Seasonal water use dynamics of Macadamia (F. Muell) orchards

Abstract

Increased demand for sustainably produced, healthy, and nutritious food has seen certain segments of the world agricultural sector flourish in the past few decades. The macadamia nut industry in particular has expanded at a tremendous rate, with more than 10 000 hectares of trees being planted annually across a range of environments. The greatest portion of these expansions occur in semi-arid areas, which are characterized by highly variable rainfall patterns, and are as a result irrigated to minimize the risk of yield, quality and income losses, brought about by water stress. The recently commercialized nature of the crop, in combination with lack of water use research specific to macadamia, has created great uncertainty amongst producers. This study has therefore firstly aimed at gaining a fundamental understanding of leaf gas exchange and macadamia transpiration (Ec) in response to a range of environmental and physiological variables, in an attempt to identify the driving variables of transpiration. Secondly, the study aimed to identify crop water use models that best incorporate the driving variables of Ec, in order to transfer results obtained from this study, to a range of growing environments. Measurements of leaf gas exchange, hydraulic conductance, canopy dimensions, weather, and Ec were made over an approximate three year period, in a fully irrigated commercial mature bearing (MB) and immature bearing (IB) macadamia orchard in the Mpumalanga province of South Africa. Leaf gas exchange measurements, included, but were not limited to net CO2 assimilation rate (A) and stomatal conductance (gs). Transpiration measurements were obtained using sap flow measurements using the heat ratio method of the heat pulse velocity technique. Macadamia A was found to be slightly lower than that of other evergreen subtropical crops, which is largely attributed to substantial stomatal and non-stomatal limitations to A. Non-stomatal limitations to A were linked to an internal light limitation resulting from the sclerophyllous nature of leaves. Stomatal limitations stem from the predominantly isohydric nature of macadamias, where gs is carefully controlled in order to maintain midday leaf water potential within certain safety margins. Isohydric behaviour suggested an underlying hydraulic limitation, which was found to exist within the stem to leaf interface of macadamias. Responses of gs to leaf vapour pressure deficit (VPDleaf) showed that gs declined as VPDleaf exceeded 2.5 kPa. The response of gs to VPDleaf, however, varied substantially throughout the season, being significantly higher during fruiting periods compared to non-fruiting periods, implying isohydrodynamic behaviour and emphasizing the influence of phenology on leaf gas exchange. Similar results were found on both fruiting and non-fruiting branches implying that an upregulation of gs at leaf level would most likely lead to an upregulation at the canopy level, which would lead to increased Ec. During fruiting periods, macadamia Ec was ~20% higher compared to non-fruiting periods, with no significant difference in weather variables or canopy size, which could act as confounding factors. Increased Ec during fruiting periods was associated with a greater response of Ec to air vapour pressure deficit (VPDair) in the 0.0 – 3.0 kPa range, which was similar to the observed increases of gs in response to VPDleaf > 1.5 kPa. An examination of transpiration crop coefficients (Kt), confirmed that increased Ec during fruiting periods stem from a physiological upregulation of gs and subsequently canopy conductance (ga). Besides physiological and phenological variables influencing macadamia Ec, physical attributes (i.e. canopy size) and weather variables remained the key driving variables of Ec. Macadamia Ec increased in a linear fashion when VPDair < 0.8 kPa, solar radiation (Rs) <0.3 MJ m-2 h-1 and reference evapotranspiration (ETo) <0.13 mm day-1, but failed to increase at the same rate when these limits were exceeded. The reduction in the rate of Ec in response to increases in environmental evaporative demand under non-limiting soil water conditions, indicates that Ec in macadamias is a supply-controlled system. Supply controlled Ec was confirmed upon examination of maximum daily recorded Ec (Ec max) in response to increases in the aforementioned weather variables, with daily Ec max failing to increase at VPDair >1.5 kPa, Rs > 15 MJ m-2 day-1 and ETo > 3.5 mm day-1. The response of Ec and Ec max to these weather variables did not vary between the two orchards, the magnitude of both Ec and Ec max, however, differed between orchards, being highest in the MB orchard. Higher Ec in the MB orchard was largely attributed to a ~60% larger canopy, with Ec in the MB orchard being ~60% more than Ec in the IB orchard. Transpiration measured in this study, however, remains site specific, and identification and validation of crop water use models were therefore needed to extrapolate data to a broader range of growing environments. The study therefore evaluated three models including the widely used FAO-56 dual crop coefficient, a canopy conductance (gc) model in conjunction with the Penman-Monteith equation, and a canopy transpiration model. The study showed, that a poor estimation of daily Kt and subsequently Ec was obtained using the FAO-56 dual crop coefficient model, which was largely attributed to overestimation of Kt and therefore Ec when daily reference evapotranspiration (ETo) rates exceeded 4.0 mm day-1, and an underestimation of Kt and Ec when ETo < 2.0 mm day-1. The model, however, provided reasonable estimates of Kt and Ec on a monthly or seasonal basis, with only slight discrepancies observed between measured and simulated Kt and Ec from January to April in each season, which was attributed to physiological upregulation of Ec in the presence of fruit. The gc estimations in conjunction with the Penman-Monteith equation, provided more accurate estimates of daily Ec in both the MB and IB orchards, compared to the empirical FAO-56 dual crop coefficient model, but was particularly sensitive to seasonal changes in leaf area index (LAI), with adjustments of maximum canopy conductance (gc max) being required to achieve accurate estimates of Ec. An adjustment for variations in LAI, however, failed to provide increased estimates of Ec during the January to April period reaffirming the phenological and physiological influence of fruit on gc and Ec during this period. Measurements of macadamia gc in this study was rather low (0.3 – 0.7 mm s-1) in relation to ga (37 - 75 mm s-1), confirming that macadamias are well coupled to the atmosphere. The high degree of coupling in macadamias implies that changes in gc would lead to direct changes in Ec, which contributed to the success of the use of a simplified Ec model. This model provided reasonable estimates of daily Ec without multiple adjustments for canopy size being needed within each of the orchards. The Ec model, similar to the other models tested, however, failed to provide reasonable estimates of Ec during the January to April period. The results from this study have shown that macadamias are predominantly isohydric in nature, a trait which ultimately dictates leaf gas exchange and Ec in this recently domesticated subtropical crop. Strict stomatal control in response to increased atmospheric evaporative demand, is also evident in the supply controlled nature of macadamia Ec, which has added to the success of mechanistic models in accurately estimating macadamia Ec. Although the study has reaffirmed that Ec is largely driven by environmental demand and canopy size, it demonstrated that physiological and phenological factors can have a significant effect on leaf level gas exchange and subsequently Ec of macadamias.