Shoot growth and leaf photosynthesis of 6-7 year-old, field-grown mango (Mangifera indica L. cv ‘Kent’) trees subjected to the following irrigation regimes were monitored during the season of 2002/2003: Co, control (±95% of field capacity, FC); DI-1 and DI-2, continuous deficit irrigation (±79% and ±69% of FC, respectively); RDI, regulated deficit irrigation (like Co except that irrigation water was withheld for 2 weeks in Dec 2002/Jan 2003 during the final stage of fruit growth); and Co-F, farm control, full irrigation when soil moisture was lower than -10 KPa). During the post-harvest flushes in 2002 and 2003, Co-F shoots showed 56% more shoot volume and dry mass than Co indicating that a large amount of irrigated water was used for such vigorous growth. There were no significant differences in the number of flowering canes between all irrigation treatments during the flowering flush in 2002. However, the pattern of decreasing numbers of flushing terminals and shoot numbers was concurrent with decreasing amounts of irrigation water applied (Co-F>Co>RDI>DI-1>DI-2). The results indicated that part of the irrigation water applied to Co-F was used for vigorous vegetative growth; while in the DI-2 treatment severe shoot growth reduction seemed to seriously hinder productivity. Midday leaf assimilations in well-irrigated ‘Kent’ mango trees in the field during winter and spring amounted to (4.5 ± 0.6 and 5.9 ± 0.3 µmol CO2 m-2 s-1, respectively) and were lower than in autumn and summer (6.4 ± 1.8 and 11.1 ± 0.8 µmol CO2 m-2 s-1, respectively). Differences in mean photosynthetic (Pn) rates between the three field-grown and well-irrigated cultivars, ‘Kent’, ‘Keitt’ and ‘Heidi’, were not significant except for January and April 2003. Young leaves (<25 DABB, days after bud break) photosynthesized only to a small extent in winter and summer at midday (0.2 ± 0.9 and 0.4 ± 0.3 µmol CO2 m-2 s-1, respectively). However, high Pn rates at midday were reached in leaves aged 90-180 DABB in summer (10.3 ± 2.0 µmol CO2 m-2 s-1) and maintained at leaves that were older than 365 DABB (10.4 ± 0.7 µmol CO2 m-2 s-1), while those in winter were lower for both leaf age groups (3.2 ± 0.6 and 3.5 ± 0.8 µmol CO2 m-2 s-1, respectively). In field-grown ‘Kent’ mango trees, water deficit reduced significantly the photosynthetic capacity in the irrigation treatment DI-2 across the entire year, especially during late spring and early summer, in comparison to the Co-F treatment, while differences between the treatments Co, DI-1, and RDI were negligible (in the range of 7.0-7.3 µmol CO2 m-2 s-1) across the entire year. Generally, the Co-F treatment maintained slightly higher photosynthetic rates (7.9 ± 3.2 µmol CO2 m-2 s-1) over the entire year than the other treatments showing that they received more water. Nevertheless, DI-2 maintained reasonable rates (6.5 ± 1.7 µmol CO2 m-2 s-1), even though they were severely stressed indicating that those trees apparently adapted to low water regimes by increasing their water use efficiency.
Dissertation (MInst Agrar (Horticulture))--University of Pretoria, 2006.