Stomatal conductance and chlorophyll fluorescence of oil palm under field conditions

Abstract
Potassium has been known to affect the stomatal movement of palms, especially during drought
stress. Previous studies have observed midday closure of oil palms stomata during the dry
season and some studies in other crops showed that potassium has a role in widening the
stomatal aperture. In this research, we would like to know whether being potassium sufficient
could widen the opening of oil palm stomata and avoid photoinhibition of photosystem II
during drought stress. A pilot study was done beforehand to observe the difference of
measurement result conducted at leaflets of different frond numbers and at different frond parts.
Different frond numbers were recorded to have quite similar photosynthetic efficiency. Leaf
senescence had influence to stomatal conductance but not to photosynthetic efficiency. Leaflets
at the top layer of middle part of frond 17 were found to be the most representative leaflets for
stomatal conductance and chlorophyll fluorescence measurement. In the actual experiment,
palms in two different plots, including a plot with sufficient potassium (plot (+)K) and a plot
with deficient potassium (plot (-)K), were set as the object of observation. Growth, chlorophyll
fluorescence, and stomatal conductance parameters of palms were observed. The growth and
chlorophyll fluorescence result indicated that palms at the plot (+)K, unexpectedly experienced
stress indicated by lower growth and PSII efficiency. This was suspected to be caused by
competition of resources from weeds surrounding the palms as a result of improper
maintenance even though the fertilizer was properly applied. We observed a difference of
stomatal conductance pattern during the day between palms of both plots. Palms at plot (+)K
showed a significant midday closure while the palms at plot (-)K did not. The high variability
of stomatal conductance in the morning period made the declining trend of stomatal
conductance of palms at plot (-)K was statistically not significant. This high variability may
have been caused by a small sample size that we had but as we noticed that it only happened
in the morning, there was a possibility that some palms could not fully open its stomata in the
morning as a result of potassium deficiency. Limited information regarding leaf potassium
content and soil potassium availability limited us from exploring the other possible causes.
This preliminary study has provided some useful information for further studies about the
behavior of stomata under field conditions. Further research with larger sample size, more
replications, and well-maintained trial, might potentially find a significant difference of
stomatal behavior and photosystem efficiency between palms with different potassium
treatment.