Photosynthesis and water use in honey mesquite
Seasonal and diurnal patterns of photosynthesis in honey mesquite (Prosopis glandulosa) growing on a clay loam and a sandy loam were studied in Lubbock, Texas for two consecutive years. The highest photosynthetic rates were measured in June when soil moisture was adequate. Photosynthetic rate of mesquite is higher than either winter deciduous trees or desert shrubs. This high photosynthetic rate is associated with high leaf nitrogen content (27 mg/g) and high light saturation requirement (photosynthetic photon flux density greater than 1.8 mmol m^-2 s^-1). The optimum temperature for photosynthesis has shown a seasonal change, 27-31 °C in spring and summer and 14-18 °C in late fall. The thermostability of photosynthetic apparatus increases during May through September.
Diurnal changes in photosynthetic rate has no unique relationship with xylem water potential. However, photosynthetic capacity is related to soil water potential. This may suggest that the primary sensor of water stress is located in the root system and root-shoot communication plays a significant role in regulating plant response to water stress.
Photosynthesis of mesquite is mostly limited by nonstomatal factors rather than stomatal factors. In the drought period, the effect of stomatal closure on photosynthesis is mainly due to increased leaf temperature more than decreased intercellular CO2 concentration.
Increased photorespiration under high temperature regimes is also responsible for the decreased net photosynthetic rates. The plants growing on the two sites are responding differently to water saturated soil. The mesquite trees on the sandy loam site tend to reach peak photosynthetic rates after soil recharge, while those on the clay loam site do not. Low soil temperature and/or poor aerobic conditions caused by slow percolation and root pruning caused by swelling and shrinking action of montmorillonitic clay may be the possible reasons for the poor response in the trees on the clay loam site.
Root total nonstructural carbohydrate (TNC) concentration of honey mesquite shows a seasonal fluctuation. The root TNC recharge in honey mesquite is positively correlated with the rate of photosynthesis and root function. This recharge is not affected by a mild water stress, but is inhibited by a severe water stress. Spring drought in west Texas does not exert a great influence on the root TNC recharge, since it only represents a mild water stress. However, this recharge is negligible during midsummer for the trees growing on the xeric sandy loam site, since the severe summer drought reduces both source strength and sink (root) strength. The rate of root TNC recharge is regulated by relative sensitivity of photosynthesis and shoot growth to water stress and phenological development of the plant. It is also affected by sink activity. The rate of root TNC recharge is higher for trees on the sandy loam site than those on the clay loam site in spring because the soil conditions are more favorable for both photosynthesis and root growth. And the opposite holds true during the midsummer, since the plants on the clay loam site suffer less severe water stress than those on the sandy loam site.
Transpiration of mesquite is regulated by strong stomatal response to internal and external factors. A close correlation between stomatal conductance and air-leaf vapor pressure difference (VPD) was found in mesquite. Stomata do not respond to bulk leaf water potential, indicating that water relation in the epidermis cells is independent of that in the mesophylls. Stomatal response to VPD is a desiccation avoidance mechanism, which allows the plant to succeed in the semiarid west Texas rangeland. Stomatal conductance is positively correlated with soil water status. This is another indication that water stress is sensed primarily by the root system. This root-shoot communication may be mediated by hydraulic resistance in soil-root-xylem system. It has been shown that hydraulic resistance is highly correlated with stomatal conductance on both sites. It is postulated that hydraulic resistance may influence the long distance transport of message from the roots to the shoots. Xylem water potential is a function of transpiration. During drought period, mesquite trees restrict transpiration so that soil water supply can be extended. As a result, photosynthesis is reduced due to insufficient transpirational cooling. The plant has evolved various means to reduce thermal load during drought periods.
Water use efficiency (WUE, A/E) at leaf level is high in mesquite (2.5-3 mmol C02/niol H2O). However, WUE at plant level will be considerably lower because large amounts of carbon are allocated to belowground plant parts. WUE varies between the plants occupying the different habitats. This ecotypic differentiation in WUE is associated with anatomical modification. Intrinsic water use efficiency (A/g) and WUE tend to increase when the plant is growing on the more xeric sandy loam site. This provides another explanation for the widespread distribution of the plant in arid/semiarid areas.
In mesquite plant, the early leaves and late leaves exhibit distinguishable characteristics in photosynthesis, leaf specific weight, leaf nitrogen content, leaf conductance and water use efficiency. The leaf morphology is also different. These changes reflect the environmental influence on leaf development and plant response to this influence. The common response of mesquite to soil water deficit is to restrict initiation of late leaves so that the plant canopy size is adjusted to match the soil water availability. The reduction in transpiring area is another effective mechanism to avoid desiccation in a drought environment. Although no difference in photosynthetic rates between the populations growing on the two sites, the productivity of mesquite trees on the clay loam site is greater than that on the sandy loam site, because of greater contribution of late leaves to the aboveground organic carbon pool.