Vapor pressure deficit (VPD) is the driving force for plant water loss. However, air relative humidity (RH) can be used as a surrogate for VPD. While plants can adapt to environments with varying RH, little is known about how they respond to sudden shifts in RH. Areas of Southern California can experience drastic shifts in RH, from 60% or greater to less than 20% in just a few hours. The effect of these shifts on avocado (Persea americana Mill.) tree productivity is a major concern to growers. We studied the effect of shifts in RH on `Hass' avocado leaf stomatal conductance (gs) and branch sap flow in trees grafted on Duke 7 clonal rootstock. Under many conditions, the avocado assimilation rate is governed by gs. When gs is high in morning (>150 mmol·m-2·s-1), the water loss generally leads to some stomatal closure in the afternoon (50% or more). Conversely, low morning gs results in a higher gs rate in the afternoon (10% to 20% stomatal closure). This relationship between morning and afternoon gs is intensified by a shift from high to low RH in the afternoon. Therefore, in a drier atmosphere in the afternoon, the afternoon depression in gs is greater, leading to an impaired assimilation capacity. We hypothesize that the afternoon decrease in gs is due to low root/shoot hydraulic conductivity since soil water is readily available. While it is possible that low hydraulic conductivity on gs is exacerbated at the graft union, sap flow of grafted trees in greenhouse studies was nearly equal to trees on their own roots (ungrafted); in fact, often the depression in the afternoon was less on grafted trees. This suggests that while avocado is not suited to areas with low RH, water flow through the roots could be an additional criterion in selecting improved rootstocks.
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