E peaked at noon or in the afternoon. During summer, constraints on gas exchange were due to high temperatures and high vapour pressure deficit (VPD) during the afternoon, causing stomatal closure. In autumn, A was correlated mainly with photosynthetically active radiation (PAR), but lower temperatures during the early morning led to lower physiological activity than during summer. During winter, low night temperatures, high day VPD and a depleted root system which was unable to cope with the evaporative demand, were the main reasons for the low physiological activity, While the diurnal variations in A and E were dependent mostly on VPD, mean daily A in winter was strongly related to T-min, as shown by regression analysis over 20 diurnal time courses (r(2) = 88%). In spring, high VPD and a root system still not fully recovered from winter due to low night temperatures, were factors limiting assimilation. During late spring, a fully recovered root system and climatic factors suitable for the physiological activity of the plant, led to higher A, E and g(s).