FIELD COMPARISONS OF PHOTOSYNTHESIS AND LEAF CONDUCTANCE IN ZIZIPHUS-MAURITIANA AND OTHER FRUIT TREE SPECIES IN ZIMBABWE

Data are presented for gas exchange in exposed leaves in field grown Z iziphus mauritiana (Lamk.) at a highland site and potted seedlings at a hotter lowland site in tropical Zimbabwe, together with indigenous a nd locally grown commercial fruit crops. The field trial at the high l and site included local Ziziphus mauritiana, introduced Indian Z. maur itiana (cv. Umran), fig and peach. In all species assimilation was hig hest early in the morning, followed by a gradual decline throughout th e remainder of the day. Leaf conductance followed the same trend as as similation for fig and peach, but in Ziziphus cv. Umran and Ziziphus M usau, conductance tracked irradiance, reaching a maximum in the middle of the day. In all species, substomatal CO;? concentrations increased with declining assimilation indicating that during high irradiance, a ssimilation was mainly controlled by mesophyll limitations rather than conductance. At the highland site both Ziziphus cv. Umran and Musau w ere highly productive, with light saturated assimilation significantly higher than in either fig or peach (P < 0.01). At the warmer lowland site, assimilation and conductance in Ziziphus Musau were also higher than in other indigenous fruit tree species growing under similar cond itions. Despite increased assimilation in Ziziphus, when compared to t he other species, there was no increase in the assimilation ratio (rat io of assimilation/conductance) which was offset by the high conductan ce values. The data indicate that under conditions where water was not limiting, young Ziziphus showed no enhanced stomatal control over wat er loss, but was highly productive (per unit leaf area) relative to th e other species.