The effect of drought on leaf gas exchange and carbohydrate supply for a semi-arid, rhizomatous oak shrub, Quercus havardii RYBD

Quercus havardii is a rhizomatous oak shrub that occupies approximately 2.7 million ha of land In western Texas, western Oklahoma, and eastern New Mexico. Due to the variable rainfall conditions that exist in the growing region, Quercus havardii should exhibit tolerance to drought conditions.

The effect of drought upon leaf gas exchange, leaf, root, and rhizome carbohydrate levels, and mycorrhizal infection was measured for Q. havardii growing in a sandy, uniform, flat site 90 km southwest of Lubbock, TX and for seedlings In the greenhouse. Pre-dawn leaf water potentials (öpd) were measured as an Indication of leaf water status. Field analyses were conducted during a prolonged, early season drought and after subsequent heavy rainfall. Fully expanded leaves located at the midpoint of the first growth flush were used for leaf gas-exchange and carbohydrate analyses. Ectomycorrhizal colonization was determined for roots of ramets at the edge of the study site.

During the late spring of 1996, Q. havardii had been exposed to drought conditions for over 1 month. Even with the extremely low soil moisture content of 0.23%, the öpd was relatively high at -1.28 MPa. The maintenance of a relatively high water potential under these conditions may have been due to an ability to reach deep water resources or to store water in the roots and especially rhizomes. However, water loss was controlled by reducing stomatal conductance (gs) during the hottest period of each day. Although net CO2 assimilation (A) was maintained at a lower value throughout the drought, when compared to the situation after the relief of drought, water stress did suppress leaf A and carbohydrate levels. Thus, Q. havardii was functioning at less than an optimum level and under some water stress.

The greenhouse study indicated that seedlings of Q. havardii are able to maintain at least some photosynthetic activity down to a öpd of about -1.4 to - 1.5 MPa, after which A is reduced to essentially zero. For öpd down to -0.9 MPa changes in A are Increasingly correlated with changes in gs. For seedlings with öpd between -0.9 and -1.3 MPa, there is some indication of biochemical dysfunction restricting A, ignoring potential patchy stomatal closure. Below -1.3 MPa, non-stomatal restrictions to A are the most likely factors leading to values of A near zero over a large range of Ci values. Unlike mature plants, the seedlings do not have the water holding capacity of the large rhizome or a root system to reach deep water sources to maintain öpd and A under prolonged drought. Therefore, the seedlings are more vulnerable to leaf water potentials below -1.4 MPa than are the mature plants. If they receive water before leaf senescence in response to stress, A recovers well after 2 days. Therefore, Q. havardii behaves as a drought postpone rather than drought tolerant species.